Recent NMR developments applied to organic–inorganic materials Christian Bonhomme a,⇑ , Christel Gervais a , Danielle Laurencin b a Laboratoire de Chimie de la Matière Condensée de Paris, UMR CNRS 7574, Université Pierre et Marie Curie, Paris 06, Collège de France, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France b Institut Charles Gerhardt de Montpellier, UMR5253, CNRS UM2 UM1 ENSCM, CC1701, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France Edited by J.W. Emsley and J. Feeney article info Article history: Received 17 July 2013 Accepted 17 October 2013 Available online 26 October 2013 Keywords: Solid state NMR Organic–inorganic materials Hybrid materials Interface Structure abstract In this contribution, the latest developments in solid state NMR are presented in the field of organic–inor- ganic (O/I) materials (or hybrid materials). Such materials involve mineral and organic (including poly- meric and biological) components, and can exhibit complex O/I interfaces. Hybrids are currently a major topic of research in nanoscience, and solid state NMR is obviously a pertinent spectroscopic tool of investigation. Its versatility allows the detailed description of the structure and texture of such com- plex materials. The article is divided in two main parts: in the first one, recent NMR methodological/ instrumental developments are presented in connection with hybrid materials. In the second part, an exhaustive overview of the major classes of O/I materials and their NMR characterization is presented. Ó 2013 Elsevier B.V. All rights reserved. Contents 1. Introduction ........................................................................................................... 2 2. Experimental and theoretical NMR developments applied to the study of organic–inorganic materials .................................. 3 2.1. Texture of porous materials ......................................................................................... 3 2.1.1. 129/131 Xe NMR ............................................................................................. 3 2.1.2. 83 Kr, 13 C, and 1 H NMR ...................................................................................... 5 2.1.3. PFG NMR ................................................................................................. 5 2.1.4. MAS PFG NMR ............................................................................................ 6 2.2. DOSY NMR ....................................................................................................... 7 2.3. Increasing NMR sensitivity .......................................................................................... 8 2.3.1. Microcoils and MAS ........................................................................................ 8 2.3.2. PHIP and MRI ............................................................................................. 9 2.3.3. DNP MAS ................................................................................................. 9 2.4. First principles calculations ........................................................................................ 11 2.4.1. Metal organic frameworks and related metal organic ligand crystalline compounds ................................... 11 2.4.2. Crystalline templated systems ............................................................................... 13 2.4.3. Interfaces in disordered systems ............................................................................. 13 2.4.4. Functionalized metallic clusters ............................................................................. 16 3. Recent applications of solid state NMR to organic–inorganic materials ........................................................... 16 3.1. Hybrid silicas .................................................................................................... 16 3.1.1. Advanced solid state NMR techniques ........................................................................ 17 3.1.2. Encapsulation of molecules ................................................................................. 18 3.1.3. Interfaces in silica derived hybrids ........................................................................... 18 0079-6565/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.pnmrs.2013.10.001 ⇑ Corresponding author. E-mail address: [email protected](C. Bonhomme). Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 Contents lists available at ScienceDirect Progress in Nuclear Magnetic Resonance Spectroscopy journal homepage: www.elsevier.com/locate/pnmrs
Transcript
Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
Contents lists available at ScienceDirect
Progress in Nuclear Magnetic Resonance Spectroscopy
journal homepage: www.elsevier .com/locate /pnmrs
Recent NMR developments applied to organic–inorganic materials
0079-6565/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.pnmrs.2013.10.001
Christian Bonhomme a,⇑, Christel Gervais a, Danielle Laurencin b
a Laboratoire de Chimie de la Matière Condensée de Paris, UMR CNRS 7574, Université Pierre et Marie Curie, Paris 06, Collège de France, 11 Place Marcelin Berthelot, 75231 Paris Cedex05, Franceb Institut Charles Gerhardt de Montpellier, UMR5253, CNRS UM2 UM1 ENSCM, CC1701, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France
Edited by J.W. Emsley and J. Feeney
a r t i c l e i n f o
Article history:Received 17 July 2013Accepted 17 October 2013Available online 26 October 2013
Keywords:Solid state NMROrganic–inorganic materialsHybrid materialsInterfaceStructure
a b s t r a c t
In this contribution, the latest developments in solid state NMR are presented in the field of organic–inor-ganic (O/I) materials (or hybrid materials). Such materials involve mineral and organic (including poly-meric and biological) components, and can exhibit complex O/I interfaces. Hybrids are currently amajor topic of research in nanoscience, and solid state NMR is obviously a pertinent spectroscopic toolof investigation. Its versatility allows the detailed description of the structure and texture of such com-plex materials. The article is divided in two main parts: in the first one, recent NMR methodological/instrumental developments are presented in connection with hybrid materials. In the second part, anexhaustive overview of the major classes of O/I materials and their NMR characterization is presented.
Organic/inorganic (O/I) materials (or hybrid materials) are nowfully integrated in the field of nanosciences [1]. They are multi-functional by nature, and can be used as innovative advancedmaterials. They can also act as precursors to new materials withhighly attractive properties in optics, electronics, membranes,functional coatings and sensors [2]. Their synthesis and character-ization require a true multidisciplinary approach, involving chem-istry, physics, materials science and processing, engineering, and insome cases biology.
From a structural point of view, hybrids show a high degree ofcomplexity, as they correspond to multicomponent assembliesexhibiting interfaces. These O/I interfaces are often difficult tocharacterize [3]. Solid state NMR is a key characterization tech-nique, as it allows one not only to investigate the local environ-ment of nuclei (through various NMR interactions, such as thechemical shift, d, or the quadrupolar interaction, Q), but also thelonger range connectivities between nuclei (using either through-bond, indirect (or scalar) spin–spin (J), or through-space, dipolar(D) interactions). In 2009, Geppi et al. [4] reviewed some applica-tions of NMR to the description of hybrid materials. First, they pro-posed a definition of an O/I material: ‘‘[it is] a material constitutedby both organic and inorganic components, where the average domainsizes of either one or both components range from nano- to microme-ters [. . .] both components exhibit a functional role in determining thefinal properties of the material’’. They then reviewed the main spec-troscopic targets for hybrid components: 29Si, 1H, 13C (and somequadrupolar nuclei such as 27Al, 17O). They selected studies relatedto the nature of O/I interfaces, local dynamics (via cross-polariza-tion Magic Angle Spinning (CP MAS) or 2H NMR), measurementof domain sizes [5], and conformation of the polymeric compo-nents. In terms of NMR sequences, 2D 1H–29Si(13C) CP MAS,1H–1H double-quantum (DQ) MAS, Nuclear Overhauser EffectSpectroscopy (NOESY), 1H–1H spin diffusion experiments andrelaxation time measurements were particularly emphasized.More recently, Alonso and Marichal published a detailed tutorialreview on solid state NMR applied specifically to micelle-tem-plated mesoporous solids [6]. All the panel of the NMR interactionswas used here for full spectroscopic characterization of these hy-brid materials at different scales. Topics such as the formation,
the structural characterization, the surface and texture of materi-als, were illustrated.
During the last 5 years, major instrumental/methodologicalNMR developments have been achieved [7,8] including: (i)ultra-high magnetic field (up to 23.5 T, m0(1H) = 1 GHz) [9]; (ii)ultra-fast magic angle spinning (up to mrot = 100 kHz) [10], (iii)extension of NMR techniques to paramagnetic samples [11,12];(iv) new decoupling/recoupling NMR schemes, including se-quences dedicated to quadrupolar nuclei [7,8,13–15]; (v) newbroadband excitation schemes [16,17]; (vi) increase in NMR sen-sitivity by orders of magnitude, including the use of microcoils[18], para-hydrogen induced polarization (PHIP) [19] and dynamicnuclear polarization (DNP) [20]. The problem of sensitivity inNMR is indeed crucial in the case of specific processing of hybridmaterials. Quoting a review article by Sanchez et al. [21]: ‘‘. . . Inour opinion, techniques that can give accurate information (eitherex situ and in situ) on the chemical speciation of periodically orga-nized mesoporous films are still lacking’’. In particular, the detailedchemical speciation of hybrid mesoporous films remains highlydifficult to establish. Here, the key problem is obviously relatedto mass-limited samples to be analyzed. For example, if weconsider a unique film of porous hybrid silica with the followingcharacteristics: surface area, S = 2 cm2; thickness = 300 nm;density, q � 1.5 g cm�3 (depending on the porosity), the order ofmagnitude of the sample mass is less than 0.1 mg. Such a massof sample is clearly challenging for conventional solid stateNMR spectroscopy.
The studies of hybrid materials have clearly benefited from theexperimental and methodological developments of NMR, by help-ing establish their structure at the atomic level as well as their tex-ture. The aim of this review is to illustrate this by selectedexamples. Thus, examples of advanced NMR characterizations ofhybrid materials will be given, focusing not only on hybrid phasessuch as those initially defined by Geppi et al. [4], but also onmaterials whose structure can be related to one of the componentsof a complex hybrid material. Indeed, we have chosen to includefor example recent solid state NMR investigations of simple metalcomplexes bearing organic ligands and coordination polymers, be-cause this can be useful for synthetic materials chemists, so thatthey know which NMR experiments can serve as a starting pointfor characterizing more complex hybrid phases.
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Section 2 deals with methodological NMR developments ap-plied to the characterization of O/I materials. 129/131Xe, pulsed fieldgradient (PFG), MAS PFG, and diffusion ordered spectroscopy(DOSY) NMR (Sections 2.1 and 2.2) are presented as pertinent spec-troscopic tools of investigation of hybrid phases. New methodsleading to a major increase in sensitivity are also presented(including microcoils under MAS, PHIP and DNP – Section 2.3). Alarge subsection is then devoted to first principles calculations ofNMR parameters (Section 2.4), following the pioneering work ofPickard and Mauri and the introduction of the GIPAW (GaugeIncluding Projector Augmented Wave) method [22]. Though tworecent reviews on GIPAW were recently published in the literature[23,24], we emphasize here applications to hybrid materials and O/I interfaces, considering both periodic and cluster density func-tional theory (DFT) approaches.
Section 3 illustrates recent applications of NMR to O/I materialsby focusing on four main families of compounds. Section 3.1 con-cerns hybrid silicas (as silica is ubiquitous as inorganic componentin a large variety of O/I materials). Several classes of silica derivedmaterials, such as bioactive silica, polyhedral silsesquioxanes(POSS), ionogels and nanoparticle networks, are reviewed. Empha-sis is stressed on new developments related to very high-field ul-tra-fast MAS NMR. Section 3.2 deals with ionic hybrid solids,including phosphate based materials, hybrid cationic/anionic clays,and functionalized micro/nanoparticles of ionic solids and metaloxides. Section 3.3 is devoted to the detailed description of bioma-terials. Natural biomaterials, such as diatoms (based on amorphoussilica), bones and teeth (based on calcium phosphates) and CaCO3
based derivatives, are first described, before focusing on syntheticbioinspired materials. Finally, Section 3.4 is related to NMR appliedto metal complexes, coordination polymers (involving phospho-nate, carboxylate and cyanide ligands, just to name a few), metalorganic frameworks (MOFs) and functionalized metalnanoparticles.
2. Experimental and theoretical NMR developments applied tothe study of organic–inorganic materials
In this section, we emphasize new instrumental/methodologi-cal developments related to solid state NMR applied to the charac-terization of O/I materials. These developments concern first thetexture of hybrids and their characterization by hyperpolarized(HP) gas and PFG NMR techniques. Gradients are also an essentialpart of DOSY, which appears as a valuable tool of investigation forfunctionalized nanoparticles. During the past 5 years, impressivedevelopments have been published on methods for improvingNMR sensitivity. Among other techniques, micro-coils, PHIP andDNP will be described, as well as illustrative applications in thefield of hybrid materials.
2.1. Texture of porous materials
The texture of organic–inorganic materials is a fundamentalcharacteristic that has to be taken into account for potential appli-cations. Among various parameters, the fine description of theporosity of the materials is of paramount importance. Severalphysico-chemical techniques can be implemented to measure theintrinsic porosity, among which is 129Xe NMR spectroscopy. In thissection, we will focus mainly on hyperpolarized (HP) 129Xe NMR, asit allows a very large increase in sensitivity. The advantage of thistechnique for the characterization of porous materials, whetherpurely inorganic or hybrid, will be demonstrated. Moreover, thetransport of matter through diffusion inside the pores of the mate-
rials is also a crucial parameter to measure. PFG methods areparticularly suited for this purpose, with or without rotation ofthe sample at the magic angle.
2.1.1. 129/131Xe NMRThermally polarized xenon NMR has been used for decades for
the characterization of porous materials such as zeolites, mesopor-ous silicas and silica glasses [25–29]. Indeed, the NMR parametersof xenon such as the isotropic chemical shift, the chemical shiftanisotropy (CSA), the linewidth and the longitudinal relaxationtime T1 are able to encode useful information related to the poros-ity and the internal surface of materials. The xenon NMR techniquehas been largely improved with the introduction of hyperpolariza-tion via optical pumping methods [30–33]. In this case, spin polar-ization can be enhanced by four orders of magnitude whencompared to that at thermal equilibrium.
Continuous flow hyperpolarized 129Xe NMR has been used tocharacterize the unique pseudo-hexagonal form of tris(o-phenyldi-oxy)cyclophosphazene (TPP) [34]. This form can be considered asan empty nanoporous structure comparable to a zeolite-like archi-tecture. Indeed, it exhibits stability at room temperature (RT) andthe open channels are accessible to gases and guest molecules. Itwas demonstrated that the shape of the nanopores is close to a reg-ular cylinder (whose diameter is actually slightly larger than thediameter of Xe � 4.4 Å). It follows that the diffusion of Xe in thenanopores is essentially fast and uniaxial. Continuous flow HP129Xe allowed: (i) recording 129Xe spectra with short recycle delays– here, 200 ms. (ii) recording 129Xe spectra with acceptable signal-to-noise (S/N) ratio with very low concentration of Xe (�1% of Xediluted in He). The symmetry of the electronic shell of Xe is clearlyreflected in the observation of the 129Xe CSA, with a clear change ofits sign when increasing the Xe % in the helium medium. At lowXe%, the CSA pattern is representative of the Xe-wall interactions,whereas the Xe–Xe interactions dominate at high Xe%. Very inter-estingly, these opposite effects vanish almost completely forXe% � 30%, and, as a consequence, an isotropic 129Xe spectrum(with almost no residual CSA) was observed. More recently [35],the kinetics of the exchange of Xe between the gas and the chan-nels was described in self-assembled L-alanyl-L-valine nanotubesusing continuous flow HP 129Xe, as well as 2D exchange spectros-copy (EXSY).
HP 129Xe NMR was successfully applied to the characterizationof silica derived mesoporous materials exhibiting amorphous walls[36]. The same approach was also applied to more organized silicawalls [37]. Variable temperature (VT) experiments clearly demon-strated the existence of various well defined Xe species: free gas,adsorbed and condensed species on the silica walls. The evolutionof d(129Xe) vs. temperature led to the evaluation of the adsorptionDH value. From a dynamical point of view, variable temperature2D HP 129Xe EXSY experiments demonstrated that Xe gas phaseaccess the mesopores on the ms timescale.
In a series of contributions, SBA (Santa Barbara) derived archi-tectures were described by means of various Xe NMR techniquesby Gedeon and coworkers (these particular mesoporous silica withperiodic 50–300 Å pores are obtained starting from triblock copoly-mers). The crystallization of Al-SBA-15 amorphous walls intoZSM-5 derived materials was studied by HP 129Xe NMR [38] leadingto the detailed characterization of the interconnection of the porousnetwork. In the case of arenesulfonic functionalized SBA-15 [39],with variable organic contents, the spectra obtained at various Xepressures and temperatures acted as pertinent fingerprints of theorganic species. It became possible to characterize the homogeneityof the mesoporous porosity towards the organic covering. Most
129Xe (ppm)160 120 80200
Fig. 1. Study of the flexibility of porous hybrid MIL-53(Al) by 129Xe NMR at83.02 MHz. The most shielded component of the spectrum corresponds to the large-pore form of the flexible MIL-53(Al). The deshielded component corresponds to thenarrow-pore form. Reproduced from Ref. [61] with permission (Copyright 2010American Chemical Society).
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interestingly, 1H spectral editing was performed by using 1H–129XeCP [32] and SPINOE [40,41] experiments. SBA structures exhibitingunimodal and bimodal porosities were also characterized by 129XeNMR spectroscopy [42]. From a more general point of view, corre-lations between d(129Xe) and pore size of mesoporous solids wereproposed by Haddad et al. [43].
In an interesting study, Liu et al. [44] demonstrated the closestacking of MCM-49 and ZSM-35 structures in co-crystallizedzeolites. Indeed, very fast exchange of Xe could be monitored inco-crystals: this exchange was even faster than in the case of ana-logues obtained under mechanical mixing. The same approach wasapplied to the study of hierarchical porous structures in mesopor-ous modified zeolites (ZSM-5) [45]. The intrinsic sensitivity of HP129Xe NMR was successfully used for the detection of very lowamounts of crystalline zeolites (<1 wt%) in amorphous aluminosil-icate gels [46]. An interesting mechanistic insight in the dissolu-tion/precipitation process of silica during water treatment ofSBA-15 and MCM-41 derived structures was proposed as well, withthe exploration of microporosity by HP 129Xe NMR spectroscopy[47].
Finally, nanometric mesoporous silica-derived thin films wereinvestigated by HP 129Xe NMR [48]. In this particular case, thespectroscopic challenge was high, as very small amounts of samplewere available, leading to an intrinsic loss in magnetization. Here,0.1–0.2 mg of mesoporous silica were typically studied and thepresence of shallow mesopores was clearly established.
In addition to silica-based porous structures, HP 129Xe NMR wasalso used successfully for the characterization of organic–inorganicwheels and MOF structures. In an elegant paper, Cheng et al. [49]considered molecular wheels, such as [Ga10(OMe)20(O2CMe)10]and [Ga18(pd)12(pdH)12(O2CMe)6(NO3)6](NO3)6 (pd2�: 1,3-propan-ediolate), as potential nanoporous materials suitable for pureone-dimension molecular diffusion studies. Such hybrid wheelsare interesting because their characteristic diameter U could betailored (U � 8.1 Å for Ga10 and U � 10.4 Å for Ga18). Based onthe observed 129Xe chemical shifts, it seemed that the interactionsbetween Xe and the walls are analogous for both structures. How-ever the modes of diffusion (single-file and one-dimensionalFickian) were clearly distinguished.
During the past years, metal organic framework (MOF) com-pounds have been investigated by advanced solid state NMR tech-niques, including 129Xe NMR spectroscopy [50]. In 2006, Böhlmannand coworkers [51] were the first to investigate a MOF structure(namely Cu3(benzene 1,3,5-tricarboxylate)2) by means of 129XeMAS NMR in connection with EPR measurements (to characterizethe state of Cu in the structure). All 129Xe experiments were per-formed without HP. Using two different synthetic protocols, twodistinct samples were studied, one with (CH2Cl2, DMF, H2O) (sam-ple B) and one with only water occupying the pores (sample A).Two resonances were observed at all loading pressures for sampleA, demonstrating the presence of pores with two different sizes.The lines are still broadened under MAS due to the influence ofparamagnetic centers in the structure. In cases where residual sol-vent molecules (CH2Cl2, DMF) were present in the porosity, onlythe larger pores were filled with Xe. Using a similar approach (sta-tic and MAS experiments without HP), Ueda et al. [52] investigatedthe xenon adsorption behavior of M = Rh- and Cu-based MOFs[M2(O2CPh)4(pyz)]n (pyz: pyrazine). In the case of M = Rh, VTexperiments, followed by the analysis of the CSA patterns, showedthat Xe2 dimers are predominant and localized mostly in extre-mely small free volumes of the structure.
In 2007, Ooms and Wasylishen [53] extended the HP continu-ous flow 129Xe NMR approach to analyze the Xe adsorptionproperties of four iso-reticular MOFs (IRMOF-1, -7, -8 and -10).
These MOFs consist of Zn4O units and exhibit cubic cages of vari-able size. It was demonstrated that the 129Xe chemical shifts aredependent on the characteristic size of the cages and thecorresponding distribution of Xe atoms.
Comotti et al. [54] proposed a detailed description of nanochan-nels exhibiting distinct cross-sections in an aluminum naphthalen-edicarboxylate (NDC) compound, Al(OH)(1,4-NDC)�2H2O. Thespace and surface of the large nanopores were explored andphysisorption energies were deduced from VT HP 129Xe NMR. Inparticular, it was shown that naphthalene species act as the mainpart of the walls available for selective adsorption. It was also dem-onstrated that the small nanochannels of the structure(3.0 � 3.0 Å2) are not accessible to Xe atoms.
One of the most spectacular properties of some selected MOFs isrelated to reversible structural transformation, flexibility and‘‘breathing’’ of the corresponding architectures upon adsorptionof gases [55]. Jiang et al. [56] characterized the flexibility of a Cu-MOF (Cu(bpy)(H2O)2(BF4)2(bpy)) by direct detection of 11B reso-nances. One of the first uses of 129Xe NMR was the study ofNi2(2,6-ndc)2(dabco) (2,6-ndc: 2,6-naphthalenedicarboxylate, dab-co: 1,4-diazabicyclo[2.2.2]octane) [57]. It was demonstrated thatthe 129Xe parameters are suitable for the characterization of struc-tural states defined by narrow and wide pores, respectively. Theapproach was extended recently to a series of compoundsM2(2,6-ndc)2(dabco) (with M = Ni, Cu, Co, Zn) [58] for the finedescription of structural flexibility and local dynamics. It has tobe stressed that in this study, a new high-pressure in situ 129Xeapparatus was used [59]. This new equipment was also used forspectroscopic studies of breathing transitions in Ni-MOFs, in com-bination with ab initio molecular dynamics (MD). Repeated poreopening/closure mechanisms were successfully characterized by129Xe NMR.
Using a newly developed thermodynamic model (in combina-tion with xenon adsorption studies), a phase diagram for Xe inMIL-53(Al) was established [60]. Thermally polarized and HP129Xe NMR experiments were implemented in order to induce alarge to narrow pore transformation. The 129Xe CSA was used forthe first time for quantitative purposes in MIL-53 materials(Fig. 1) [61].
In contrast to the 129Xe isotope, the literature related to 131Xe(I = 3/2, natural abundance: 21.2%) is much more restricted, thoughsome rare studies related to porous materials have been published[62–64]. Such studies were clearly hindered by the low c(131Xe)
A
B
pore wall
liquid phase
Fig. 2. Diffusion in a cylindrical pore (the walls of the pore are represented in grey)investigated by PFG NMR. The filling of the pore is considered as partial (scheme(A): wetting surface, scheme (B): nonwetting surface). Diffusion motion in theliquid phase (in blue) is represented by solid lines, whereas molecular flights in thegaseous phase are represented by dashed lines. Adapted from Ref. [82] withpermission (Copyright 2012 American Chemical Society). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)
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and the broadening of the lines (typically, kHz linewidths), leadingto a very small intrinsic sensitivity of the experiments. Recently,Stupic et al. [65] revisited the preparation and properties of HP131Xe with large spin polarization (up to 2.2%). The signal enhance-ment was estimated to �5000 times the thermal equilibriumpolarization (at 9.4 T). Here, spin-exchange optical pumping wasused to produce alkali metal free HP 131Xe. It was further demon-strated that H2O molecules had a major impact on the T1(131Xe)relaxation time.
2.1.2. 83Kr, 13C, and 1H NMRHydrated surfaces (of hydrophilic borosilicates and hydropho-
bic siliconized glasses) have been successfully studied by HP 83Krspectroscopy [66] under near atmospheric conditions. 83Kr hasthe following characteristics: I = 9/2, natural abundance: 11.5%,and a lower gyromagnetic ratio than 129Xe. Most interestingly,83Kr has a quadrupole moment affecting 83Kr relaxation and line-shapes through the modulation of the electronic environment ofKr by surface adsorption. These highly sensitive effects weresuccessfully used for the detailed description of macroporousmaterials (for such materials, d(129Xe) is usually not pertinent asa spectroscopic probe) [67].
In the framework of CO2 sequestration, MOFs can be consideredas potential candidates. Among them, MOFs with exposed Mg2+
cations sites are suitable for improved capture of CO2. The dynam-ics of CO2 in MOFs exhibiting open metal sites is still debated. In arecent contribution, Kong et al. [68] used 13C-enriched CO2 and VTstatic 13C NMR to investigate the local dynamics of carbon dioxidemolecules in Mg2(dobdc) (H4dobdc: 2,5-dihydroxyterephthalicacid). A careful analysis of the 13C CSA powder patterns allowedthe authors to propose realistic rotations of CO2 species and to dis-tinguish several populations of adsorbed molecules. Activationenergies and correlation times (T1 measurements) were derivedas well. Such a study can be seen as a preliminary work leadingto strong structural constraints for future DFT and MD calculationsrelated to CO2 adsorption. Pinto and coworkers [69] also imple-mented 13C and 15N MAS NMR to probe the amine-CO2 bondingin amine-modified nanoporous materials (porous clay heterostruc-tures or PCH). Carbamate and carbamic acid were clearly detected.Their stability was studied subsequently and a possible mechanismof CO2 activation was proposed.
Due to its very high intrinsic sensitivity, 1H NMR is a tool ofchoice for the characterization of porous hybrid materials. Adsorp-tion of water molecules in the MOF structure Cu3(BTC)2 (BTC:benzene 1,3,5-tricarboxylate) was recently monitored by 1H MASNMR. [70]. In this case, adsorbed water species are stronglysubjected to the 63/65Cu–1H hyperfine coupling, leading to a strongdeshielding of the proton resonance (up to 11 ppm). 1H spectrawere also found suitable for following the stability of the MOF asa function of time. Grzech and coworkers [71] studied the irrevers-ible binding of hydrogen in Cu3(BTC)2 (containing unsaturated Cu2+
coordination sites). 1H NMR studies demonstrated clearly that H2
molecules are split, leading to Cu0 particles, conversion of theBTC linkers to their acidic form, and final collapse of the structure.A loss of structural integrity was also demonstrated by 1H and 13CNMR in the case of the reaction of Cu3(BTC)2 with ammonia vapor(in particular, specific Cu(II)-induced paramagnetic shifts werepresent in both 1H and 13C spectra) [72]. Furthermore, it was re-cently shown that MOF compounds have the ability to exhibitglass-like properties. Indeed, Besara et al. [73] used 1H NMR exper-iments to establish an unusual order–disorder phase transition at156 K in ((CH3)2NH2)Zn(HCOO)3, following the local dynamics ofðCH3Þ2NHþ2 cations vs. temperature.
Finally, we mention here a very detailed review by Koptyug,dealing with the applications of MRI to study mass transport inporous media [74]. Very recently, synergistic diffusion-diffraction
patterns were implemented for the first time to improve substan-tially the spatial resolution of MRI in the case of porous media [75].
2.1.3. PFG NMRPFG NMR [76–79] is a versatile tool of investigation for probing
dynamics in porous media, leading to a deeper understanding ofdiffusion processes in materials. The ability to monitor the typicallength of the diffusion path by adapting the pulse sequences makesPFG NMR particularly appealing for the study of hierarchicalporous materials (exhibiting nano-, micro- and mesoporosity)[80]. Indeed, the explored diffusion paths range from <100 nm upto 100 lm. It follows that PFG NMR is one of the most versatilein situ techniques of investigation of mass transfer in open struc-tures and host–guest systems. This includes the determination ofboth intra- and inter-crystalline diffusion contributions. Mostinterestingly, PFG NMR is highly sensitive to the presence ofmesopores interconnected to micropores [80,81]. In this particularcase, it has been demonstrated that the intra-crystalline diffusioncan be modeled by the weighted mean of the related micro- andmesoporosity diffusion coefficients.
As an illustrative example of PFG-NMR, Zeigermann et al. [82]studied the diffusion of alkanes in mesoporous silica exhibiting abimodal pore structure. The spherical mesopores (diameterU � 20 nm) were interconnected by much smaller (U � 2–3 nm)worm-like mesopores. PFG NMR was applied to the study of cyclo-hexane diffusion for variable gas pressures (on adsorption anddesorption). In all cases, a unique effective self-diffusion coeffi-cient, Deff, could be extracted by the fitting of the PFG NMR data.In these experiments, the typical time scale explored was the ms.It followed that during this characteristic time, all cyclohexanespecies experienced a displacement exceeding by far the typicallength scale of the material (estimated here by a ‘‘unit cell’’ of�50 nm). Such an observation implied that the condition of afast-exchange regime between distinct regions in the materialwas fulfilled (Fig. 2). Moreover, it was elegantly demonstrated thatthese regions could be described unambiguously by different diffu-sivities. In other words, the extension of the fast-exchange modelwas established for diffusion in hierarchical mesoporous materials.Importantly, the parameters involved in the fast-exchange modelwere precisely defined from a physical–chemical point of view.
Guest-molecules such as n-hexane were also used to probemesopore connectivity in activated carbons [83,84]. In this case,it was demonstrated that the diffusivities were highly dependenton the sample ’’history’’ (in terms of pressure increase/decrease).Very recently, Adem et al. [85] implemented PFG NMR to study dif-
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fusion properties of hexane in pseudomorphic mesoporous silica.Interestingly, it was possible to monitor the modifications of theporous network during the synthesis and to estimate the timeneeded for the porous channels to run through the silica particles.The morphology of the pores was also studied in depth.
The first study of MOFs by PFG NMR was performed on MOF-5[86]. The mobility of methane, ethane, n-hexane and benzene wasstudied. Fast exchange of gas molecules with the surrounding gasatmosphere was clearly demonstrated. The mobility of benzenein MOF-5 was further investigated by Hertel et al. [87] An interest-ing comparison between MD computed self-diffusion coefficientsand experimental data obtained by PFG NMR was presented re-cently in the case of selected chain molecules diffusing in IR-MOF-1 [88].
PFG NMR has also been applied in the field of cement physicalchemistry. Internal post curing (IPC) of cements and concretes isan interesting topic of research (mixtures of additives capable ofdelaying the release of water in fresh cements or concretes).Nevertheless, the mechanisms related to IPC are not well under-stood. PFG NMR (in combination with NMR relaxometry) areundoubtly useful (non-destructive) tools of investigation for IPC.As an example, Friedemann et al. [89] studied IPC of cement pastesincluding alginate spheres. Such alginate spheres contained �98%of water. Very interestingly, it was demonstrated by PFG NMR thatthe water coming from the alginate particles was distributedhomogeneously into the cement phase (for both dormant andaccelerated periods).
Modeling is an essential part of the in-depth analysis of PFGNMR data. As an example, self-diffusion of various liquids (polarand non-polar) in nanopores was described using a three-phasemodel including capillary condensed and adsorbed species, as wellas a gaseous phase [90]. In particular, it was demonstrated that thetransport of molecules in pores was clearly influenced by surfaceinteractions. The contribution of both mechanisms (diffusion inthe gas phase and diffusion in the layers on adsorbed molecules)was found to be dependent on pore loading, pore size and mor-phology, and characterized by complex trends for the effectiveself-diffusion coefficient [91].
In 2006, a fundamental contribution by Valiullin et al. [92] ex-plored for the first time MD and mass transport involved in theadsorption hysteresis of mesoporous materials (here, Vycor porousglass). The authors used a combination of PFG NMR and Monte-Carlo simulations for the precise description of adsorption/desorp-tion dynamics and self-diffusivity. Outside the hysteresis region, itwas demonstrated that pure diffusion dominated the relaxationprocesses. In the hysteresis region and for long times scales, amuch slower relaxation related to activated dynamics was ob-served (corresponding to a rearrangement of the adsorbate densityin the Vycor glass).
Instrumental developments of PFG NMR are also an essentialpart of the widespread use of this technique. In 2012, Schlayerand co-workers extended the PFG NMR approach to X-nucleiself-diffusion studies in various mesoporous and microporousmaterials [93]. A standard X-observe NMR probe was modifiedby adding a z-gradient coil (Maxwell-pair like), leading to gradientsup to �16 T m�1 (maximum gradient current: 100 A). Exceptionalsensitivity was obtained for 7Li and 133Cs spectra in mesocellularsilica foams (MCF) [94,95]. It was demonstrated that: (i) Cs+ andLi+ self-diffusion coefficients are reduced by �4–5 when comparedwith data obtained for bulk solutions; (ii) the tortuosity, s, of thenetwork could be estimated to be �4.5 (with s defined as Dbulk
DMCF).
Such a tortuosity implies that the cations have to multiply theirdiffusion path by �2 in order to mimic the same path as in the bulksolution. The modified X-probe was also used for the study of dif-fusion (and relaxation measurements) of CO2 and CH4 in the
‘‘Cu(BTC)’’ MOF. The measured self-diffusion values were conse-quently compared to MD predictions [96]. Some discrepancieswere observed but not fully understood. Finally, the behavior ofthe strongest adsorption sites was carefully described in samplesexhibiting both CO2 and CH4 co-adsorption.
From a more methodological point of view, Adem et al. [97]demonstrated that the use of dedicated pulse sequences couldhave an impact on the interpretation of the PFG NMR results. Inthe case of NaX zeolite crystals, the authors compared the stimu-lated spin-echo [98] and a 13-interval bipolar pulse sequence[99]. The size of the restricted diffusion domains was systemati-cally underestimated by the stimulated spin-echo sequence, whencompared to analogous data obtained with the 13-interval bipolarsequence. Such a result demonstrated unambiguously the presenceof internal field gradients within the NaX structure (attributed to aheterogeneous distribution of Al atoms). Although such fields(which are due to a large mismatch in magnetic susceptibility)were known to perturb potentially PFG NMR measurements[100], their presence in zeolites was demonstrated here for the firsttime.
Using single pulsed field gradient methodology, it can be provedthat only porous systems exhibiting coherent organization (or‘‘ensemble anisotropy’’) can be studied unambiguously [101]. A pri-ori, this is not the case for randomly oriented anisotropic compart-ments and perfectly isotropic spheres. In a recent work, it wasdemonstrated that angular bipolar double PFG methodology issuitable for the description of highly heterogeneous systems. Per-tinent applications related to emulsions, rocks, and biological spec-imens were reported [102,103]. In this experiment, twoindependent gradient vectors G1 and G2 with a relative angle Ware applied during the pulse sequence: from the experimentalpoint of view, G1 is fixed and the direction of the second gradientG2 is modified with respect to G1 [104].
2.1.4. MAS PFG NMRSome severe limitations of PFG NMR have been stressed in the
past, namely the line broadening due to dipolar interactions andinhomogeneities in the magnetic susceptibility of the samples.The typical time which can be used for the application of the gra-dient pulses is essentially limited by the characteristic T2 decays.Combining PFG NMR with fast macroscopic reorientation of thesample at the magic angle leads to efficient averaging of theseinteractions and consequently to: (i) an enhancement of sensitivityfor small molecular trajectories; (ii) an enhancement of resolutionfor the corresponding spectra. The application of field gradientsalong the spinning axis (at hm = 54.74� from the external magneticfield B0) enables diffusion studies along this particular axis. Theenhancement of resolution (due to fast MAS) leads to spectroscopicinformation that can be used efficiently for multi-component diffu-sion studies. MAS PFG NMR was initiated by Cory et al. [105] andMaas et al. [106]. To the best of our knowledge, the first report ofMAS PFG NMR applied to the diffusion of n-butane in silicalite-1was proposed by Pampel et al. in 2005 [107] and extended subse-quently to other systems [108] and in combination with molecularsimulations [109].
Recently, MAS PFG NMR methodology was applied to the studyof diffusion in MOFs. Gratz et al. [110] followed the two-compo-nent diffusion of hexane and benzene adsorbed in MOF-5. In thiscontribution, the authors emphasized the role of precise settingsconcerning the hardware, the sample alignment, the temperaturecalibration and the gradient properties (strength of the Eddy cur-rents). For several hydrocarbons in MOF-5, a biexponential behav-ior (involving two distinct diffusion coefficients) was observed forthe proton decays of n-hexane and adsorbed benzene. Anothercomponent of the proton spectrum (8.6 ppm) corresponding to
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lattice phenyl species was not observed in the MAS PFG NMRexperiment because of very efficient relaxation. It is also men-tioned that care must be taken when comparing the obtained re-sults with data already published in the literature. Thus, theloading of the molecules and the corresponding occupation ofspace in the MOF structure must be comparable for safe compari-sons to be made. In ZIF-8 crystals, self diffusion coefficients weremeasured separately by MAS PFG NMR [111]. In this case, ZIF-8was loaded with an ethene/ethane mixture (with a typical loadingof four molecules per cavity). It was observed that Dethene/Deth-
ane = 5.5: this result is related to the different size of the corre-sponding molecules. Interestingly, the microscopic diffusivitiesobtained by MAS PFG NMR were comparable to mesoscopic valuesextracted from infrared microscopy [76].
In recent years, other applications of MAS PFG NMR were alsoproposed in the literature. Sharifi et al. [112] implemented a com-bined spectroscopic approach (including impedance spectroscopy,MAS NMR and MAS PFG NMR) to study hybrid organic–inorganicmembranes for PEMFC (proton exchange membrane fuel cell)applications. In this particular case, the resolution achieved byMAS is of prime importance as it allows 1H signals related tonon-diffusing methylene groups and to other diffusing hydroge-nated moieties to be distinguished. An interesting feature of thiscontribution lies in the careful comparison of impedance spectros-copy measurements and MAS PFG NMR results. Here, proton con-ductivity in mesoporous silica was explained by structuraldiffusion (formation and decomposition of Eigen ions from- andto Zündel ions). Another interesting application of MAS PFG NMRin the field of polymer fuel cell membranes was proposed by Jen-
Fig. 3. Studies of polymer fuel cell membranes by means of HRMAS (upper panel) and Mfree and associated water and solvent in the membrane are given vs. the gradient strengtSociety).
kins and coworkers (Fig. 3) [113]. MAS enabled the spectral resolu-tion of mixed solvent systems. Interestingly, a high resolutionmagic angle spinning (HRMAS) probe was used in this work.2D1H NOESY spectroscopy was used in combination with diffusionmeasurements.
The effect of confinement on the diffusion of molecules in mes-oporous materials has been also explored using the MAS PFG NMRmethodology. Romanova et al. [114] studied the diffusion of nema-tic liquid crystals in porous glasses with mean pore diameters of 30and 200 nm. NMR techniques have been applied previously tostudy the reorientation dynamics of confined liquid crystals[115], but self-diffusion was rarely investigated, probably becausein the case of liquid crystals, the anisotropic nature of the rota-tional dynamics prevents full averaging of NMR interactions (lead-ing to rather strong residual dipolar interactions). Consequently,the combination of MAS with PFG is clearly an advantage. It wasdemonstrated that the diffusivities in confined media and in thebulk state are comparable quantitatively.
2.2. DOSY NMR
One general route to hybrid materials is related to the assem-bling of defined nano-building blocks (NBB) (such as metal oxo-clusters or nano-layered materials including clays and LDH layereddouble hydroxides) [116]. Surface functionalization plays a partic-ular role and the local description of the NBB surface becomes animportant spectroscopic target. DOSY has emerged recently as anin situ pertinent tool of investigation [117,118]. DOSY relies onpulsed field gradient methods and is used for the measurement
AS PFG (lower panel) techniques. In the lower panel, the normalized signal decay ofh. Reproduced from Ref. [113] with permission (Copyright 2012 American Chemical
8 C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
of self-diffusion coefficients. Using this method, NBB (and nanopar-ticles in general) can be discriminated through their size. Thequantification of adsorbed and free ligands can be achieved easily,and the dynamics of chemical exchange can be characterized aswell. The first DOSY experiments were applied to the characteriza-tion of gold clusters (protected by a ligand monolayer) [119] andpolypropylene sulfide nanoparticles (NP) [120]. One prototypeDOSY study applied to nanomaterials was proposed by Ribotet al. [121] in the case of CeO2 nanoparticles treated with lauricacid, C11H23COOH. The interactions between NPs and mixtures ofligands can also be studied by DOSY NMR [122], as well as nano-meter sized quantum dots [123], and polyoxometalates (Fig. 4)[124,125]. Recently, new hybrid core–shell star-like architecturesmade of poly(n-butylacrylate) grown from well-defined titaniumoxo-clusters have been well-characterized by DOSY NMR [126].
Some caution has to be taken when interpreting the DOSY re-sults. From a mathematical point of view, it has been demonstratedthat the generation of the diffusion dimension is highly dependenton statistics. Indeed, the inverse Laplace transform is used, which isan ill-posed mathematical problem. It follows that numerical ap-proaches such as multi-exponential least squares fit or maximiza-tion of entropy have to be implemented. Furthermore, from theexperimental point of view, differences in T1 and T2 have to be ta-ken into account explicitly by modifications of the standard DOSYexperiment [127].
2.3. Increasing NMR sensitivity
In an article entitled ‘‘NMR Spectroscopy: Pushing the Limits ofSensitivity’’ [128], Spiess focused on the use of promising NMRtechniques in terms of their increased intrinsic sensitivity, includ-ing HP noble gases (see Section 2.1.1), DNP, mechanical detectionof magnetic resonance signals [129], planar microslot waveguide
Mo O
S
Fig. 4. Examples of structures of polyoxometalates based on Mo. Insert: correlation betwRef. [124] with permission (Copyright 2009 American Chemical Society).
NMR probes [130] and micro-coils for MAS NMR applications. Inthis section, we will focus on the latest developments related tothe use of micro-coils, PHIP and DNP experiments. In all cases, alarge increase in sensitivity is observed: this experimental factopens new avenues for the study of O/I materials and their corre-sponding O/I interfaces.
2.3.1. Microcoils and MASThe use of microcoils with MAS has been developed recently in
two directions: (i) the piggyback scheme [131] and (ii) the magic-angle coil spinning scheme or MACS [132,133]. It is well knownthat microcoils lead to a dramatic increase in sensitivity for smallsamples [134]. They have been primarily used in solution stateNMR and applications in solid state NMR are much more recent[18]. In scheme (i), a thin sample tube is attached to a rotor and in-serted in a microcoil. In scheme (ii), the microcoil and a chip capac-itor are located inside the rotor and spun together at the magicangle. One major advantage of MACS is that it can be adapted tostandard MAS probes, without any hardware modification. Thispoint should popularize the use of MACS for mass-limited samples,in connection with recent progress in microfabrication of MACS in-serts by wirebonding [135]. So far, MACS has been successfullyused for the characterization of small samples, including nanolitertissue biopsies [136,137] and inorganic materials (43Ca NMR)[138].
In the case of hybrid materials, the first MACS spectra wereencouraging, starting from mesoporous hybrid silica-derived thinfilms (<100 lg). 1D 1H and 2D 1H–1H Back-to-Back (BABA) [139]spectra were obtained. Future progress in lithographic and coilmicrofabrication technologies, possibly in connection with micro-fluidics and on-chip laboratory techniques [140], should lead toMACS having an important role to play in the study of hybrid films,which are obtained only in very small amounts. At this stage, we
een diffusion (determined by DOSY NMR) and mass of the clusters. Reproduced from
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should mention that the disk MAS approach [18] could also bewell-suited for non-destructive, high-resolution studies of ‘‘flat’’samples. In this context, a first study of a sputtered LiCoO2 batterymaterial by 7Li MAS NMR was published recently by Inukai et al.[141].
2.3.2. PHIP and MRIIt is well established that hydrogenation of unsaturated species
with parahydrogen can lead to the so-called para-hydrogen in-duced polarization effect (PHIP) [19]. An estimation of theenhancement of the NMR signals shows that it is several ordersof magnitude. Most PHIP studies have been related to homoge-neous hydrogenation reactions with transition-metal derivatives[142,143]. Recently, Kovtunov et al. [144] extended the use of PHIPto heterogeneous catalysis (heterogeneous gas phase hydrogena-tion of propene and propyne) by studying an IRMOF-3 structuremodified by a Au(III) Schiff base complex (Fig. 5). It was clearlydemonstrated that in this reaction both atoms of a H2 moleculeare transferred as a pair to a unique product molecule, as shownby the strongly enhanced signals assigned to the methyl and meth-ylene groups of propane. This result is of great importance as itestablishes a bridge between homogeneous and heterogeneouscatalysis [145,146] and confirmed the nature of the reactionmechanisms.
PHIP was also used in combination with standard solid stateNMR to study the leaching properties of immobilized catalysts[147]. In this particular case, phosphine-modified mesoporoussilica (SBA-15) was used as a support and the Wilkinson’s catalystas a model system (RhCl(PPh3)3). PHIP experiments were suitablefor differentiating the coordinated/adsorbed catalytic species atthe mesoporous silica surface. Metal-containing zeolitic catalystshave also been investigated by PHIP. As an example, Henninget al. [148] used parahydrogen for the hydrogenation of propenein Rh-containing zeolite Y. For the first time, PHIP was combinedwith in situ MAS NMR under continuous flow conditions.
Finally, we mention two recent developments of PHIP whichcould be of great interest for the study of hybrid materials. Stan-dard PHIP has several drawbacks, such as a loss of polarization asa result of spin–lattice relaxation, and a rather limited amount ofdissolved H2 (due to the restricted gas–liquid interface of the cor-responding bubbles). In an interesting contribution, Roth et al.[149] used a hollow-fiber membrane technique, exhibiting a verylarge gas–liquid interface. In this case, the dissolution of parahy-
Fig. 5. PHIP at 7.1 T in heterogeneous hydrogenation using a modified IRMOF-3structure. Reproduced from Ref. [144] with permission (Copyright 2010 AmericanChemical Society).
drogen is continuous in the reaction mixture, enhancing the con-version rates and minimizing relaxation issues. This PHIPcontinuous approach should lead to interesting developments in1H and 13C enhanced NMR spectroscopy and MRI. Zhivonitkoet al. [150] have demonstrated the use of PHIP to characterize amicrofluidic gas reactor. Very interestingly, PHIP was combinedhere with remote-detection MRI [151–153] in order to circumventsensitivity losses, and to allow the visualization of gas flow in amicrofluidic device.
2.3.3. DNP MASDynamic Nuclear Polarization (DNP) is based on the saturation
of the EPR transition of radicals leading to polarization enhance-ment of nearby nuclei [20,154]. The first reported applicationswere in low field experiments. During the past two decades, Griffinand co-workers have extended DNP applications by combininghigh magnetic fields, high-power, a high frequency microwavesource (gyrotron), fast MAS at �90 K, and by using organic radicalslike TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) [155] orTOTAPOL (1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol)[156] as sources of polarization. The main high-field applicationswere devoted to the study of bio-solids and proteins [157–159].In 2010, Lesage et al. [160] demonstrated the novel impact ofDNP to study hybrid mesostructured materials (obtained bysol–gel methods and templating routes) (Fig. 6). In this case, polar-ization was transferred from the protons of the solvent (impreg-nating the solid sample) to nuclei such as 13C (indirect DNP). Spindiffusion between protons was also involved in the process. Manyexperimental details are described in this contribution, and indeed,sample preparation is one of the most important points of the DNP
Fig. 6. DNP SENS (Surface Enhanced NMR Spectroscopy) of 13C at 100.6 MHzapplied to the characterization of mesoporous silica-derived materials. Reproducedfrom Ref. [160] with permission (Copyright 2010 American Chemical Society).
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approach. Here, incipient wetness impregnation with a solutioncontaining the radicals (TEMPO or TOTAPOL) was performed. Itwas assumed that at �90 K a disordered glassy phase was formedupon freezing water molecules in the pores of the materials (evenin the absence of glycerol as a co-solvent). In the case of the birad-ical TOTAPOL, very large 13C enhancements were observed, e � 50(defined as the intensity ratio of the signal when the microwavesare on to when they are off). In the case of phenol- or imidazoliumfunctionalized mesoporous silica, 2D 1H–13C HETCOR experimentswere recorded in less than 4 h. Most importantly, the authorsstressed that DNP opens new avenues for the detailed study of sur-faces in materials, leading to the concept of Surface Enhanced NMRSpectroscopy (SENS). The DNP approach was then extended to 29SiNMR in functionalized silica exhibiting Tn and Qn units [161].Sol–gel derived and post-grafted materials were compared interms of surface bonding modes. A gain of time of a factor �400was observed leading to the fast acquisition of 2D correlationexperiments. Moreover, the combination of DNP with CPMG (CarrPurcell Meiboom Gill) acquisition under optimum conditions led toa sensitivity enhancement factor of �100 (or in other words, a�10,000 fold reduction in experimental time) [162]. Lafon et al.[163] studied 13C and 29Si DNP efficiency in experiments on meso-porous silica loaded with surfactant. The silica was functionalizedwith 3-(N-phenylureido)propyl groups and cetyltrimethylammo-nium bromide (CTAB) was used as a surfactant. It was demon-strated that 1H magnetization was transported into the poresfollowing a one-dimensional 1H–1H spin diffusion process. In thiscase, e values were rather limited (e � 8 for 29Si and 13C aftercross-polarization from 1H). In the absence of CTAB, e values in-creased to �30 because of the presence of the radicals in themesopores.
High magnetic field 29Si direct DNP was implemented for thefirst time by Lafon et al. [164] in the case of porous silica. Markeddifferences were observed between direct and indirect (through 1Hcross polarization) 29Si MAS DNP spectra. It is assumed that indi-rect DNP is sensitive to subsurface species. This result is of greatimportance as it opens the way to the study of non-protonated sil-ica derived materials. However, one should note that in the case ofdirect DNP, the 29Si polarization build-up can require hundreds ofseconds (while only a few seconds are usually necessary for indi-rect CP DNP). The 29Si direct/indirect DNP methodology was re-cently implemented for the study of dispersed nanoparticles oflaponite [165]. An interesting discrimination of the 29Si sites wasobtained. Indeed, the signal of 29Si nuclei in close vicinity to un-paired electrons were enhanced by direct DNP, whereas the signalof more remote sites were enhanced by indirect 1H–29Si CP DNP.
1H–29Si CP DNP SENS was also implemented for the detailedcharacterization of metal-surface interactions and their potentialrole in stabilizing immobilized Ru-NHC alkene metathesis catalysts(NHC: N-heterocyclic carbene) [166]. In this case, the study tar-geted mainly the ligand orientation towards the correspondingsurface. Rossini et al. [167] recently published the first DNP spectraof MOFs (a series of three functionalized MOFs derived from MIL-68) [168]. Here, the study was quite challenging, as the pore sizeof MIL-68 (�1.6 nm) is much smaller than the characteristic poresize of the mesoporous materials described above in this section(�6 nm). Moreover, in the case of proline functionalized MIL-68,the one-dimensional porosity was blocked; it followed that thefree diffusion of the radicals was forbidden. Nevertheless, DNPenhancements were observed for all samples, with an overall sen-sitivity gain between 10 and 30 (with respect to a dry sample atroom temperature). This corresponded to a reduction in experi-mental time between 100 and 900, respectively for 1H–13C and1H–15N CP DNP MAS experiments.
More recently, DNP MAS has been extended to the study ofquadrupolar nuclei. The surface of c-alumina nanoparticles was
characterized by indirect 27Al DNP (MAS and MQ-MAS) [169]. Theradical concentration (TOTAPOL in this case) appeared as a crucialparameter and had to be reduced when compared to typical valuesused for mesoporous silicas. The extent of wetting (defined as themass of solvent divided by the total mass of the sample) had also tobe accurately adjusted for optimal DNP efficiency. The surface ofthe nanoparticles was investigated by 1H–27Al CP DNP MAS exper-iments, leading to a reduction of �400 in experimental time.Further resolution was achieved by implementing 27Al CP DNPMQ-MAS experiments (acquired in �50 h). Surface penta-coordi-nated Al sites were not observed. This important result suggeststhat such sites may be coordinated by water molecules i.e. the sol-vent of the TOTAPOL radicals. We quote here an important sen-tence of the article [169], which may be generalized to all DNPexperiments at high magnetic field: ‘‘. . . in order to achieve a DNPenhancement, our wetted, glassy and frozen sample is observed underconditions that greatly differ from a dry powder at room tempera-ture’’. High surface mesoporous alumina, Al2O3, was further inves-tigated by 2D 27Al–27Al dipolar correlations, under DNP [170]. Inthis particular case, the true sensitivity enhancement corre-sponded to 4–5 orders of magnitude. It has to be stressed that longimpregnation times were necessary for diffusion of TOTAPOL intothe sample to produce an optimal DNP effect. 5-fold coordinatedAl atoms were clearly distinguished in the case of high surfacemesoporous alumina (unlike the study of c-alumina nanoparticlespresented above [169]). The enhancement e was estimated to be�184, corresponding to a saving in time of a factor of �34,000.Homonuclear dipolar recoupling experiments required �4 h (in-stead of the 15 years which would be required for standard solidstate NMR conditions!). Most interestingly, full assignment of the27Al–27Al correlations indicated that AlV sites do connect AlIV andAlVI sites at the surface of the material. From the experimentalpoint of view, CP DNP efficiency was clearly enhanced by low tem-perature, as fast relaxation of quadrupolar nuclei was efficientlyquenched during the RF spin-lock. Finally, we mention that DNPMAS has also been successfully applied to 17O, for the study of inor-ganic derivatives [171].
During the past 3 years, several methodological aspects of DNPMAS were raised in the literature. First, new radicals were pro-posed exhibiting improved DNP efficiency. Highly concentratedTEMPO (monoradical) solutions were initially used. Improvementin DNP efficiency was achieved by using biradical polarizing agents(as shown by Griffin et al.) [172], and now the biradical TOTAPOL isone of the most employed exogenous polarizing agents in solidstate DNP NMR. However, the structure of TOTAPOL is flexible,restricting consequently the matching condition in cross-effect(CE) DNP to a fraction of the biradicals present. In order to circum-vent this problem, Griffin, Tordo and co-workers introduced amuch more rigid molecule, bTbK (bis-TEMPO-bisketal), whose effi-ciency was found to be 1.4 times larger than TOTAPOL [173]. Oneof the key parameters to take into account for the design of newpolarizing agents is T1e, the electron spin–lattice relaxation time(the saturation of the EPR transition is facilitated when T1e is long).In 2012, Zagdoun et al. [174a] proposed a new biradical, bCTbK,(bis-cyclohexyl-TEMPO-bisketal) corresponding to a bulky deriva-tive of bTbK. For such a polarizing agent, T1e at 100 K is twice aslong as that for bTbK. Impressive 1H, 13C, 29Si and 15N DNP spectrawere obtained using bCTbK for stepwise functionalized hybridmaterials (mesostructured silica). Other more bulky and even morepromising nitroxide biradicals for DNP were recently introducedby the same group [174b].
Another topic of interest in DNP NMR applied to hybrid materi-als is related to the solvents used for dissolving the radicals.Whereas mixtures of H2O/D2O were frequently used for this pur-pose, the investigation of water incompatible materials calls fornew experimental procedures. It has been clearly demonstrated
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 11
that non-aqueous solvents can be used in combination with birad-icals. Zagdoun et al. [175] investigated a series of organic solventsfor the study of a hybrid mesoporous material exhibiting phenolicgroups as surface species. Halogenated solvents led to the best DNPenhancements, 1,1,2,2-tetrachloroethane (EtCl4) being one of themost promising. We mention here that EtCl4 was used as a DNPsolvent in the case of MIL-68 derived structures [167], becauseMIL-68 is known to react chemically with water. Finally, the useof non-aqueous solvents opens the way to the synthesis and poten-tial use of new hydrophobic biradicals for DNP NMR.
Recent studies were also oriented towards specific sample prep-arations. Takahashi et al. [176] developed a new matrix-freeapproach leading to a uniform distribution of the radicals aroundthe microcrystals of interest. In this case, the linewidths of the res-onance lines remained almost unchanged, in contrast with thestandard frozen-solution preparation. This led to substantial DNPgain and allowed them to implement fast (62 h) naturalabundance 13C–13C homonuclear dipolar correlation experiments.Interestingly, Zagdoun et al. [177] demonstrated that theincorporation of deuterated functional groups within hybrid mate-rials could lead to improved SENS. It is well known that nuclei inmethyl groups in solid samples are characterized by short T1 val-ues, which leads to a drastic reduction of DNP efficiency. The pres-ence of deuterons instead of protons restores the DNP efficiency.Moreover, it was demonstrated that the size and the polarity ofthe chemical groups present onto the surface of the materialshad a great impact on the overall DNP efficiency. Indeed, suchgroups tend to modulate the detrimental paramagnetic effects ofthe polarizing agents. Solvent-free DNP at high field was alsodeveloped by using endogeneous TEMPO molecules [178]. Here,the polarizing agents are incorporated into porous inorganic mate-rials through co-condensation processes. Such an approach leads toefficient direct 29Si DNP transfer (whereas indirect 1H–29Si CP DNPremained rather inefficient). This solvent-free spin-labelling tech-nique was already mentioned by Griffin et al. in the case of thebiradical TOTAPOL [156] and by Vitzhum et al. [179].
The precise measurement of the DNP enhancement is one of themost complex problems of SENS (Surface Enhanced NMR Spectros-copy). It depends on multiple experimental/instrumental variableswhose impacts on the DNP process have to be carefully quantified.One of the most used parameters is e, the standard DNP enhance-ment. It is simply defined as the ratio of the intensities obtainedwith and without microwave irradiation (MW on/off). Rossiniet al. [162] introduced an overall sensitivity factor, R, which isthe product of e, h and (K)1/2, where h stands for the quantificationof the reduction of the DNP signal through paramagnetic quench-ing and K = T1degassed/T1DNP/TOTAPOL. As a rule of thumb, h decreaseswhen the concentration of biradicals increased. The R parameterwas further refined by taking into account the Boltzmann enhance-ment factor at low temperature, leading to: R� � (2.8)R (thenumerical factor corresponds here to the Boltzmann enhancementfor T = 105 K compared to T = 298 K). Later, Vitzhum et al. intro-duced a global DNP factor, called eglobal [179]. The eglobal factorwas defined as the product of R (see above) by edilution, where edilution
stands for the fraction of molecules of interest in the DNP experi-ment. The experimental time saving was proportional to e2
global.Takahashi et al. [176] introduced the absolute sensitivity ratio(ASR) with the following definition: ASR = (R�) � vLW � vweight -� vseq � vex, where vLW is the ratio of the linewidths, vweight theratio of the effective sample weights, vseq the ratio of the effectivemagnetization after decays during pulse sequences, and vex corre-sponds to extra effects (different probe and/or different magneticfield). The authors showed a very good agreement between calcu-lated and experimental ASR values, demonstrating the rationaliza-tion of the DNP enhancement factor. Finally, Kobayashi et al. [180]have stressed the notions of per scan escan, and per unit time etime,
taking into account the combined effects of microwave irradiation,radical concentration, solvents, temperature, and enhancement byother techniques such as CPMG.
To close this section on DNP under MAS, we mention recenttheoretical work related to the study of various DNP transfer mech-anisms, using the density matrix approach [181].
2.4. First principles calculations
Solid-state NMR experiments offer new perspectives in terms ofstructural characterization of hybrid materials but in some cases,the interpretation of the high-resolution spectra produced usingthe various techniques described in this article can still pose a con-siderable challenge. Indeed, many different nuclei with a widevariety of coordination environments are present and for some ofthem, there is relatively little information available in the litera-ture. An additional tool is necessary to fully exploit the informationavailable, creating a link between structural models and experi-mental NMR data: first-principles calculations can provide thisbridge and enable a more complete interpretation of NMRspectroscopy data.
Two main types of methods have been used to perform NMRcalculations on hybrid materials:
– Quantum chemical calculations with a ‘‘cluster approach’’ usingthe gauge-including atomic orbital method [182,183]. Typically,the Gaussian suite of codes [184] is used with either theRestricted Hartree–Fock (RHF) method or hybrid density func-tional theory (B3LYP) [185,186]. Some authors also use theAmsterdam Density Functionnal (ADF) package [187] whichalso allows GIAO [188] (Gauge Including Atomic Orbitals)shielding calculations.
– Periodic and plane-wave pseudopotential approach using PBE(Perdew, Burke, and Ernzerhof) generalized gradient approxi-mation [189,190] with the GIPAW (Gauge Including ProjectedAugmented Wave) [22] method which permits the reproduc-tion of the results of a fully-converged all-electron calculation.Typically, these calculations are performed with the CASTEP[191], PARATEC [192] or Quantum Espresso [193] codes usingeither norm-conserving [194] or ultrasoft [195]pseudopotentials.
In addition, these NMR computations can be run on the crystalstructure data provided by XRD or on structural models providedby MD in the case of disordered systems [23,24]. In the followingsection, we will show how NMR calculations can be used to helpunderstand the experimental spectra acquired in a range of hybridsolids including MOFs, templated microporous or mesoporous net-works, and more generally organic (bio)molecules/inorganicinterfaces.
2.4.1. Metal organic frameworks and related metal organic ligandcrystalline compounds
Among the metal ions bound to organic linkers in MOFs andcoordination polymers, some correspond to ‘‘exotic’’ nuclei froman NMR point of view, having low receptivities and/or large quad-rupole moments. NMR studies on some of these isotopes have beenimplemented, as illustrated below.
Schurko et al. [196] investigated by 15N and 109Ag solid stateNMR the structure of layered silver sulfonate materials reactedwith primary amines. 109Ag is a spin 1/2 with a low gyromagneticratio (c) and consequently a low receptivity combined with longlongitudinal relaxation time constants (T1) resulting in excessivelylong acquisition times. These difficulties were overcome usingCross-Polarization (CP) techniques [197–199]. Ab initio calculationsof silver and nitrogen chemical shielding as well as J-coupling ten-
12 C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
sors helped the authors to propose structural models for the mate-rials formed. They are largely comprised of layers of silver-diaminecations after elimination of the original metal organic coordinationnetwork. These calculations were performed with the Gaussiancode. [184] using crystal structure data as starting points afteroptimization of hydrogen atoms positions.
The same group used solid state NMR coupled with ab initiocalculations to study a number of organometallic complexes focus-ing on the transition metal. For example, cyclopentadienyl chloridecompounds were investigated by 47/49Ti NMR at moderate and ul-tra high field [200]. Titanium isotopes have a low natural abun-dance (5.51% for 49Ti, 7.28% for 47Ti) and small gyromagneticratios. However the main experimental difficulty is that they havemoderately large quadrupole moments. Titanium NMR has theadditional difficulty that the isotopes have almost identicalgyromagnetic ratios, so that both resonances are observed in thespectra. These factors combine to produce relatively low intrinsicdetection sensitivity. Therefore, the DFS (double frequency sweep)[201] signal enhancement scheme was used as well as the QCPMG(Quadrupolar Carr Purcell Meiboom Gill) [202] pulse sequence foracquisition at a field of 9.4 T. DFS consists in increasing the sensi-tivity of central transition spectra by manipulation of the satellitetransitions prior to the initial pulse while application of theQCPMG sequence generates echo trains, resulting in a manifoldof ‘‘spikelets’’ increasing the sensitivity. Electric field gradient(EFG) and chemical shift (CS) tensors were calculated with bothRHF and B3LYP using Gaussian 03 [184] and CASTEP as a compar-ison, and the relative orientation of the tensors were used to relatethe NMR parameters to the molecular and electronic structures ofthe metallocene complexes. Similarly, Schurko et al. investigatedexperimentally and theoretically lanthanum-containing metallo-cenes (139La: I = 7/2, large quadrupole moment and low c, usingthe QCPMG pulse sequence) [203], cisplatin and four relatedsquare-planar compounds [204] (195Pt: I = 1/2, very large CSAs;14N: I = 1, 35Cl, I = 3/2, low c, using CPMG and WURST (Wideband,Uniform Rate, and Smooth Truncation) excitation [205]), niobiumcyclopentadienyl complexes [206] (93Nb: I = 9/2, moderate quad-rupole moment, using QCMPG), zirconocene compounds [207](91Zr: I = 5/2, moderate Q, low natural abundance and low c, usingWURST-QCPMG), and polymeric potassium metallocenes [208](39K: I = 3/2, moderate quadrupole moment but significant EFGsin non spherically symmetric electronic environments and verylow c, using DFS and QCPMG). In the later example, variable tem-perature 39K NMR experiments provided temperature-dependentchanges in quadrupolar parameters which can be rationalized interms of alterations of bond distances and angles with tempera-ture. Similarly, an investigation of structure and dynamics in so-dium metallocenes was performed using 23Na solid state NMR,VT XRD and computational modeling allowing the authors topropose that the sodium atom in CpNa undergoes an anisotropic,temperature-dependent, low frequency motion within the abcrystallographic plane [209]. Finally, 63Cu/65Cu solid-state NMRexperiments have been conducted on a series of organometalliccopper complexes [210] (63Cu/65Cu: I = 3/2, large quadrupolarinteractions, using QCPMG and Nuclear Quadrupole Resonance(NQR)) providing, by combination with the calculations of 63/65CuEFG and CS tensors, relationships between NMR interaction tensorparameters, the magnitudes and orientations of the principalcomponents, and molecular structure and symmetry (Fig. 7).Moreover, residual dipolar coupling in the 31P NMR spectra ofcomplexes with Cu–P spins pairs enabled the determination ofthe sign of quadrupolar coupling constant (CQ) as well as theorientation of the Cu–P vectors with respect to the EFG tensorframes.
In the field of MOFs, zeolitic imidazolate frameworks (ZIFs) con-taining tetrahedrally-coordinated zinc sites have been investigatedby 67Zn NMR at high field (21.1 T) [211] (67Zn: I = 5/2, low c, lownatural abundance (4.1%) and a relatively large nuclear quadrupolemoment). MAS and static spectra were recorded, using QCPMG andWURST in the latter case. Interpretation of spectra and in particularvalues of quadrupolar coupling constants, CQ, were obtainedthanks to GIAO method at the B3LYP level of theory in Gaussian03 [184], cluster calculations showing that spectra are sensitiveto the Zn local environment and to the guest species present inthe cavities of this MOF. Moreover, 67Zn NMR data were combinedwith molecular dynamics simulation, giving detailed informationon the distribution and the dynamics of the guest species. It shouldbe noted that 67Zn NMR spectra were also recorded on other O/Ihybrids such as the fluorinated layered compound Zn3Al2F12-
�[HAmTAZ]6, and compared to GIPAW calculated values with theaim of helping in the resolution of the structure [212]. Indeed,structure determination from X-ray powder diffraction data stillremains a challenge for the light elements (F or H) and the so called‘‘SMARTER’’ [213] (Structural elucidation by nuclear MAgneticResonance, computation modEling, and X-ray diffraction) strategyappears to be a successful method for the accurate determinationof structural parameters.
25Mg static NMR spectra of CPO-27-Mg [214], on dehydratedsamples and on samples at different levels of rehydration [215]showed that removal of water bound to the magnesium introducesdisorder leading to an increase of quadrupolar coupling constantsrelated to the distortion of the local symmetry. These results weresupported by DFT cluster calculations showing the influence of theMg–OH2 bond length on 25Mg diso and CQ parameters. The sameauthors studied non-equivalent Mg sites with close local environ-ments in microporous a-Mg3(HCOO)6, that could be unambigu-ously assigned [216] using a combination of two-dimensionalhigh-resolution multiple-quantum magic angle spinning(MQ-MAS) [217] at 21.1 T and GIPAW calculations. Similarly, thescandium analogue of MIL-53 was investigated by 13C and 45Sc(I = 7/2, moderate quadrupole moment) solid state NMR [218].MQ-MAS 45Sc spectra were recorded at high field (20.0 T) to re-solve signals from the two scandium sites expected in the struc-tures. To help interpretation, NMR parameters were calculatedwith the CASTEP code for both hydrated and rehydrated structuresafter structural optimization. In addition, 2H wideline NMR spectraof dehydrated MIL-53(Sc) were measured over the temperaturerange 298–373 K, with the results indicating strongly restrictedmotion of the phenyl rings of the terephthalate consistent with anarrow channel opening.
NMR investigations of MOFs and coordination networks havealso been centered on the organic ligands rather than on the metal-lic center when the latter is not ‘‘NMR friendly’’. For instance, 13CCP MAS and 1H high field and ultra-fast (62.5 kHz) MAS DoubleQuantum–Single Quantum (DQ–SQ) NMR spectra using the R121
2
recoupling pulse sequence [219] were recorded on UiO–66(Zr)functionalized MOF [220]. This allowed all NMR signals to be as-signed, and validated the proposed crystalline structures with thehelp of DFT calculations using the CASTEP code.
A first step in the NMR analysis of biological hybrid materialscontaining cations such as Ca2+, Mg2+or Sr2+ was proposed withthe investigation of model compounds such as Ca-carboxylates[221–223], Mg-carboxylates [224–226], and Sr-malonate [227].For Ca2+, in the case of calcium benzoate trihydrate, static cal-cium-43 NMR lineshapes were obtained which have complexshapes because of the presence of anisotropic NMR interactionsof similar magnitude (CSA and EFG), and the full interpretation ofthe spectra required DFT GIPAW calculations [221]. In addition,
Fig. 7. 65Cu EFG tensor orientation for (A) [(PhCN)4Cu]+ and EFG and CS tensororientation for (B) CpCuPEt3, (C) CpCuPPh3, (D) Cp�CuPPh3, and (E) Cp⁄CuPPh3. Allorientations are from RHF/6–31++G⁄⁄ calculations. Protons were removed forclarity. Reproduced from Ref. [210] with permission (Copyright 2007 AmericanChemical Society).
Fig. 8. 91Zr NMR spectra of (A) ZrPOF-pyr and (B) ZrPOF-Q1 (Q = quinoline) at
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 13
an NMR investigation of the coordination environment of Ca2+ wascarried out, using high resolution 13C–43Ca MAS NMR experimentssuch as TRAPDOR (transfer of population double resonance) [228]and heteronuclear J-spin-echoes, showing that it is possible to dis-criminate between carbon atoms according to their calciumenvironments.
25Mg has a natural abundance of 10.0%, a small c and a non-negligible quadrupole moment (I = 5/2). Therefore, spectra wererecorded at different fields including very high ones, using some-times in addition QCPMG [225] or enhancement schemes[225,226] such as FSG-RAPT (Frequency Switch Gaussian – Rotor-Assisted Population Transfer) [229,230]. A good agreement wasobtained between GIPAW calculated NMR parameters andexperimental values.
Finally, 87Sr NMR spectra of 87Sr-labeled strontium malonatewere recorded with DFS, QCPMG and WURST excitation (87Sr:I = 9/2, large quadrupole moment, small c) [227]. It was shownthat GIPAW DFT calculations can accurately compute 87Sr NMRparameters in a wide range of Sr crystalline phases (Sr–phosphates,–silicates, –phenylphosphonate, –phenylboronate).
21.1 T. DFT calculations on the different sites present in model clusters constructedfrom the crystal structure are also presented as dashed lines: (A) model clusters
used were ZrðOPO3Þ14�6
h i; ZrF2ðOPO3Þ10�
4
h i, and ZrFðOPO3Þ12�
5
h ifor Zr1, Zr2, and Zr3
sites; (B) model cluster ZrðOPO3Þ14�6
h iwas used for calculation of Zr1, Zr2, and Zr3
sites while a ZrF4ðOPO3Þ6�2
h icluster displaying a [ZrO2F4] octahedral unit was used
for Zr4. Reproduced from Ref. [233] with permission (Copyright 2012 AmericanChemical Society).
2.4.2. Crystalline templated systemsCharacterization of aluminum phosphate analogues of zeolites
can be achieved with a multi-technique approach combining X-ray diffraction data with solid-state NMR experiments and DFT cal-culations. Typically, high-resolution 27Al and 31P NMR experimentscan enable the resolution and identification of the different Al and
P sites in the structures. In many cases, 27Al MQ-MAS NMR exper-iments are required to separate the quadrupolar-broadened alumi-num resonances which are unresolved in the one-dimensionalspectrum. Unambiguous assignments of the different resonancescan then be obtained with GIPAW calculations. This approach hasbeen successfully applied to the characterization of STA-15 [231]and AlPO-53 [232] as-made materials (still containing ammoniumcation templates) starting from crystal structures after optimiza-tion of the structure geometries. It is important to notice nonethe-less that in some cases, NMR parameters calculated from publishedcrystal structures can be in poor agreement with experiment. Forinstance, in the case of AlPO-53, the 13C NMR spectra show addi-tional peaks that may be associated with disorder of the methyl-amine template, suggesting that the proposed structure in theliterature may not provide a complete picture.
Templated phosphate-based systems have also been investi-gated by combining NMR and DFT. For instance, layered andthree-dimensional zirconium phosphates containing pyridine (j(C5-
H6N)4(H2O)2j[Zr12–P16O60(OH)4F8]) (ZrPOF-pyr) or 1,4-dimethylpi-perazine ([C6H16N2]0.5Zr(H0.5PO4)2�H2O) were studied by 91ZrSSNMR, using both MAS and static QCPMG [233]. In the first case,the non-framework species (pyridinium ions and water molecules)could be located in the channels of the framework while the sec-ond system consists of zirconium phosphate layers with the pro-tonated 1,4-dimethylpiperazine and water molecules in between.Theoretical calculations on the crystal structures using model clus-ter approaches in both CASTEP and Gaussian 03 [184] were quiteuseful in aiding spectral assignments and interpretation, in partic-ular when multiple Zr sites exist (Fig. 8).
2.4.3. Interfaces in disordered systemsModeling the structure of hybrid materials consisting of amor-
phous inorganic matrices and disordered organic molecules ispotentially very difficult. Therefore, some authors used NMR calcu-lations on simplified systems to help assign the NMR spectrarecorded on the real systems. For instance, Constantino et al. inves-tigated by 13C CP MAS NMR Mg/Al or Zn/Al LDH host materials
14 C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
intercalated with Pravastatin, a cholesterol-lowering drug [234].Experimental data were interpreted in light of DFT calculationsperformed on crystalline sodium Pravastatin. Similarly, the inter-calation of a tripeptide (Glutathione, GSH) in LDH by ionexchange was studied by different spectroscopies including IR,Raman, UV and 13C NMR [235]. The results were compared totheoretical vibrational and NMR data obtained on the optimizedgeometry of the tripeptide using Gaussian 03 [184]. This studyshows that after hydrogen transfer, GSH in the water solvent canbe intercalated into LDHs via ion exchange. Moreover, Mali et al.studied by 1H MAS, 13C CP MAS and 1H–29Si HETCOR NMR modeldrug-delivery systems consisting of non-functionalized or APTES-functionalized SBA-15 mesoporous silica matrices loaded withindomethacin (IMC) molecules (a nonsteroidal anti-inflammatorydrug) [236]. 1H and 13C NMR signals of encapsulated IMC were as-signed on the basis of GIPAW calculations performed on the crys-talline structures of IMC-a and c polymorphs. Finally, periodicmesoporous organosilicas (PMO) obtained by co-condensation oftetraethyl orthosilicate and an organic building block derived from1,4-diazabutadiene (DAB) ligand, R–N@C(Ph)–(Ph)C@N–R(R = (CH2)3Si(OEt)3) were characterized by 13C and 29Si MAS NMR[237]. Experimental values were compared with those calculatedwith GIAO method at the B3LYP level using Gaussian 03 [184] onboth cis- and trans-geometries of model compounds of the diaz-abutadiene modified ligand, suggesting that the structure of theligand is preserved in the final material.
Instead of focusing on the organic part of the hybrid systems (asin the examples given just above), other authors consider specifi-cally the inorganic matrix. For example, computed models of alamellar silicate surfactant mesophase were generated [238] toinvestigate the structure of ordered silicate frameworks containingdisordered alkyl chains of surfactant (this disordered organic phasepreventing the measurement of ordering over molecular lengthscales by inducing strong amorphous background X-ray scatteringat wide angles). 29Si chemical shifts were then computed usingGIPAW and found to be in reasonable agreement with the experi-ments (mainly 2D correlation 29Si NMR sequences to determineconnectivities between silicon sites) demonstrating that the modelstructures provide a self consistent description of the lamellarmaterial. Very recently, this approach was refined by integratingtopological information (numbers, relative populations, and con-nectivities of distinct 29Si sites) with new distance and local geom-etry constraints between 29Si sites into a general algorithm toexplore possible framework structures of a layered C16N+Me2Et-sil-icate [239]. For this purpose, Si–O–Si distance constraints weredetermined by fitting 29Si{29Si} DQ intensity build-up curves andby comparing experimental and DFT calculated 2J(29Si–O–29Si)couplings.
More challenging, combined NMR experimental/theoreticalstudies of hybrids based on the modeling of the complete systems(that is both the inorganic and organic parts in interaction) havealso been conducted. For example, the immobilization of differentphenols (catechol, hydroquinone and anthrarobin) at the surface ofhydrothermally synthesized titania nanotubes was investigated by13C solid state NMR [240], with the help of NMR calculations(obtained by GIAO method, B3LYP hybrid functional and usingGaussian 03 [184]) on clusters built using the Materials Studiomolecular modeling software [241]. This study allowed to proposepreferential chelation modes (monodentate or bidentate) for thevarious molecules. Chiche et al. studied the growth of boehmiteparticles (AlO(OH)) in the presence of xylitol (C5H12O5) [242].Molecular modeling was shown to be crucial for describing thenature of the interactions between the boehmite surface and thepolyol molecules at the atomic level: the VASP code [243,244],based on a periodic DFT approach in which electron–ion interac-tion is described by the Projector Augmented-Wave (PAW) method
[245], was used for this purpose. Various models for the surfacescleaved from bulk boehmite, and adsorbed water molecules andxylitol moieties were included. DFT calculations of 13C NMRparameters predict a global deshielding of the xylitol adsorbedon boehmite when compared to the pure substance. More recently,the nature of the Brønsted acidic sites at the surface of amorphoussilica-alumina, and their behavior in the presence of molecules ofvarious basic strengths (CO, pyridine, lutidine and ammonia) werestudied by a combination of DFT modeling of the surfaces and29Si/27Al GIPAW calculations [246].
In the field of catalysis, surface complexes obtained by graftingZr CHt
2Bu� �
4; WðBCtBuÞ CHt2Bu
� �3; CH3ReO3 or Hf CHt
2Bu� �
4 onc-alumina have been investigated using a combination of spectro-scopic methods (including NMR) and DFT calculations [247–249].The trihydrated (110) surface of c-Al2O3 was represented by afour-layer slab and during the optimizations performed with theVASP code, the two uppermost layers were allowed to relaxtogether with the grafted organometallic fragments to generatepossible configurations of grafting. These models were used for13C GIPAW calculations combined with DFT calculations with theB3LYP hybrid functional using Gaussian 03 [184].
In the case of Hf, the importance of relativistic effects on theshielding was highlighted. Indeed, better agreement is obtainedfor computed 13C NMR parameters (when compared to experi-ments) when spin–orbit coupling is introduced into the zeroth-order regular approximation (ZORA) method [250]. The structureand grafting properties of c-alumina (with pretreatment tempera-ture) were also studied by NMR and DFT calculations [251].
Apart from alumina, silica-based hybrid catalysts have alsobeen the subject of various NMR/DFT combined studies. For exam-ple, MD of organometallic complexes grafted at the surface ofamorphous silica were investigated by comparing experimental13C chemical shift anisotropies with values predicted by DFT calcu-lations [252]. In addition, dynamics were quantified by the deter-mination of dipolar and chemical shift order parameters.
More generally, in the field of hybrid mesoporous silica materi-als, MCM-41 phases functionalized with chloroalkylsilanes werecharacterized by solid-state NMR, in particular using two-dimensional 1H–13C and 1H–29Si HETCOR experiments [253]. Inaddition, molecular clusters representing the various hybrid envi-ronments were built using Materials Studio molecular modelingsoftware and the corresponding 1H, 13C and 29Si chemical shift val-ues were calculated with GIAO/B3LYP method (using Gaussian 03[184]), showing an excellent agreement with experimental valuesand demonstrating the relevance of this approach. Similarly, thegrafting of chlorodiphenylphosphines on mesoporous MCM-41and SBA-15 silica matrices was investigated by comparing 31PNMR experimental chemical shifts with theoretical ones calculatedwith GIAO method (using the hybrid functional B3PW91 in Gauss-ian 03 [184]) on clusters in which mesoporous silica is representedwith different SixOyHz models (x = 13, y = 20, H = 28; x = 14, y = 21,H = 22; x = 12, y = 20, H = 16) [254]. This demonstrated the oxida-tion of the phosphine after grafting and the influence of the hydro-gen bonding network on the NMR spectra. Aerosil silica graftedwith homoleptic benzyl derivatives of titanium and zirconiumhave also been investigated by a combination of spectroscopiesand theoretical calculations [255]. Experimental 1H and 13C NMRparameters were compared to those calculated with GIAO method(using the hybrid functional B3PW91 in Gaussian 03 [184]) on tita-nium and zirconium complexes grafted onto polyoligosilsesquiox-ane derivative-type surface models. Finally, ultra high fields (17.6,20.0 and 23.5 T) and ultrafast (>60 kHz) 27Al MAS NMR spectrawere recorded to determine the nature and structure of the Alsurface sites when triethylaluminum is reacted with SBA-15mesoporous silica [256]. In parallel, a large number of potentialAl sites on silica have been screened by DFT calculations, combin-
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 15
ing cluster models (obtained with the TURBOMOLE code)[257,258] and periodic approaches (using the VASP code and the{001} surface of cristobalite to represent the mesoporous silicawalls) and calculating the corresponding 27Al NMR parameterswith the GIPAW method (CASTEP code). This combination of ap-proaches showed that Al species are not present in the form ofmonomers but as Al dimers with a tetrahedral environment forthe Al atoms on the silica surface. More detailed descriptions andadditional references regarding simulations in catalysis and sur-face organometallic chemistry can be found in a recent review bySautet and Delbecq [259].
In the field of mesoporous systems, cationic surfactant/silicainterfaces of mesostructured CTA+-templated materials (hexagonal(p6mm), cubic (Ia3d), and lamellar structures) were studied by 14NMAS NMR using both experimental and theoretical approaches[260]: surfactant/silica interactions were studied by Born–Oppenheimer molecular dynamics (BOMD) coupled with the Den-sity Functional Theory augmented with an empirical dispersionterm (DFT-D) [261,262]. This DFT-D approach is indeed necessaryto account for the intramolecular dispersion interaction within theC16 alkane chains. A Si8O16H8 spherosilicate was used as a simplifiedmodel of the silica surface and the BOMD DFT-D calculations wereperformed with the deMon2k program [263]. For each model, the2H and 14N EFG tensor elements as well as 13C CSA values were com-puted (using Gaussian 03 [184]) demonstrating the sensitivity of the
A
CFig. 9. Calculated 13C and 15N NMR chemical shift values for various configurations of ggeminal silanols, (B) glycine carboxylate in interaction with vicinal silanols, (C) glycine cand (D) glycine carboxylate in interaction with vicinal silanols in the presence of interfaChemical Society).
14N quadrupolar parameters to the head-group’s interactions andsurfactant mobility. It is worth noting that 14N NMR was also usedvery recently by these authors to investigate the local order inMFI-type (mordenite framework inverted) zeolites templated withtetrapropylammonium cations (TPA+) [264].
Regarding zeolites, a series of doubly charged structure-direct-ing agents based on two methylimidazolium moieties linked by alinear bridge of n = 3–6 methylene groups has been used in thesynthesis of silica zeolites in the presence of fluoride [265]. Thezeolite structures obtained depend on the n values and the effectof the spacer length on structure direction (generating eitherTON, Theta 1, or MFI phases) was investigated. In particular, theinfluence of the imidazolium rings orientations was investigatedby 19F MAS NMR. The location and interaction energies of the dif-ferent imidazolium derivatives were studied by molecularmechanics simulations, as implemented in the Forcite Module inthe Materials Studio software [266]. Periodic boundary conditionswere applied in all the calculations and complementary GIPAWcalculations of 19F NMR parameters suggest that fluorides residealways in the cages, the different 19F resonances observed beingdue to the different orientation of the closest imidazolium rings.
In the previous examples, the silica matrix structure was de-scribed computationally either by clusters or periodically using acrystalline phase. An alternative way to describe amorphous silicain hybrid systems is to use a periodic model as the one proposed by
B
Dlycine adsorption on the silica surface: (A) glycine carboxylate in interaction witharboxylate in interaction with geminal silanols in the presence of interfacial water,cial water. Reproduced from Ref. [270] with permission (Copyright 2013 American
Fig. 10. (A) {1H}13C HETCOR (CP) MAS NMR spectrum at 9.4 T of cysteine coatednanoparticles (B) Bilayer model of cysteine on gold. Bonds capable of largeamplitude motions are marked with arrows. Adapted from Ref. [272] withpermission (Copyright 2012 American Chemical Society).
16 C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
Tielens et al. for an amorphous hydroxylated silica surface [267].Starting from models used in a molecular mechanics calculation[268], a slab containing 26 SiO2 units was extracted and 13 watermolecules added as terminating OH groups. The atomic positionsand cell dimensions were optimized using first-principles MD at400 K with the VASP code. This slab was then used as a startingpoint to mimic the interactions between silica and dipalmitoyl-phosphatidylcholine (DPPC) molecules in silica-encapsulatedliposomes [269]. Several optimized configurations of the DPPChead groups were obtained in the vicinity of the silica surface. Insome of them, water molecules were added, modifying signifi-cantly the hydrogen bonding network. In a subsequent step, NMRparameters were calculated for the various configurations andcompared to VT 31P NMR experiments. It was demonstrated thatthe 31P chemical shift is highly sensitive to the geometrical charac-teristics of a given configuration. Based on this parameter, severalof the configurations could be excluded safely. This amorphousslab was also used very recently to investigate the absorption ofglycine in a mesoporous MCM-41 silica [270]. Two-dimensional1H–X HETCOR CP MAS experiments (X = 13C, 15N, and 29Si) were re-corded for investigating the proximities between silanols, water,and glycine molecules. In parallel, various configurations of glycineadsorption at the silica surface were modeled, taking explicitly intoaccount possible interfacial water molecules. Finally, and in orderto determine the relevance of the models, calculations of NMRparameters were performed using the GIPAW method for allinvolved nuclei and compared with experimental data (Fig. 9).
2.4.4. Functionalized metallic clustersOrganically modified colloidal metallic particles have also been
investigated by combining NMR experiments (usually 1H and 13C)and calculations. For instance, the influence of the core charge statein Au25½SðCH2Þ2Ph�z18 ðz ¼ �1;0;þ1Þ gold clusters was investigatedby 1H solution state NMR (both 1D and 2D experiments) and 13Cexperiments [271]. In particular, the strong effect of the paramag-netic z = 0 form on the chemical shift values of nuclei in the ligandswas supported by DFT calculations (using Gaussian 03 [184]),showing a strong correlation between diso and the extent of spindelocalization. Still in the field of gold nanoparticles, their interac-tion with L-cysteine was studied by solid-state NMR (Fig. 10),suggesting a bilayer molecule boundary around the coated nano-particles [272,273] and the same authors confirmed these resultsby a DFT calculation [274]. They showed that in the bilayer model,the cysteine zwitterion structure is stabilized via an H-bondingnetwork between inner and outer cysteine layers. Moreover, thethree types of carbon atoms of the inner and outer layers presenta similar trend in terms of charge changes as observed in solid-state NMR spectra. In addition, a quantum mechanics/molecularmechanics (QM/MM) approach has been used to investigate thio-lated gold clusters such as Au25(SCH2–R)18 and Au38(SCH2–R)24
[275] using the Gaussian-09 suite and the two-layer ONIOMscheme (Our own N-layered Integrated molecular Orbital andmolecular Mechanics) [276,277]. The QM/MM calculated 1H and13C isotropic chemical shifts agree very well with DFT calculationsand experiments [271]. This method is computationally lessdemanding and allows the study of properties of large clustersby describing only a small region of the system at a high QM level.
Heptanuclear silver clusters, [Ag7(H){E2P(OR)2}6] (E = Se, S),which are precursors of silver nanoparticles, have also been char-acterized by 1H and 109Ag solution state NMR [278]. This showedthe presence of a hydride within the structure (by observation ofa JAg�H scalar coupling), while the proposed X-ray structures wereconfirmed by comparison between experimental and calculated 1Hchemical shift values (GIAO method, using Gaussian 03 [184]).Finally, colloidal Pt nanoparticles have been synthesized through
an intermediate Pt2Al2 based complex [279]. Chemical shift datafrom 13C NMR spectra of the latter were compared with calculatedvalues in different cluster models proposed for this complex. Inaddition, spin–orbit corrections to the 13C nuclear shieldings werecomputed separately by the combined finite-perturbation/SOS-DFPT approach [280] using modified versions of the deMon-KS[281,282] and deMon-NMR [283] codes. A plausible quadruplybridged structure was proposed.
3. Recent applications of solid state NMR to organic–inorganicmaterials
3.1. Hybrid silicas
Silica is of paramount importance in the field of hybrid materi-als as it corresponds to the most used inorganic component. Recentprogress in hybrid materials science, including synthesis strategiesof silica derived materials, silica based biohybrids and bioceramics,and properties of silica-based hybrid materials were recently re-viewed [2]. Here, our goal is to present new instrumental/method-ological solid state NMR developments applied to the detaileddescription of hybrid silicas (in terms of structure and dynamics).
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 17
Advanced NMR techniques will be described, including ultra-fastMAS at very high field, MAS-J-derived experiments, and high reso-lution NMR of quadrupolar nuclei (17O) at the surface of silicamaterials. The effects of confinement in silica mesoporous archi-tectures (such as MCM-41 and SBA-15) will be reviewed with aparticular emphasis on the characterization of organic/inorganicinterfaces. Finally, important classes of silica-derived materials,such as polyhedral silsesquioxanes, ionogels and nanoparticle net-works will be considered.
3.1.1. Advanced solid state NMR techniquesHere, we focus mainly on NMR developments recently applied
to the study of hybrid silicas, excluding DNP MAS NMR (seeSection 2.3.3). Such developments concern mainly J and D hetero-nuclear correlations as well as ultra-fast MAS experiments at highmagnetic field [284].
A very important step in understanding the silica-surfactantinteractions was achieved by Baccile et al. [285] 2D 1H–29Si CPHETCOR experiments were performed under Lee-Goldburg (LG)irradiation. Under LG conditions, 1H spin-diffusion was efficientlysuppressed, leading to a much easier interpretation of the 2Dexperiments. Nevertheless, such experiments remained time-consuming (1–3 days). Interestingly, the authors also implementeda 1D double cross-polarization sequence 1H–29Si–1H, based on twoCP contact times, tCP1 and tCP2. The first contact time tCP1 was opti-mized to maximize the 29Si magnetization. The value of tCP2 wasprogressively incremented in order to discriminate between pro-tons depending on their 1H–29Si internuclear distances. It has tobe stressed here that an efficient suppression of unwanted residual1H magnetization was inserted in the sequence after the first CPcontact. Moreover, the structure of the adsorbed water layerswas analyzed by 1H MAS NMR by comparing spectra recorded ondehydrated and partially rehydrated samples. Homonuclear1H–1H correlation experiments combined with double cross polar-ization were also implemented for a full description of materialsobtained under acidic and basic conditions. The same solid stateNMR approach was then extended to the study of surface interac-tions between pollutants and silica [286]. In order to select mobilecomponents of the hybrid materials, J-derived experiments wereimplemented such as 1H–13C MAS-J-INEPT.
Trebosc et al. [287] pointed out important experimentalimprovements, such as the role of the MAS rotation frequency.Under ultra-fast MAS (here, 40 kHz), highly resolved 1H NMRspectra of MCM-41 were obtained without any multiple pulses se-quence for homonuclear decoupling. It allowed to record 1H–29SiCP HETCOR correlations at 40 kHz MAS, as well as 1H–1H 2D ex-change and recoupling (RFDR) experiments. Realistic representa-
Fig. 11. NMR methodology applied to 17O surface-labeled silica: speciation and interactio17O). Reproduced from Ref. [290] with permission (Copyright 2012 American Chemical
tions of silanol groups (isolated and geminal) and watermolecules were consequently proposed. The same ultra-fast MASapproach was then applied to the study of organically functional-ized mesoporous silicas (with allyl groups covalently bonded tothe surface of MCM-41) [288]. Despite the small rotor volume(<10 lL), natural abundance 1H–13C CP HETCOR spectra were ac-quired, using low power 1H decoupling. In the case of 1H–29Si CPHETCOR experiments, the sensitivity was further increased byimplementing the CPMG type of acquisition (the increase in sensi-tivity was estimated to a factor of 10). In this case, low powerdecoupling was an essential aspect, as long CPMG acquisition peri-ods were necessary.
In a very detailed paper, Cadars et al. [289] used sophisticatedNMR pulse sequences in order to study surfactant-templated silicalayers (prepared with 50 % enrichment in 29Si). 29Si–29Si z-filteredrefocused MAS-J-INADEQUATE experiments were implemented atvariable temperature. Elongated DQ lineshapes were observed inthe spectra and assigned to static structural disorder. It was assumedthat the static disorder was related to the freezing of the surfactantheadgroup motions at the surface of the material. In other words,29Si nuclei are sensitive to local structural disorder (directly) andto the dynamics of surfactant molecules (indirectly). Local dynamicswere also extensively studied by VT NMR, detailed analysis of 29Silineshapes under MAS, and measurement of T02ð29SiÞ (T02 stands herefor the transverse dephasing time, obtained via a selective spin-echo29Si experiment under MAS). Most importantly, it was demonstratedthat dynamics and disordering effects on the NMR observables couldbe disentangled and analyzed independently.
Recently, Merle et al. [290] implemented 17O MAS NMR experi-ments to characterize the structure of supported catalysts and theirinteractions with silica (flame silica Aerosil 200) (Fig. 11). Flame sil-ica was first surface labeled with 17O (siloxane and silanol groups)and studied by 17O MQ-MAS and 1H–17O HMQC experiments. TheMQ-MAS spectrum distinguished between Si–17O–Si and Si–17OHgroups, whereas the HMQC spectrum revealed the presence of bothisolated and hydrogen-bonded Si–17OH moieties (1JOH � 100 Hz). Ina second step, 17O NMR studies were conducted on organometallicspecies grafted on SiO2 (zirconium tetralkyl, Zr CHt
2Bu� �
4
� �, tungsten
trisalkylalkylidene W BCtBu� �
CHt2Bu
� �3
� �� � �). The Si–17OH, Si–17O–
Si and Si–17O–M signals were very sensitive to slight local modifica-tions (as the ranges of d(17O) and CQ(17O) are large). Clearly, 17O NMRappears as a new interesting tool for investigation of the details ofmetal-support interactions, a key issue in heterogeneous catalysis.
In 2008, Blanc and coworkers [291] published a tutorial reviewdevoted to the characterization of active sites of well-defined het-erogeneous catalysts by means of solid sate NMR. The main topicswere high resolution 1H NMR of surface species, homonuclear/het-
ns through 17O MQ-MAS and 1H–17O HMQC experiments at 9.4 and 18.8 T MHz (forSociety).
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eronuclear connectivities and proximities on surfaces, and thedetermination of local geometry of surface species by the precisemeasurement of (through-bond) J coupling constants. The follow-ing NMR pulse schemes were emphasized: 1H delayed acquisition(to improve spectral resolution), high resolution 1H–1H constanttime experiment, DQ and TQ correlations, 1H–13C HETCOR and J-re-solved correlation experiments. The selected examples were re-lated to zirconium hydride complexes [292], tantalumimidoamidoamino derivatives [293] and rhenium complexes[294] grafted on silica based materials.
3.1.2. Encapsulation of moleculesMesoporous silica-based materials have extensively been used
for the encapsulation of different kinds of molecules, includingdrugs (therapeutics). In an important contribution, Azaïs et al.[295] studied in depth the confinement of ibuprofen in MCM-41 matrices. Ibuprofen is a crystalline solid at room temperatureand acts as an anti-inflammatory drug. By varying the porediameter of the MCM-41 structure (namely, 35 Å and 116 Å)and the temperature of the experiments (room temperatureand 223 K), several physical states of ibuprofen were evidenced.At room temperature, the mobility of the ibuprofen molecules isfound to be very high, especially in the largest pores (116 Å). Al-most no interaction between ibuprofen molecules and the silicasurface was observed, in agreement with fast release of the drugin simulated biological fluids. Interestingly, solution-state derivedNMR techniques, such as 13C MAS-J-INEPT [284], were used tocharacterize the mobile fraction of ibuprofen. At low temperature(223 K), the crystallization of ibuprofen occurred in the largestpores, whereas a glassy state was observed in the smallest ones.The approach combining data from solution-state and standardsolid state NMR techniques was then extended to benzoic acidand lauric acid encapsulated in MCM-41 silicas [296]. The panelof NMR experiments included 13C CP MAS, 1H–13C MAS-J-HMQC,1H–29Si–1H double CP MAS and 1H EXSY. Moreover, 1H spin-diffusion based experiments allowed the proximities betweenentrapped molecules and silica surface to be estimated. Power-gated heteronuclear 1H–13C NOE experiments were furtherimplemented for the detailed study of benzoic acid confined inMCM-41 structures [297]. It was clearly demonstrated thatconfinement effects lead to a decrease of the values ofthermodynamic parameters, such as the phase transition temper-
Fig. 12. 2D 1H–1H MAS NOESY spectrum at 300.13 MHz of MCM-41 (loaded withbenzoic acid molecules): the two populations are denoted A and B, whichcorrespond to the bimodal nature of the silica matrix with large (100 Å) andnarrow pores (45 Å). Reproduced from Ref. [297] with permission (Copyright 2010American Chemical Society).
ature. In the case of silica samples exhibiting a bimodal poredistribution, 2D 1H NOESY experiments showed the presence oftwo benzoic acid populations (in small and large pores,respectively) (Fig. 12).
Phenylphosphonic acid was also used as a model molecule forencapsulation studies in SBA-15 derived materials [298]. The incip-ient wetness impregnation method was used, leading to highincorporation of the molecule (up to 380 mg/g). In native SBA-15,the phenylphosphonic acid molecules were found to be highlymobile, as already observed in the case of small organic molecules.In aminopropyl-modified SBA-15, the molecules were found to beimmobilized, in strong interaction with the aminopropyl groups.Such interactions were carefully characterized by 2D 1H–1H BABAdouble quantum experiments performed under ultra-fast MAS(67 kHz).
Finally, we mention here an important contribution by Grün-berg et al. [299] related to the application of 1H NMR methodologyto the study of H-bonding of water confined in MCM-41.
3.1.3. Interfaces in silica derived hybridsIn a series a papers, Alonso and coworkers implemented a com-
plete solid state NMR methodology to gain new insight in thestructure of hybrid organic–inorganic materials, with a particularemphasis on O/I interfaces. As an example, alkoxides containingamine groups and epoxy samples containing alkoxides weremixed, leading to highly cross-linked hybrids [300]. Solid stateNMR techniques were successfully used to demonstrate thechemical homogeneity at the nanometer scale and to evaluatethe residual mobility in the dried materials. Thiol-functionalizedmicrometer mesoporous spheres were obtained through sol–geland spray-drying processes [301]. The positioning of the thiolfunctions at the surface of the pores was demonstrated by carefulanalysis of 2D 1H–1H MAS exchange spectra. Moreover, 14N NMRwas implemented in order to obtain additional information onthe local environment of the polar head groups in the materials(see below). 29Si, 1H and 31P MAS NMR have also been used forthe study of spray-dried porous silica microspheres functionalizedby phosphonic acid groups [302].
In an important contribution, Alonso et al. [303] stressed theadvantages of 1H solid state NMR methodologies for the descrip-tion of materials textured by self-assembled amphiphiles. Theystudied the local structure around the organic and inorganiccomponents, describing the hydrophobic chains, the confinementeffects due to encapsulation, the spatial distribution of chemicalgroups, and the characterization of counter-ions of the polar headgroups. 1H NMR characterization was used at different lengthscales, by studying 1H–1H dipolar interactions (0.1–1 nm) and 1Hspin diffusion (1–100 nm). Such an approach clearly opens newavenues for the detailed description of interfaces in O/I materials.This contribution also highlighted the potentialities of more exoticNMR nuclei, such as 14N (I = 1; 99.6% natural abundance) and 81Br(I = 3/2; 49.3% natural abundance). Such nuclei act as spectroscopictargets for the head groups of the surfactants (14N) and the coun-ter-anions of cationic surfactants (81Br). A systematic study ofn-alkyltrimethylammonium bromide crystals was undertaken(Fig. 13) [304]. For these derivatives, the values of CQ(14N) are mod-erate (�100 kHz) and standard MAS experiments were adequatefor a direct observation. The values of CQ(81Br) are much larger(5–10 MHz), but NMR experiments remained feasible at very highmagnetic field. An empirical correlation established that CQ(81Br)values were linearly related to R (rN�Br)�3 (in other words, smallvariations in N–Br distances may have strong effects on CQ). Thestudy was then extended to mesoporous materials. However, insuch materials, the O/I interfaces are most probably disordered.This means that a distribution of charge near Br� anions has tobe considered: consequently the authors failed to detect 81Br
Fig. 13. A systematic study of n-alkyltrimethylammonium bromide crystals by 81Br(17.6 T) and 14N (9.4 T) solid state NMR. The bromine anions are represented withlarge spheres, whereas C and N correspond to the backbone of the molecule,respectively. Reproduced from Ref. [304] with permission (Copyright 2009 Amer-ican Chemical Society).
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 19
resonances by direct excitation in such materials. Interestingly,indirect 1H detection of 81Br resonances was successful by using1H{81Br} TRAPDOR experiments. As expected, Br� anions werefound to be distributed in close vicinity to the polar head groupsand the alkyl chains of the surfactants, as well as to the organicgroups bound to the hybrid silica network.
Finally, it is worth mentioning here that complete overviews ofNMR methods for the characterization of interfaces in micelle-templated mesoporous solids [6] and hybrid mesostructured mem-branes were published recently [305]. In the latter case, models forthe spatial organizations of hybrid membranes obtained for vari-ous compositions were proposed.
3.1.4. Bioactive silica derived hybrid materialsThe in vitro alteration of bioactive glasses in the presence of
simulated biological fluids (such as SBF – Simulated Body Fluid)is an interesting topic of research [306]. The spectroscopic targetis here the silicate speciation in the bulk, in the pores and at thesurface of mesoporous bioactive glasses. 1H MAS, 29Si MAS and1H–29Si CP MAS experiments have been mainly implemented forthis purpose [307]. Interestingly, the biomimetic mineralizationof hydroxyapatite on bioactive glasses was followed by multinu-clear 31P, 29Si, 23Na, and 13C solid state NMR [308]. In a very recentcontribution, Gonzalez et al. [309] introduced for the first timebioactive hybrid materials exhibiting amine dendrimers as nano-building blocks. The dendrimers were functionalized with alkoxy-silane groups and the final materials were obtained throughstandard sol–gel process. The integrity of the organic network inall hybrids was carefully checked by 1H–13C CP MAS experiments.In order to enhance the bioactive response, phosphorus containingspecies (such as triethylphosphate and diethylphosphatoethyltri-ethoxysilane) were incorporated during the sol–gel process. Thenature of the phosphorus species in the materials was determinedby 31P MAS NMR. Other examples of silica-based hybrid biomate-rials and biominerals are presented in Section 3.3.
3.1.5. Polyhedral silsesquioxanesPolyhedral silsesquioxanes (POSS) represent an interesting class
of organosilicon derivatives, with tuned nanometer size structures.Indeed, they correspond to cubane shaped clusters T8 ((RSiO1.5)8),decameric T10, and dodecameric T12 entities. In all cases, the silicacore can be considered as rigid whereas the organic component (R)makes POSS suitable for the synthesis of hybrid nanocompositematerials. The applications of POSS are mainly oriented towards
catalysis [310], innovative porous media [311] and encapsulants[312]. Many synthetic routes have been proposed so far in the lit-erature [313] and solution/solid state NMR has been a fundamentaltool of investigation for the characterization of synthetic POSS. In-deed, mixtures of T8, T10, and T12 architectures with variable func-tionalization are often obtained. The main spectroscopic NMRtargets for POSS studies are 1H, 29Si and 13C. 29Si MAS/ CP MASNMR experiments are well suited for the description of the silicacore [314] whereas 1H and 13C MAS/CP MAS NMR experimentshelp to characterize the organic part of the hybrid materials. Asan example, Zhang et al. [315] studied the copolymerization of(HSiO1.5)8 and ((HSiMe2O)SiO1.5)8 with octavinylsilsesquioxanesthrough hydrosilylation. The spectroscopic goal here was toidentify linear Si–CH2–CH2–Si fragments (b-hydrosilylation) and/or Si–CH(CH3)–Si bridges (a-hydrosilylation). The CP dynamicswere analyzed by variable contact time experiments taking into ac-count multi-regimes during the cross polarization process [316].
In a recent contribution, Zhao et al. [317] extended the conceptof quantitative cross polarization (QCP) [318] to the study ofisobutyl-POSS derivatives. QCP is based on cross polarization andcross depolarization, restricting the number of experiments tothree for quantitative purposes. Indeed, a complete variable con-tact time CP experiment for quantification is usually time consum-ing as it involves a rather large number of contact time increments.Moreover, the authors demonstrated the useful implementation ofQCP to determine the average number of reacted vinyl groups inthe case of octavinyl POSS nanocomposite.
Solid state NMR allowed also the study of local dynamics inPOSS-derived polymeric networks. Epoxy networks reinforced byPOSS acted here as a standard example of nanocomposites exhibit-ing a hierarchical structure [319]. The size of the domains in thenanocomposites was determined by 1H–1H MAS spin-diffusionexperiments (Fig. 14). Various ‘‘domain selective’’ relaxation andrecoupling sequences were implemented in order to investigatemolecular/local dynamics (T1 and T1q
13C relaxation experiments,2D T1 filtered amplitude modulated PISEMA experiment for theselection of rigid domains, and 2D inverse T1 filtered direct polar-ization amplitude modulation PISEMA experiment for the selectionof highly mobile domains). Most interestingly, NMR studiesallowed the assignment of the contribution of defined molecularsegments to physical properties of the nanocomposites, such asthe glass transition temperature, and the storage shear modulus.
Very recently, Lin and Kuo [320] studied an interesting class ofhybrids, involving linear polypeptides grafted onto POSS. The effectof POSS incorporation was to enhance the a-helical conformationin the solid state as demonstrated by 13C solid state NMR.
3.1.6. Ionogels and nanoparticle networksIonic liquids are most commonly defined as ionic compounds
which are liquid below 100�C. They often contain salts of an organ-ic cation (phosphonium, imidazolium, pyridinium, etc.), and can beused as alternative solvents [321]. It has been shown that ionic liq-uids can be efficiently confined in a silica-derived network, leadingto ionogels. As a consequence of the confinement at the nanometerlevel, phase transitions are strongly modified, although somemolecular mobility is maintained in the materials [322]. By 1HNMR spectroscopy, it was demonstrated that highly resolved spec-tra can be obtained at very low spinning frequencies (mrot = 0.4kHz). Indeed, CSA and dipolar interactions are efficiently sup-pressed, in agreement with a quasi-liquid behavior for the confinedionic liquid. The glass transition temperatures were tentativelymeasured by low temperature 1H NMR and compared to DSC re-sults. These transition temperatures were shown to be significantlylower in ionogels in comparison to pure ionic liquids. A detailedinvestigation of the confinement effect of ionic liquids in silicawas performed by Le Bideau et al. [323] MAS NMR and relaxation
Fig. 14. 500.18 MHz 1H–1H MAS correlation spectra of POSS-reinforced networks(spin-diffusion). DGEBA: diglycidyl ether of bisphenol A. D2000: poly (oxypropyl-ene) diamine. POSSOct,E1: isooctyl monoepoxide POSS. POSSPh,E1: phenyl monoep-oxide POSS. POSS,E4: tetraepoxide POSS. POSS,E8: octaepoxide POSS. Reproducedwith permission from Ref. [319] (Copyright 2008 American Chemical Society).
Fig. 15. 2D 1H{FSLG}–31P HETCOR NMR spectrum at 9.4 T (31P) of (C2H5NH3)–[Ti(H1.5PO4)(PO4)]2. Reproduced from Ref. [338] with permission (Copyright 2008American Chemical Society).
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times studies of a series of ionogels offered new insight in thedynamics of the confined ionic liquid. The 1H linewidths underslow/moderate MAS (63 kHz) showed unambiguously the reduc-tion of the mobility of the ionic liquid when decreasing the sizeof the pores of the silica-based matrix. The remaining linewidthwas assigned to a distribution of 1H chemical shifts. Relaxationtime measurements demonstrated clearly that below 270 K, theconfined ionic liquid has a liquid-like behavior, whereas crystalli-zation was observed for the corresponding pure sample. More re-cently, Viau et al. [324] synthesized new monolith ionogelswhich involve tetrafluoroborate anions. Highly condensed
mesoporous silicas were obtained, exhibiting some fluorinatedsites at the surface of the materials. Such sites were fully character-ized by 19F MAS and 19F–29Si CP MAS experiments.
Neouze has developed a new synthetic approach involvingnanoparticle networks bridged by ionic liquid derivatives [325].The main work was initially related to silica nanoparticles, but re-cently the synthetic approach was extended to porous titania nano-particle networks [326]. 1H–15N CP MAS experiments weresuccessfully implemented to characterize the bridging imidazoliumunits between the titania nanoparticles. The assignment of the 15Nresonances was performed by using 1H–15N HMBC experimentscarried out on methylimidazole and butylmethylimidazolium.
3.2. Hybrid materials involving an ionic solid as inorganic component
3.2.1. Hybrid phosphate-based materialsPhosphates have also been extensively used as building blocks
for the preparation of hybrid O/I phases. Phosphates have mostoften been combined with other building units like aluminates,titanates or vanadates, one of the popular examples being the alu-minophosphate family of mesoporous materials (AlPOs). A selec-tion of recent examples of solid state NMR characterizations ofhybrid phosphate-based architectures is given below.
Most solid state NMR characterizations of aluminophosphatephases (whether hybrid or purely inorganic) were initiallyconducted in order to gain an insight into the structure of the alu-minophosphate framework, using for example 27Al MQ-MAS,27Al{31P} REDOR, 31P{27Al} TRAPDOR, 27Al–31P CP HETCOR and31P–31P HOMCOR experiments [327–331]. It should be noted thatmore robust 27Al–31P correlation experiments have been developedrecently [15,332–334], which should find broader applications inthe near future for the characterization of hybrid AlPOs. A few thor-ough characterizations of hybrid AlPO phases have been reported[231,232,328,335–337], in which the organic and/or mineral com-ponents were analyzed by NMR, with, in some cases, a specific focuson the structure at the organic–mineral interface. For example, Ma-fra et al. characterized the local environment of the two independentmethylamine species present in the microporous aluminophos-phate IST-1, using high resolution solid state NMR experimentsbased on FSLG decoupling, such as 1H{FSLG}–1H HOMCOR and 1H–{13C/31P/27Al} HETCOR experiments, all carried out on a 9.4 T magnet(400 MHz) [328].
Among the different hybrid titanium phosphate phases pre-pared to date, n-alkylammonium intercalated layered c-titaniumphosphate phases have been the most thoroughly studied byNMR [338,339]. For example, in the case of a hexylammoniumintercalated phase, it was shown using 1H{FSLG}–31P HETCOR
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experiments that the N–H protons are closer to the surface phos-phate groups than the alkyl protons. This information was comple-mentary to the powder X-ray diffraction data, and allowed astructural model to be proposed [339]. More recently, an ethylam-monium intercalated phase was characterized using high resolu-tion 1H correlation experiments (Fig. 15) [338]. Evidence of avery strong inter-layer P—O� � �H� � �O–P bond was obtained from2D 1H{FSLG}–31P HETCOR spectra recorded with a very short con-tact time (CT = 0.1 ms), and 13C{31P} REDOR experiments were thencarried out to gain insight into the orientation of the ethylammo-nium residues between the Ti-phosphate layers.
Hybrid Zr [340–342], In [343,344], Mg [345], Sc [346], Ga [347–349], V [350–354] and Zn [355–358] phosphate phases have alsobeen characterized by solid state NMR. While in some cases onlythe 31P spectra were recorded [343,344,346,356,357], in othersthe local environment of the metals [345,347,349,350] or the or-ganic component of the hybrids were also examined[348,349,353,355,358]. One recent example concerns the prepara-tion of a hybrid Zn-phosphate phase using histidine as a template[358]. The crystalline sample was characterized using 1D 1H–13C,1H–15N CP MAS experiments, as well as 2D 1H–1H single quan-tum/double quantum and 1H–13C and 1H–31P HETCOR CP MASanalyses, providing proof of the protonation of the histidine aminefunction, and of the presence of an HPO2�
4 anion in the structure.Finally, it is worth mentioning that in the case of layered Zr-phos-phate phases, the dynamics of intercalated pyrazine molecules hasalso been studied using 2H NMR (perdeuterated pyrazine specieshaving been intercalated during the synthesis of the layered mate-rial) [340].
3.2.2. Hybrid cationic claysClays are a large family of minerals, which are classically di-
vided into two groups: cationic clays, commonly found in nature,and anionic clays, rarer in natural environments but relatively easyto synthesize. Here, NMR studies of hybrid cationic clays will bedescribed, while studies on anionic clays can be found inSection 3.2.3.
Cationic clays are formed of negatively charged alumino-silicatelayers, with cations and water in the interlayer space [359]. Exam-ples of common cationic clays are montmorillonite, hectorite, andkaolinite. It has been shown that in addition to simple inorganiccations like sodium, organocations can also be intercalated in theinterlayer space. Furthermore, a wide variety of surfactant/clay hy-brids and polymer/clay composite materials have been described,and the surface functionalization of the clay nanoplatelets has alsobeen studied.
The intercalation of ‘‘small’’ organocations between the layersof clay materials has been studied by NMR [360–362]. For example,Bisio and co-workers looked into the structure of CTA+-intercalatedsaponite clays, which had been prepared using two differentsynthetic routes [360]. 13C CP MAS NMR experiments providedevidence of the interaction between the positively charged ammo-nium end-function and the anionic planes, and also revealed vari-ations in the local environments and conformations of theintercalated organocation, depending on the synthetic routechosen. In addition, direct-excitation 27Al and 29Si MAS NMRexperiments showed variations in the composition of the alumino-silicate layers. 1H–13C HETCOR experiments were carried out toinvestigate the mode of organization of the surfactant moleculesbetween the layers. Evidence of a close proximity between themethylene carbon atoms of the cetyl chains and the silanol func-tions of the anionic layers was observed, suggesting that the CTABcations lie horizontally between the planes. Comotti and co-work-ers also studied the structure of a hectorite phase intercalated bytetraethylammonium and octadecylammonium ions using multi-nuclear solid state NMR experiments, as well as hyperpolarized
129Xe NMR (see Section 2.1.1) [362]. The intimate proximity ofthe organic and mineral components of the material was demon-strated using 2D 1H–29Si HETCOR experiments. In the case of tet-raethylammonium intercalated phases, the presence of voids inthe interlamellar space between the loosely packed cations wasdemonstrated using 129Xe NMR spectroscopy, and this wasexploited to intercalate gases like methane, benzene and CO2. Inthe case of CO2, 13C NMR experiments were carried out to provethe intercalation of this molecule between the layers.
The work by Comotti et al. described above [362] is one of themany examples concerning the intercalation of gases in cationicclays. Along this line, another study worth mentioning here con-cerns the characterization of montmorillonite phases intercalatedby methane hydrate clusters, using 27Al, 29Si, 23Na, and 13C MAS so-lid state NMR [363]. While no changes in 27Al and 29Si NMR spectrawere observed before/after intercalation, variations in 23Na NMRchemical shifts occurred after intercalation of methane-hydrates,and 13C NMR spectra were also recorded to study the differentCH4 hydrate environments in the samples.
The surface functionalization of clay nanosheets using organosi-lanes is another synthetic route which has led to hybrid cationicclays. Solid state NMR has been used to analyze the structure ofthese materials [364–366]. For example, the functionalization ofkaolinite using 3-aminopropyltriethoxysilane (APTES) was carriedout as part of the development of new electrochemical sensors,and it led to hybrid materials which were characterized using 13CMAS and 1H–29Si CP MAS experiments [364]. The NMR spectra sug-gested the formation of two different grafted moieties: organosilyland ethoxy groups. More recently, Mueller and co-workers func-tionalized kaolinite and montmorillonite clays using (3,3,3-triflu-oropropyl)dimethylchlorosilane, this molecule being meant toselectively react with the non-hydrogen bonded Q3–Si hydroxylsites [365]. 1H–29Si CP MAS spectra were recorded, showing thepresence of a signal at �13 ppm which is evidence of a covalentgrafting of the organic molecule at the platelet clay surface. Quan-titative 19F MAS NMR experiments were then carried out, fromwhich the number of reactive hydroxyl sites was found. An alterna-tive strategy for the preparation of organoclay-like structures hasbeen proposed, which consists in directly mixing organosilaneand inorganic precursors [366]. The organic and mineral compo-nents of a talc-like organo clay of this type were characterized by13C and 29Si MAS NMR experiments.
In relation to the silane-functionalized organoclays, the porousclay heterostructures (PCH) are also worth mentioning here. Theseare prepared by surfactant-templated polymerization of silica pre-cursors between the galleries of pillared aluminum clays. As shownby Pinto et al., the functionalization of the internal surface of thesilica mesopores of these materials can be performed using orga-nosilanes like APTES [69,367], and 13C and 29Si NMR experimentscan be used to study this grafting. In addition, the activation ofCO2 by reaction with the amino terminal group of APTES was stud-ied using isotopically-enriched 13CO2 as a reagent, and analyzingthe structure of the resulting materials by 13C NMR spectroscopy[69].
A large number of polymer/clay nanocomposites have been pre-pared, which can exhibit excellent mechanical and thermal proper-ties. Solid state NMR has been used to characterize these hybridcomposites, in order to get information on (i) the mode of associa-tion between the organic and mineral components [368–372], (ii)the thickness of the mineral nanosheets in the composite [373],(iii) the degree of dispersion of the mineral platelets in the organicmatrix [374–379], (iv) the dynamics of the polymer chains at themineral surface [370,371,380,381], (v) the possible long-term deg-radation of the polymer by reaction with the clay mineral [382].Schmidt-Rohr and co-workers developed a series of NMR experi-ments adapted to the analysis of the proximity between the
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polymer chains and the mineral platelets, including 2D 1H–29Si and1H–13C HETCOR experiments (with 1H spin diffusion before CP), and2D 1H–1H spin diffusion experiments [369,370]. For example, in thecase of hectorite clay platelets intercalated by poly(styrene-ethyl-ene oxide) block copolymers [369], 2D 1H–29Si HETCOR spectra (re-corded with 1H spin diffusion before CP, and then enhancing thedetected 29Si signal using multiple Hahn echos) revealed the closeproximity between Si atoms of hectorite and polyethylene oxide(PEO) protons, suggesting the intercalation of the PEO block onlybetween the silicate galleries. This was then confirmed using1H–13C HETCOR experiments, in which correlations between sili-cate Si–OH protons and 13C resonances of PEO (but not polystyrene)were observed. The same group developed the HARDSHIP experi-ment (heteronuclear recoupling by strong homonuclear interac-tions of protons), which allows to evaluate particle thickness innanocomposites, and was tested on various hybrid phases includingpolyvinyl alcohol/hectorite hybrids [373]. More recently, thedynamics of PEO chains adsorbed at the surface of laponite plateletswere examined by Lorthioir et al. [371,380], using 1H and 13C solidstate NMR experiments, including VT measurements (Fig. 16).Although the motions of intercalated PEO chains were found to beslower than in an amorphous phase of neat PEO, reorientations ofthe polymer segments were shown to occur over the tens of micro-seconds timescale, and the intercalated PEO chains were also foundto exhibit dynamical heterogeneities.
3.2.3. Hybrid anionic claysLayered double hydroxides are a family of anionic clays, of
general formula M2þ1�xM3þ
x ðOHÞ2h ixþ
ðAn�Þx=n � yH2O, where M2+ aredivalent cations like Mg2+, Ca2+ or Zn2+, M3+ trivalent cations likeAl3+, Fe3+ and Cr3+, and An� anions like NO�3 ; CO2�
3 ; Cl�, etc. Thesematerials have a lamellar structure, with the anions and watermolecules located between the positively charged sheets ofmixed-metal hydroxides. It has been shown that in addition tosimple inorganic anions, a wide variety of larger organic anionscan be intercalated, such as siRNA [383], drugs (naproxen [384],Pravastatin [234], and L-Dopa [385], etc.), phospholipids [386], por-phyrins [387], and some dyes [388].
In the case of hybrid LDH phases, solid state NMR has been usedto characterize both the organic and mineral components. Forexample, 13C CP MAS experiments are commonly used to demon-strate the intercalation of the intact guest molecule and to discussthe mode of interaction with the inorganic layers [384,388–394].Furthermore, 31P solid state NMR has been used to characterizephosphonate-intercalated LDH [392,395], and 1H NMR has beenapplied to surfactant-modified LDH [396,397]. 27Al MAS andMQ-MAS NMR experiments have most often been used to charac-terize the mineral phase in LDH hybrids [393,396–398]. Recentstudies have shown that more complex NMR experiments mayprovide additional information on the structure of the inorganic
Fig. 16. Temperature dependence of the half-height line width d m1/2 of the PEO 13CNMR peak in the 75.03 MHz 13C spectrum of PEO/laponite (30/70) nanocomposite( ) and for neat PEO ( ). For neat semicrystalline PEO, a short recycle delay wasused to probe the amorphous chain segments. Reproduced from Ref. [371] withpermission (Copyright 2009 American Chemical Society).
layers [399,400], and these may become more common in the fu-ture. However, to the best of our knowledge, no NMR correlationexperiments have been reported yet, as a way of probing the spa-tial proximity between the organic and mineral components ofLDH hybrids.
3.2.4. Functionalized micro/nanoparticles of metal oxides and otherionic solids
The functionalization of inorganic micro/nanoparticles is a verycommon procedure for the preparation of hybrid building-blocks,combining the properties of the inorganic core with those of theorganic molecule grafted onto their surface. Examples of NMRcharacterizations of surface-functionalized metal oxide, metalfluoride, and other ionic solid particles will be given here. In mostcases, solid state NMR has been used (i) as a proof of the success ofthe grafting, and/or (ii) as means to rule out the presence of un-wanted by-products. It should be noted that the particular caseof functionalized SiO2-NP has already been developed inSection 3.1, while functionalized metal-NP will be looked into inSection 3.4.4.
NMR characterizations have been carried out on metal oxideparticles such as TiO2, Al2O3, SnO2, HfO2 and Al-coated SiO2, func-tionalized mainly by carboxylates and phosphonates [401–409].Generally, the presence of broader resonances on the 13C and 31PMAS spectra, in comparison with crystalline samples containingthese organic species, are interpreted as a proof of the functional-ization. In particular, in the case of surface modifications byphosphonate coupling agents, the presence of sharp resonancesis evidence of unwanted dissolution–precipitation reactions[409,410]. More insight into the grafting mode of phosphonatesat the surface of TiO2 particles was obtained by Mutin and co-workers [403]. Surface functionalization was performed using17O-labeled phosphonic acids, and 17O MAS NMR spectra wererecorded at two different magnetic fields, bringing direct evidenceof the formation of P–O–Ti bonds.
The surface functionalization of calcium phosphate (mainlyhydroxyapatite), zinc phosphate, and calcium carbonate particleshas also been the object of solid state NMR characterizations. Con-cerning calcium phosphates, a series of studies have been carriedout on phases purposely synthesized to shed light on biominerali-zation processes or to prepare novel biomaterials; these exampleswill be developed in Section 3.3. Calcium phosphate particles func-tionalized by carboxylate or phosphonate-like molecules have alsobeen prepared for more fundamental investigations, and character-ized by 31P, 13C and/or 1H MAS NMR, in order to obtain informationon the mode of grafting [411–414]. Similarly, carboxylate andphosphonate-functionalized calcium carbonate particles have beenstudied by NMR [410,415,416]: for example, in the case of phos-phonate modified calcite, 31P–1H CP MAS experiments wereperformed to study the grafting process [410]. ConcerningZn-phosphate nanoparticles, one recent study by Schmedt aufder Günne and co-workers has shown that for diethylene–glycolcoated particles, 31P{1H} REDOR experiments in combination withnumerical simulations can be used to discriminate homogeneousnanoparticles from core–shell nanoparticles [417].
Other hybrid inorganic nanoparticles have also been investi-gated by solid state NMR. For example, palmitate-functionalizedindium-phosphide quantum dots have been characterized using31P MAS and 1H–31P CP MAS experiments, revealing that the sur-face phosphorous atoms are oxidized [418]. In the case of relatedInP/ZnS core/shell particles, it was observed that the amount ofoxidized phosphorous species was actually higher than for pureInP particles [419]. 1H–13C CP MAS experiments were then usedto characterize the grafted palmitate, and the broad 13C resonanceof the carbonyl region was interpreted as a proof of fact that thesespecies play the role of ligands attached to the surface of the
139La (ppm)
A
B
C
D
E
F
Fig. 17. 56.46 MHz 139La MAS NMR spectra of (A): LaF3, (B) undoped LaF3 NPs, (C) 5% doped (Yb3+) LaF3 NPs, (D) 10% doped (Yb3+) LaF3 NPs, (E) 5% doped (Y3+) LaF3 NPs, (F)10% doped (Y3+) LaF3 NPs. Inserts: schematic representation of Ln3+-doped LaF3 nanoparticles (NPs) stabilized by di-n-octadecyldithiophosphate ligands (Ln = Yb, Y).Reproduced from Ref. [420] with permission (Copyright 2007 American Chemical Society).
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 23
quantum dot nanoparticle [419]. The extensive NMR study of pureor doped LaF3 nanoparticles functionalized at their surface by di-n-octadecyldithiophosphate ligands is also worth mentioning here[420]. In this work, the core of the NP was characterized by 139Laand 19F MAS NMR experiments (Fig. 17), and also 45Sc MAS NMRin the case of Sc-doped particles. The surface-grafted species werestudied using 1H and 31P solid state NMR. In particular, in the 1HNMR spectra of NP-doped by paramagnetic impurities, resonanceswere found to be broader than in the non-doped analogues, andthis was seen as a proof of the attachment of the molecules atthe NP surface. Furthermore, 31P NMR experiments provided evi-dence of the partial hydrolysis of the dithiophosphate ligands((RO)2P(S)S) into thiophosphate ((RO)2P(O)S) and phosphate((RO)2P(O)O) functional groups. Additional 19F–31P CP MAS exper-iments were then carried out, showing that the thiophosphate andphosphate species were actually the closest to the fluorine (andthus to the NP surface).
3.2.5. Other hybrid materialsA large number of other hybrid O/I phases has been investigated
by NMR, including hybrid cements, fluorinated hybrid structures,glass–polymer and nanoparticle–polymer composites. Recentexamples of the characterization of these phases will be presentedin this sub-section.
Cements are an important class of structural materials, thealuminate/silicate phases having been the most studied to date.Controlling the mechanism of formation and the hydration kineticsof aluminate/silicate cements is important to achieve the materialproperties required, particularly for large scale construction pro-jects. It has been shown that the addition of saccharide molecules
to the slurry can slow down the hydration processes of cementsand modify their rheological properties. In order to understandthe underlying molecular mechanisms, Chmelka and co-workersperformed a series of NMR studies to characterize the surfaceinteractions of different saccharides (glucose, sucrose and malto-dextrin) in aluminate, silicate, and alumino-silicate slurries[421,422]. A variety of 27Al, 29Si and 13C NMR experiments werecarried out, including 2D 1H–13C and 1H–27Al HETCOR experi-ments. For example, in the case of tricalcium silicate and aluminatephases, the degree of hydration in the presence of 1% weight ofsaccharide was estimated using 29Si and 27Al MAS NMR experi-ments. Quantitative analyses of the spectra showed that sucroseis the most effective inhibitor of the hydration. 13C solid stateNMR characterizations (carried out on a sample prepared using13C-enriched saccharides) provided evidence of the degradationof glucose in these phases, but not sucrose. Using 2D 1H–13C HET-COR experiments, evidence of the close proximities between theglucose degradation products and the hydrated aluminates specieswas obtained, while no such proximity was observed for sucrose.In a different context, it should be noted that hybrid cements havealso been synthesized for biomedical applications, and that solidstate NMR has also been used to provide information on the organ-ic or mineral components [423,424]. Further details on these‘‘biocements’’ can be found in Section 3.3.
Another class of hybrids for which NMR has provided usefulstructural information is the family of crystalline fluorinated hy-brid materials (with partial or complete fluorination), which alsoincludes some fluorinated MOF structures [212,425–430].Multinuclear solid state NMR can be used to complement X-raydiffraction characterizations, in particular to confirm the choice
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of the space group. The Zn3Al2F12[HAmTAZ]6 phase (HAmTAZ = 3-amino-1,2,4-triazole) is particularly noteworthy [212], as it is oneof the rare examples of a material for which NMR characterizationof the local environments around every single element present inthe material was carried out (1H, 13C, 15N, 19F, 27Al, 67Zn).
Finally, it is worth noting that many other hybrids based on an‘‘ionic solid’’ inorganic component have been described, such asmesoporous tantalum oxide/rubidium fulleride composites [431],polymer/titania composites for photovoltaic applications [432a],carbon-modified TiO2 phases [432b], solid acid electrolytes[433,434], and glass/polyamide nanocomposites [435,436]. In thelatter case, a series of 13C MAS and 1H–13C CP MAS NMR experi-ments were carried out in order to discriminate the signals comingfrom bulk-type nylon-6 from those corresponding to the polyam-ide domains in close proximity with the inorganic particle surfaces[435].
3.3. Biomaterials
Hybrid organic–inorganic materials are present in a number ofliving organisms, the inorganic component being formed throughbiomineralisation processes. In order to rationalize the uniqueproperties of these materials, much research has been carried outto determine their structure, notably at the molecular scale. In thiscontext, solid state NMR has been used as an atomic level charac-terization tool, and has provided insight into the organic andmineral components, as well as the organic/mineral interface.Recent examples of NMR studies of natural biomaterials will firstbe presented below. NMR investigations on hybrid biomimetic/bioinspired materials and functional hybrid biomaterials will thenbe detailed. Additional examples on the application of solution andsolid-state NMR techniques to the study of biomineralization canalso be found in the recent review by Brunner and co-workers[437].
3.3.1. Natural biomaterials3.3.1.1. Diatoms and other living organisms with siliceous-basedbiominerals. Siliceous biominerals can be found in a large numberof living microorganisms, such as diatoms, plants and sponges[438–440]. In the case of diatoms, silica is taken up in the formof Si(OH)4 by the silicon transporter proteins (SITs) and stored inthe ‘‘silicon pools’’ as precondensed species, which are thought tobe stabilized by organic biomolecules. Silica then infills the so-called silica deposition vesicle (SDV), in which SiO2 condensationand patterning occurs, leading to formation of mineralized frus-tules (i.e. the diatom cell walls) [439,441]. The biomineralizationof silica has been found to be under genetic control, and to involvea large number of biomolecules, such as phosphoproteins (silaffins,silacidins) and long-chain polyamines [439]. Other organic mole-cules also become intimately entrapped within the silica walls[442], as further detailed below.
The siliceous component of diatoms has been analyzed by 29Sisolid state NMR spectroscopy, using single-pulse MAS and CPMAS pulse sequences, both at natural abundance [437,442,443],or after 29Si-enrichment [437,441,442,444]. In the latter case, dia-toms were grown in artificial sea water containing Na2
29SiO3
[437,441], or 29SiO2 [442,444]. The broad 29Si resonances revealthe amorphous nature of the silica phase. The chemical shifts ofthe individual spectral components were found hardly to varyaccording to the diatom species [437]. The Q speciation deter-mined from quantitative 29Si NMR experiments shows the pre-dominance of Q4 species (�70–75%). A more detailed analysisshowed that the Q4/Q3 ratio actually differs depending on whetherthe complete diatom cell is analyzed by 29Si NMR, or only the cellwalls [437]. This was interpreted as showing the presence of a sil-ica sol inside the cells, with a lower condensation degree than in
the walls [437,441,445]. It should be noted that an experimentalprotocol has been proposed to best analyze the mineral phase ofintact diatoms, avoiding any unwanted further condensation ofthe silica species present initially [441]. In the case of 29Si labeleddiatoms, more advanced 2D 1H–29Si HETCOR experiments havealso been carried out [444]. These revealed the expected proximityof 1H of the silanol groups with the Q3 Si species, although the 2Dspectra were also observed to be dependent on the hydration stateof the sample [437]. In a recent study, Q4/Q3 ratios have also beenderived from CP MAS spectra, in order to understand the influenceof external parameters (such as the salinity of the growth medium)on the final Q speciation in diatoms [443]. Finally, in the case ofplants like wheat and rice, siliceous components have also beenanalyzed using 29Si NMR, revealing the presence of broad reso-nances and a predominance of Q4 units [437,446,447]. A recentstudy showed that in the case of Phragmites australis plants, thedegree of condensation of silica varies from the leaf shealths tothe leaf tips [448].
NMR analyses of the organic components of diatoms have alsobeen carried out on solid samples, either using solid state NMRequipment [437,442,444,449] or HRMAS setups [450,451]. A verythorough study is the work by Coradin and co-workers on Thalass-iosira pseudonana diatoms, isotopically enriched in 13C and 15N[442]. In this study, whole diatom cells were first characterizedby 1H, 31P and 29Si solid state NMR, but this gave very complexspectra. Consequently, the cells were washed with sodium dodecylsulfate (SDS) and EDTA, in order to remove the cellular materialslocated inside the frustule, and to allow characterization only ofthe organic species associated to the silica matrix. 1H, 13C, 31Pand 15N NMR experiments were carried out on the resulting mate-rial, revealing that the biomolecules associated to the siliceousphase are mainly glucose-based carbohydrates, lipids, and pro-teins/amines. Based on 15N IRCP (inversion-recovery CP) MASexperiments, it was suggested that these organic species presentdifferences in ‘‘mobility’’, depending on whether they are embed-ded inside the silica matrix, or whether they are only weakly inter-acting with the cell walls. Further chemical treatment of thematerial using H2O2 allowed the visualization of the signals com-ing from the species most intimately interacting with the frustules.The 13C and 15N NMR spectra obtained reflected mainly the pres-ence of lipids and chitin. It should be noted that the 31P MASNMR spectra were also of interest in this study, bringing evidenceof glycerol 1,2-cyclic phosphate and polyphosphates in the cellularmaterial inside the frustules of non-washed diatom cells. In a morerecent study, 13C MAS and 1H–13C HETCOR experiments were car-ried out at natural abundance on three different diatom species,revealing variations in the carbohydrate/aminoacid ratio [437].
Although the studies presented above provided information onthe organic phase of diatoms [437,442], they did not specificallyfocus on characterizing the interactions taking place at the organ-ic–mineral interface. Chmelka and coworkers tried to address thisissue by characterizing 13C, 29Si (and 15N) isotopically enrichedThalassiosira pseudonana diatoms [444]. After careful comparisonof different cross-polarization transfer methods on a series of mod-el samples, an adiabatic-passage Hartmann–Hahn cross-polariza-tion (APHH-CP) experiment was carried out to probe 13C–29Siproximities, and thereby try to determine which organic functionsare closest to the silica surface. Despite several days of acquisition,only a very weak signal assigned to carboxylate groups at the silicasurface was observed. Access to higher magnetic fields may helpprovide more definite answers on the nature of the organic–mineral interface in diatom shells.
3.3.1.2. Bone, teeth and other calcium-phosphate-based biominer-als. Bone tissue has three main constituents: an organic phase(mainly composed of collagen), a calcium-phosphate mineral
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 25
phase (containing mainly nanocrystals of carbonated hydroxyapa-tite), and water.
Initially, NMR investigations of bone tissue focused on studyingthe structure of the mineral phase, notably through 31P MAS and1H–31P CP MAS solid state NMR experiments [452]. The 31P NMRparameters and CP build-up curves were compared to syntheticcalcium phosphate phases likely to be present under physiologicalconditions, including carbonate-substituted apatites, brushite,octa-calcium phosphate, and hydroxyapatite [452–457]. Two-dimensional 1H–31P HETCOR experiments actually provided thefirst unambiguous evidence of the presence of a significant amountof hydroxyl groups within the bone apatite crystallites [455]. Sim-ilar 2D experiments were later carried out on a series of equinejoint bone tissues, but little difference was observed in the mineralcomponent between the healthy or diseased samples [458]. Con-cerning NMR studies of dental tissues [459–461], Kolodziejskiand co-workers used 31P ? 1H CP MAS NMR experiments in orderto estimate the OH content in enamel, dentin, and cementum ofhuman teeth [461]. More recently, it was shown that 2D 1H–31PHETCOR experiments can be carried out in �3 h on a single mousetooth at ultra-high magnetic fields (17.6 T), using small volumerotors (1.3 mm diameter) [460].
In addition to analyzing the structure of the mineral phase atthe atomic level, Schmidt-Rohr and co-workers have shown theusefulness of performing HARDSHIP (heteronuclear recouplingwith dephasing by strong homonuclear interactions of protons)NMR experiments to determine the thickness of the apatite crys-tallites in bone [373,462]. Furthermore, the local environment ofother ions present in the mineral phase of bone has been analyzedby NMR. In particular, it was shown recently that despite the verylow-receptivity of calcium-43, it is possible to use 43Ca MAS NMRexperiments to characterize bone and dental tissues [463,464],provided that complementary Ca K-edge X-ray absorption spec-troscopy analyses are performed to ensure proper interpretationof the NMR data [464]. The local structure around Na+, which isone of the main substituents to Ca2+, has also been analyzed using1H–23Na R3-HMQC [465,466] experiments, bringing direct evi-dence of the close proximity between some Na+ sites and the OHgroups of the apatite lattice [464]. Solid state NMR has also beenused to study CO2�
3 in enamel and bone [467,468], and F� substit-uents in bone tissues [469–471]. To the best of our knowledge, nohigh field/high resolution 19F NMR experiments have yet been pub-lished on bone or dental samples.
Natural-abundance 13C CP MAS NMR experiments on bone yieldsignals coming mainly from the collagen matrix [452,472–478].The linewidth of individual 13C peaks was found to depend onthe hydration state of the bone sample [475], with significantline-broadening occurring upon dehydration. Thus, controllingthe hydration state of bone samples during 13C NMR experimentsis important in order to maintain the integrity of the sample duringthe measurement [475,478]. In a recent study, Ramamoorthy andco-workers showed that by soaking bone samples into a PBS (phos-phate buffer saline) solution containing paramagnetic Cu(II) cen-ters, it was possible to reduce the 1H T1 by a factor �2–3, andthus favour faster acquisition of 13C CP MAS spectra at naturalabundance [474]. However, some of the 13C collagen resonancesappeared to be more affected by the presence of nearby Cu2+ ions,making the interpretation of these spectra somewhat difficult.
The NMR characterization of the organic–mineral interfacestructure in bone and other calcium-phosphate based biomineralshas been the focus of several studies over the past eight years. Duerand co-workers were the first to enter the field in 2005 [479],showing that 13C{31P} REDOR experiments are well suited to thecharacterization of organic–mineral interfaces, because most ofthe phosphorus in bone is located in the mineral phase, while mostof the carbon is in the organic matrix. Particular attention was paid
to the signal centered at �76 ppm, which showed significantdephasing after re-introduction of 31P recoupling pulses. Basedon the comparison with 13C NMR spectra of a series of calcified tis-sues, the signal was suggested to correspond to a carbon signalfrom glycosaminoglycans [476]. Later on, Schmidt-Rohr and co-workers questioned this assignment showing that it may ratherbe due to citrate ions intimately adsorbed at the mineral surface[480]. Although there still seems to be some debate around the ex-act nature of this signal, the significant dephasing of this peak in13C{31P} REDOR experiments has been observed not only in bone(Fig. 18) [473,476,478,480], but also in other calcified tissues suchas teeth [481], calcified plaque and cartilage [482,483], and apatitekidney stones [484a]. It has also been suggested that becauseREDOR specifically focuses on the study of the organic–mineralinterface, it could be used as a tool for measuring bone quality[473].
Water is another important component of bone tissue, which ismeant to be present within the organic matrix, but also at the or-ganic–mineral interface. It has been shown that 1H T2 relaxationmeasurements can allow 1H signals from collagen, collagen-boundwater, and pore water to be differentiated, and to report on thehydration state of bone [485]. The possible application of such 1HNMR measurements to predict fracture risk has been discussed,and their advantage over more traditional bone mineral densitymeasurements demonstrated. On the other hand, 1H MAS NMRand 1H–31P LG-CP MAS experiments have been carried out as well,bringing evidence of the presence of tightly bound water at thesurface of bone mineral platelets [486,487].
Just as in the case of diatoms, one important question to addresswhen it comes to the MAS NMR study of bone is the form underwhich the samples are studied. Indeed, while solid state NMRexperiments are most often carried out on dry powdered samples,this can be problematic for biological tissues such as bone. As men-tioned previously, upon dehydration, it was shown that 13C NMRlinewidths and 13C{31P} dephasings could be affected [475,478].Therefore, recent characterizations have been performed on‘‘wet’’ samples, and have used temperature regulation during theexperiment, in order to avoid any excessive heating of the sampleduring the spinning. Moreover, it was recently suggested by Nikelet al. that intact samples (rather than powders) are preferable for13C{31P} REDOR analyses of the organic–mineral interface structure[473], and an experimental protocol was proposed for the repro-ducible spinning of intact human bone samples of different age.Today, in order to broaden the range of applications of solid stateNMR studies of bone and dental tissues, it seems necessary to gotowards the analysis of smaller samples, because the size of thesamples studied does reveal some of the heterogeneities detectedin common biochemical and histological analyses at micrometerscales. The use of small MAS rotors (diameter <1.3 mm) [460] orof techniques like MACS [133] should help the move in thisdirection.
Finally, it is worth mentioning here that although most NMRstudies of Ca-phosphate based biominerals have concerned boneand dental tissues, solid state NMR experiments have also beenperformed on cartilage [482,488], apatite-based kidney stones[484], ivory [489], the shells of ibliform barnacles [490] and lingu-liform brachiopods [491]. In the latter case, the material is not anapatite/collagen composite, but rather a chitin/apatite hybridmaterial [491].
3.3.1.3. Calcium carbonate based biominerals. Calcium carbonate is abiomineral of great importance for a large number of species [492].It is for example present as nacre in the inner shells of mollusksand in oyster pearls. In Nature, calcium carbonate can be foundin five different crystalline forms: three anhydrous polymorphs(calcite, aragonite, and vaterite), and two hydrated phases (mono-
13C Chemical Shift (ppm)020406080100120140160180200
A
B
C
Fig. 18. 13C{31P} REDOR NMR spectra at 14.1 T of an ‘‘intact’’ bone sample from a 77-year old woman donor, recorded to highlight the C atoms closest to the mineral surface.Spectra recorded with (red) and without (black) 31P recoupling pulses are compared; the difference spectra are in grey. Spectra were processed by: (A) simple Fouriertransform. (B) Conventional exponential multiplication (40 Hz line broadening). (C) Singular value decomposition (SVD) (de-noising procedure) followed by Fouriertransform, without line broadening. Adapted from Ref. [473] with permission (Copyright 2012 American Chemical Society).
26 C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
hydrocalcite (CaCO3�H2O) and ikaite (CaCO3�6H2O)). Amorphouscalcium carbonate (ACC) phases have also been observed [493].In natural biominerals, calcium carbonate phases were found toform in presence of biopolymers like chitin, silk-like proteins,and acidic proteins like caspartin and calprismin [494,495].
Several solid state NMR studies of nacre have been reported[496–498]. The mineral phase mainly comprises the aragonitepolymorph, although high resolution TEM and solid state NMR pro-vide evidence of an ACC component as well [496,497]. Indeed,using direct-excitation 13C NMR experiments and 1H ? 13C CPMAS experiments at variable contact times, evidence of the pres-ence of different CO2�
3 local environments was obtained [496].The amorphous component was further described by recording2D 1H–13C HETCOR spectra, which showed that it contains HCO�3anions, as well as a significant proportion of water. Althoughsignals coming from the organic component of nacre can also beidentified from CP MAS spectra [496,498], no direct evidence of or-ganic–mineral interactions could yet be obtained by NMR. One ofthe reasons for this is that characterizing these interfaces byNMR in the absence of isotope enrichment is not as straightfor-ward as in bone: (i) the organic/mineral balance in natural nacrephases has been shown to be much lower than in bone; and (ii)the only isotope specific to the mineral phase is 43Ca, which is anextremely challenging nucleus for NMR. Nevertheless, it was ar-gued that the presence of an ACC layer around the aragonite plate-lets ruled out the possibility of having epitaxial growth of nacre innatural biominerals, and that this layer may help stabilize the[001] surface of aragonite [497].
In some species of crustaceans, calcium is stored under the formof ACC within reservoir organs called gastroliths. Solid state NMR
characterizations have been carried out in order to understandthe structure of crayfish and lobster gastroliths [499,500], and todetermine how ACC is stabilized in vivo. The inorganic componentof the gastroliths was analyzed using 13C direct excitation and1H–13C CP MAS experiments. The strong similarities in the CO2�
3
signal in these spectra (both in the peak-width and position) re-vealed that all carbonate ions are in hydrogen-rich environments[499,500], in agreement with the fact that biogenic ACC is knownto be highly hydrated. By comparing the 13C NMR spectra, as wellas other characterizations (such as Ca K-edge X-ray absorptionspectroscopy and pair-distribution function analyses), it was foundthat the structure of the ACC phase in gastroliths is very similar tothe one obtained in synthetic ACC materials [500]. The only appar-ent difference between fresh synthetic samples and natural onesconcerns the water mobility, as suggested from 1H solid stateNMR spectra. In the case of crayfish gastroliths, Schmidt and co-workers focused more specifically on trying to establish the originof the stabilization of ACC [499]. Analyses of the 1H–13C CP MASNMR spectra of gastroliths before and after decalcification, and ofthe 1H, 13C and 31P NMR spectra of the solution recovered afterdecalcification revealed the presence of chitin, but also of otherspecies like citrate, phosphoenolpyruvate, and inorganic phos-phate. Using 2D 31P–13C HETCOR experiments, it was demon-strated that the inorganic phosphate and phosphoenolpyruvateanions are dispersed within the ACC phase, meaning that theyprobably play a significant role in the stabilization of biogenicACC. Using 13C{31P} REDOR analyses, it was then shown that mostcarbonates are within 8–9 Å from a phosphorous atom. Interest-ingly, REDOR studies also revealed the close proximity of citratewith the phosphorous-bearing anions, and it was hypothesized
Fig. 19. Solid state NMR spectroscopy at 7.0 T applied to the in situ observation ofthe structure and composition of biomineralized calcite (from Emiliania huxleyi). S0
corresponds to the acquisition of the spectrum with recoupling pulses turned off.DS is defined by S0 � SR, where SR stands for the acquisition with recoupling pulses.Reproduced from Ref. [501] with permission (Copyright 2008 American ChemicalSociety).
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 27
that citrate–Ca2+–phosphate (or citrate–Ca2+–phosphoenolpyr-uvate) complexes may be dispersed within the ACC phase. Itshould be noted that as for bone specimens, it has been shown thatit is preferable to perform solid state NMR measurements on intactspecimens (the grinding being as minimal as possible, and per-formed only to allow stable rotation of the rotor). Indeed, thiscan help avoid unwanted crystallization of the ACC phase [499].Recent 31P NMR studies suggested that a minor amorphouscalcium phosphate phase may also be present in lobster gastroliths[500].
The biogenic calcium carbonate phase present in the coccolithsof a unicellular alga (Emiliana huxleyi) was analyzed by solid stateNMR (Fig. 19) [501]. The phytoplankton strain was grown in a 13Cand 15N enriched medium. Based on direct-excitation 13C and1H–13C CP MAS experiments, carried out on both intact and NaO-Cl treated samples (the NaOCl treatment allowing to removesome of the biomolecules), it was shown that two different car-bonate environments are present, corresponding to a highly crys-talline calcite environment and a more disordered heterogenousphase. Comparison of the direct excitation and CP MAS spectraalso showed that 13C resonances arising mainly from the mineralphase are observed by direct excitation, while more selectiveexcitation of the organic phase is possible by CP using short con-tact times. This observation was important in view of the subse-quent REDOR studies. Indeed, 13C{31P} and 13C{15N} REDORexperiments were then carried out, in which the 13C signals wereexcited either by direct excitation or by CP. Analysis of thespectra showed that the 31P resonances in intact samples mainlyresult from the presence of phosphorous inside the calcite lattice,and that interestingly, some nitrogen species are also present in-side this crystalline phase. It was suggested that this could becaused by NO�3 and PO3�
4 defects in the mineral phase [501].The presence of other trace elements like boron within two coralaragonites and one foraminiferal calcite was demonstrated using11B solid state NMR [502]. More recently, solid state NMR hasbeen used to characterize adult sea urchin spine [503]. In thiscase, 13C NMR characterizations were also performed using bothdirect-excitation and CP schemes, and evidence for the presenceof three different carbonate local environments was obtained,one of which was assigned to a more ‘‘amorphous’’ type of phase.Based on the different characterizations made, a novel model forthe structure of the hybrid material in sea urchin spine was pro-posed. Finally, in some cases, 13C NMR has been used in a moresystematic way to compare the organic matrix only (after
decalcification of the hybrid material), as shown in a recent studyon a series of Verongida sponges [504].
3.3.1.4. Other hybrid biominerals. A variety of other natural calcifiedbiominerals have been identified in living organisms, some ofwhich actually correspond to pathological calcifications [505]. Afew of them have been studied by NMR. For example, it was shownthat Ca-oxalate renal stones can be characterized using 13C CP MASexperiments [506] and natural abundance 43Ca solid state NMR[507]. Furthermore, 31P and 13C solid state NMR analyses of stru-vite ((NH4)MgPO4�6H2O) based stones have also been reported[484,506]. Further studies of such biogenic minerals can beexpected in the future.
3.3.2. Synthetic bio-inspired materials and coatingsThe study of natural biocomposites has inspired materials
chemists to synthesize a series of bio-inspired or bio-mimetic O/Imaterials, which have been prepared either to help understandthe intrinsic structure of natural materials, or to try to reproducesome of their unique properties. In many cases, solid state NMRhas been used as a tool to characterize the organic and/or mineralcomponent of the material. Below, a few examples of such hybridphases will be presented, starting with the materials involvinginorganic siliceous matrices first, and then following with syn-thetic calcium phosphate hybrids.
3.3.2.1. Siliceous based materials. Using sol–gel chemistry, a widevariety of biopolymers have been incorporated into silica-basedmatrixes, such as chitosan [508], gelatin [509], and chitin [510].Furthermore, the use of poly(c-glutamic acid) in the preparationof siliceous hybrids for bone regeneration applications has beendescribed [511,512]. Synthetic procedures have been developed,allowing the proper dispersion of the polymer within the silica ma-trix, and, in some cases [508,509,511], a covalent attachment be-tween the organic and mineral components of the materials. 29Sisolid state NMR has been used to determine the degree of conden-sation of the silane (and organosilane) fragments by looking at theQn and Tn (see the list of acronyms) speciation, and to establishhow the condensation can be affected by synthetic parameters,such as the percentage of polymer introduced in the reaction med-ium [508,509]. The biopolymer itself has been characterized by 13C,15N, and/or 1H solid state NMR experiments [508,510]. In the caseof chitin/silica nanocomposites [510], Alonso et al. used 1H spin-diffusion experiments to analyze the organic–mineral interface,and they demonstrated that the chitin and silica components ofthe materials are not phase separated at the nanoscale.
3.3.2.2. Calcium phosphate based materials. A variety of polymer/calcium phosphate hybrids have been characterized by solid stateNMR, which involve either biopolymers (such as collagen, chon-droitin sulfate, dermatan sulfate, hyaluronic acid, dextran sulfate,polygalacturonic acid, poly-L-glutamate, poly-L-asparagine, poly-L-lysine, and polyleucine/polylysine copolymers) [513–517], orother kinds of synthetic polymers ((modified) Pluronic F127. . .)[518–520]. Some NMR characterizations have specifically focusedon the analysis of the mineral phase [513,519,520], for exampleto characterize the disordered Ca-phosphate layer at the surfaceof apatite nanocrystallites [519]. The key objective of most NMRcharacterizations has been however to demonstrate the intimatemixing of the organic and mineral components of these materials,and thus the formation of a nanocomposite. For this purpose, notonly 13C{31P} REDOR experiments were used (for the same reasonsas exposed above in the section on NMR studies of bone) [514,515],but also NMR experiments involving 1H–1H spin diffusion[516,518–520]. In addition, Schmidt-Rohr and co-workers showedthat the thickness of the apatite nanocrystals in such hybrid phases
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can be determined by combining several different NMR experi-ments, including 31P{1H} HARDSHIP and 1H–31P HETCOR with 1Hspin diffusion for example [462,519].
To obtain a better understanding of biomineralization pro-cesses, the mode of adsorption of proteins or small peptides atthe surface of hydroxyapatite has been widely studied, and itwas shown that solid state NMR can provide valuable informationon the positioning, conformation and dynamics of proteins at theapatite surface [437,521–523]. The most comprehensive exampleto date is the study by Drobny and co-workers on the adsorptionof statherin [437,521,522], which is a small protein (43 aminoacids) known to control the nucleation and growth of hydroxyap-atite crystals in saliva. A series of statherin proteins selectively en-riched in 13C/15N at selected amino acid positions were preparedfor NMR studies, the enrichment being chosen so as to allow a spe-cific distance to be measured. After adsorption on hydroxyapatite,high resolution solid state NMR experiments were performed, suchas 13C{31P} REDOR or DQ DRAWS [524], in order to determinegeometrical features of the adsorbed protein (inter-atomic bonddistances, torsion angles. . .) [525–532]. One of the conclusions ofthis work was that both ‘‘acidic’’ (e.g. glutamic acid) and ‘‘basic’’(e.g. arginine) amino acid residues interact intimately with thehydroxyapatite surface [525,527]. It was also shown that the modeof adsorption of statherin onto this surface can be modeled usingthe Rosetta program (and the RosettaSurface.nmr program)[533,534], using the results from the NMR experiments as geomet-rical constraints in the calculations. Amelogenins are anotherfamily of proteins present in the developing enamel matrix, whoseinteraction with apatite crystallites has been investigated by solidstate NMR [523,535–537]. In this context, Shaw and co-workers re-cently showed that changes in the ionic strength of the proteinsolution during adsorption led to modifications of the amelo-genin/apatite interfaces [523].
Bisphosphonate based drugs like Alendronate and Zolendronateare important in the treatment of osteoporosis and their mode ofinteraction at the surface of synthetic calcium-phosphate biomate-rials [424,538–540] and bone mineral [541,542] has been studiedby NMR. For example, Fayon and co-workers recently studied apa-tite-doped alendronate cements using 31P 2D DQ–SQ correlationexperiments, showing the close proximity between the orthophos-phate groups of the apatite matrix and the phosphonate functions[424]. Oldfield and co-workers synthesized a series of bisphospho-nates isotopically enriched in 13C, 15N and 2H, and compared theirmode of adsorption and dynamics at the surface of human boneapatite [542]. 2H NMR experiments, in particular, revealed thatthe bisphosphonate side-chains undergo motions, which wereidentified in the case of pamidronate as a hopping of the –CH2-NHþ3 terminal group between different surface phosphate positions[542].
3.4. Hybrid structures involving metal complexes and coordinationnetworks
3.4.1. Simple metal complexes and clustersWith the increasing chemical and structural complexity of hy-
brid materials, the need to expand the range of characterizationtechniques likely to help understand in more depth their structureand reactivity has emerged. In particular, being able to describe indetail the local environment of (trace) metal ions in complex hy-brid phases now appears as essential, for example in fields suchas supported organometallic catalysis. As a first step, a series of so-lid state NMR studies have already been carried out on possibleprecursors and building blocks present in the hybrid phases. Inparticular, as mentioned in Section 2.4 (computational studies),metal complexes bearing organic ligands have been studied, with
the specific aim of determining which NMR parameters are mostlikely to inform on the local structure around the metal.
Solid state NMR studies of metal complexes or clusters bearingorganic ligands still mainly focus on characterizing these ligands(using 13C or 13P NMR for example) [543–546]. However, recentexamples show that direct observation of the metal ions is alsopossible in a number of cases. Solid state NMR can indeed be usedto analyze the metal ions in organic complexes containing alkaline-earth metal ions (e.g. Mg2+, Ca2+, Sr2+. . .), transition metal ions (e.g.Ti4+, Zr4+, Cu+, Co2+, Ag+. . .) as well as metal ions of groups 13 and14 (e.g. In3+, Ga3+, Pb2+, Sn2+, Sn4+, etc.). Although most NMR exper-iments used for characterizing these nuclei are still ‘‘non-routine’’,valuable information can be obtained on the local structure, geom-etry, and also on the electronic environment of the metal, asillustrated below.
The sensitivity of the NMR parameters of metal ions to subtlechanges in their coordination environment has most often beenstudied on a series of metal complexes. For example, the local envi-ronment of Cu(I) in bis-triphenylphosphine copper complexes ofgeneral formula [(PPh3)2CuX] (X = O2C–CH3�nFn, O2C–Ph, NO3,BH4, etc.) was studied using 65Cu NMR [547]. These complexesare of interest in relation with Cu-catalysts used in organic synthe-sis. Variations in the 65Cu CQ parameters (which were determinedfrom both NQR and WURST QCPMG NMR experiments) in [(PPh3)2-
Cu(O2C–CH3�nFn)] were discussed in relation with the decrease inP–Cu–P bond angle and progressive change in denticity of the car-boxylate ligand as n increases. The influence of the nature of the‘‘X’’ ligands on the quadrupolar parameters was also discussedfor this family of compounds. Another example concerns the workby Wasylishen et al. on a series of gallium and indium triarylphos-phine trihalide adducts [548,549]. These compounds are of interestas precursors for the preparation of semiconductors like InP andGaP, which are extensively used in light-emitting diodes. Static115In and 71/69Ga NMR spectra were recorded at different magneticfields (and notably at 21.1 T), in order to extract the quadrupolarand CSA parameters. Similar trends were observed in the variationsof 115In and 71Ga chemical shielding as a function of the nature ofthe halide. In addition, for a given triarylphosphine ligand, thechemical shift spans measured for iodide adducts are always thelargest. However, no clear relationship between the quadrupolarcoupling constants CQ and the structure of the adducts could beestablished. It should be noted that as part of these studies,1J115In�31P and 1J71Ga�31P coupling constants were also measured,by recording 31P MAS NMR spectra. In the case of the indiumadducts, it was concluded that the 1J115In�31P values generallyincrease as the basicity of the triarylphosphine ligands increases.
Schurko et al. have studied Zr and Ti-metallocenes[200,207,550a] which are known to be active in olefin polymeriza-tion. The extensive 91Zr, 47/49Ti and also 35Cl solid state NMR stud-ies of Zr and Ti cyclopentadienyl complexes (some of whichactually crystallize as coordination polymers) have shown that (i)47/49Ti and 91Zr NMR parameters are sensitive to the presence ofsubstituents on the cyclopentadiene ligand, (ii) 91Zr quadrupolarand chemical shielding parameters are sensitive to the nature ofthe ‘‘heteroligand’’ bound to Zr (i.e. Cl, Br, etc.), and (iii) the 35Clquadrupolar and chemical shift tensor parameters are very sensi-tive to the mode of binding of the chlorine ligand (e.g. bridgingor terminal). More recently, 35Cl NMR and NQR experiments werealso applied to characterize chlorine ligands in a series of transitionmetal organometallic complexes [550b].
For some metal complexes, extensive molecular orbital calcula-tions may be needed to assist in the interpretation of the NMRspectra and to fully understand the electronic environment of themetal. This was the case in particular for the study of a family ofvanadium catechol complexes, because the catechol ligands (also
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called o-dioxolene) are redox-active. Schurko and co-workersmeasured the 51V NMR parameters of a series of vanadium-cate-chol complexes [551a], revealing that the chemical shift tensor ismuch more sensitive than the quadrupolar parameters to subtlechanges in the local electronic environment of the metal. Usingquantum chemical calculations, the influence of electron-donatingor electron-withdrawing substituents on the HOMO–LUMO gapwas determined, and the contributions of these orbitals to the vari-ations in 51V chemical shielding parameters were then analyzed.Another example concerns the study of three Pt-bisdithiolene com-plexes of general formula ½PtðtfdÞ2�
z�½NEt4�þz (with tfd = S2C2(CF3)2,and z = 0, 1 and 2) [551b]. The complexes corresponding to z = 0and 2 are diamagnetic, and they were characterized using multinu-clear solid state NMR (195Pt, 13C and 19F NMR). 195Pt CSA parame-ters were extracted from the simulation of 195Pt MAS NMRspectra, showing differences between both oxidation states inthese complexes. DFT calculations were then carried out, revealingthat the changes in the chemical shift tensors can be related tovariations in the populations and character of the valence molecu-lar orbitals, rather than to geometrical changes in the complexes.
3.4.2. Coordination polymersWhen multidentate ligands are involved, crystallization in the
solid state can lead to the formation of coordination polymers ofvarious dimensionalities (1D, 2D or 3D). The most popular exampleto date concerns the family of MOFs, which will be discussed inmore detail in the next section.
As for ‘‘isolated’’ metal complexes, coordination polymers haveattracted much attention in the field of hybrid materials, for severalreasons: (i) they can be an actual component of a hybrid material[552,553]; (ii) they can serve as structural models of certain compo-nents of hybrid materials, and their spectroscopic characterizationcan thus help understand the structure of complex hybrid phases[410,541]; (iii) they can form as by-products during the synthesisof hybrid materials [410,554]. Solid state NMR is a valuable toolfor characterization of coordination-polymer networks, and a fewrecent examples will be given below, for networks involving organ-ic ‘‘oxoanionic’’ ligands (e.g. phosphonates, carboxylates, sulfo-nates, and boronates), and then cyanide ligands. Although NMRcharacterizations of coordination networks involving other ligandssuch as azides and oximes have also been carried out [555], theseexamples will not be discussed in detail here.
3.4.2.1. Recent examples of phosphonate, carboxylate, sulfonate andboronate coordination polymer networks. Multidentate anionicligands such as phosphonates (R–PO2(OH)� or R—PO2�
3 ), carboxyl-ates (R–COO�), sulfonates (R—SO�3 Þ, and more recently boronatesR—BðOHÞ�3� �
are prone to the preparation of crystalline coordina-tion polymers, and many structures involving these ligands havebeen described.
Concerning metal phosphonate coordination polymers, 31P solidstate NMR has been used as a tool of investigation of (i) the numberof inequivalent phosphonates in the crystal structures [556–560];(ii) the degree of protonation of the phosphonate ligand (i.e. R–PO2(OH)� or R � PO2�
3 ) [541,560], (iii) the binding mode of thephosphonate ligand [561], (iv) the orientation of the chemicalshielding tensor [562] and (v) the purity of the material prepared.Recent examples concern the studies of Ln-phosphonate structuresby Rocha and co-workers [556,557,563], and of Ln-phosphonatoe-thanesulfonate structures by Stock and co-workers [564], in which31P MAS NMR was notably used to characterize some of thesephases in order to determine the number of non-equivalent P sitesin the material. In addition to 31P NMR, Zima and co-workers used1D as well as 2D (NOESY and BABA) 1H NMR experiments to char-acterize a Zr-sulfophenylphosphonate phase (Zr(HO3S–C6H4–PO3)2�2H2O) [565]. This found evidence of the presence of ‘‘mobile’’
protons within the structure, which complements the proton-con-ductivity measurements performed on these materials. Finally,analyses of the organic chain bound to the phosphonate (using13C NMR) [560,563,566], and of the local structure around the cat-ions in phosphonate networks (through 23Na, 43Ca, 113Cd and 87SrNMR experiments for example) [227,557,558,560] have beenreported.
In the case of metal carboxylates and sulfonates, apart from 13CNMR characterizations of the organic ligand [221,226,563,567],other NMR studies have been made. For example, in the case ofmetal organophosphine and metal organophosphonium frame-works, which were prepared in relation to gas storage applicationsby reacting the tris(para-carboxylated) triphenylphosphine ligand(or its methylated derivative) with Zn2+, 31P solid state NMR wasused to determine the oxidation state of the phosphorous in thematerials, and to check for the formation of phosphine-oxideby-products [568]. Furthermore, in the case of Ca-benzoate, itwas shown using 13C–43Ca correlation experiments that C� � �Ca dis-tances reaching 5.6 Å can be probed [221]. Finally, the structuralchanges of two lamellar Ag-sulfonate coordination networks([Ag(4-pyridinesulfonate)]4 and [Ag(p-toluenesulfonate)]) byreaction with primary amines were investigated using 109Ag andalso 15N NMR (in cases when 15N-enriched amines were used inthe reactions) [196], in order to establish whether the aminesintercalate between the layers of the lamellar phase, or lead tomore pronounced structural changes, by coordination to Ag+ ions.By analyzing the splitting due to 1J109Ag�14N and 1J109Ag�15N cou-plings in the starting materials and in the products, it was con-cluded that the final compound is not a simple intercalationcompound.
More recently, a series of investigations have been initiated onboronate coordination polymers. Three phenylboronate structureswere described (M(C6H5–B(OH)3)2, M = Ca, Ba; and Sr(C6H5–B(OH)3)2�H2O) [569], as well as one butylboronate phase (Ca(C4H9–B(OH)3)2) [570]. The Ca and Sr phenylboronate structures arecomposed of chains of metal ions interconnected through boronateligands; these chains then interacting with each other throughH-bonds to form a 2D layered structure. In the case of Ba-phenyl-boronate and Ca-butylboronate, the interconnection between thecations and boronate ligands directly leads to a 2-dimensionalorganization of the material. Multinuclear NMR characterizationswere performed on all these compounds. 1H, 13C and 11B NMRspectra were recorded for each sample, and additional 43Ca and87Sr experiments were carried out on the Ca and Sr boronates[227,569,570]. The NMR parameters extracted from these spectrawere compared to those calculated using the DFT GIPAW method,in order to help locate the H atoms in the crystal structure. This re-vealed the importance of (i) recording spectra of the metal ions(despite the fact that 43Ca and 87Sr NMR are very challenging),and (ii) performing 13C NMR measurements at different tempera-tures (in the case of alkylboronates in particular), in order todetermine the best structural model for the material.
3.4.2.2. Coordination polymers involving cyanide ligands. Cyanide li-gands (CN�) are commonly used for the preparation of coordina-tion polymers [571–573], one of the most well-known examplesbeing the ‘‘Prussian blue’’ structure, which was initially discoveredin 1704. Cyanides are very versatile ligands: they are capable ofbinding through the C or N atom to a wide variety of metals, andthey can be used as bridging ligands between different metalcations. The preparation of 1D, 2D and 3D metal cyanide-basedcoordination polymers is an active field of research, and many dif-ferent crystalline structures have been reported, with interestingmagnetic [574,575], optical [576,577], and thermal expansionproperties [578,579]. The ‘‘shaping’’ of these metal cyanide phaseshas been examined early on, notably through the preparation of
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nanoparticles [580,581] and thin films [582–584]. More recently,synthetic procedures have been developed to form hybrid materi-als associating metal cyanides to organic (e.g. polymers like chito-san and alginate) [552,585] or inorganic phases (e.g. silica, layereddouble hydroxide. . .) [586,587].
The physical properties of coordination networks involving cya-nide ligands often rely on the mode of coordination of the cyanideand on its local environment within the crystal structure. X-ray dif-fraction is well-suited to the study of crystalline materials, but dif-ficulties can arise in determining the exact binding mode ofcyanide ligands because of the high similarity in the scatteringproperties of C and N. Even more difficulties are encountered forpoorly crystalline and disordered systems. Solid state NMR studiesof coordination polymers involving cyanide ligands are still scarce(less than 20), despite the fact that the work carried out by Kroe-ker, Wasylishen and Lescouezec shows how 15N and 13C NMRcan provide clues on the local structure around cyanides (seebelow).
In addition to Prussian blue-type precursors like Cs2K[M(CN)6](M = Fe, Mn) [588], coordination networks involving other ligandsapart from the cyanides were also studied by 13C and 15N NMR, like[Hdabco][Au(CN)4] (dabco = 1,4-diazabicyclo[2.2.2]octane) [589],just to name a few. Some of these NMR studies have been carriedout at natural abundance [589–591], using MAS and CP MAS pulsesequences (Fig. 20). Other studies have been performed on materi-als containing 13C and/or 15N isotopically enriched cyanide ligands[588,592–594], allowing static spectra to be recorded [592], fromwhich 13C–15N dipolar couplings (and thus C–N bond distances)were determined. It should be noted that there are also a smallnumber of examples in which solid state NMR spectra of the metalions present in the coordination-polymer network (e.g. Pb2+, Co3+,Cd2+, etc.) [595–599] were recorded in order to gain information onthe structure/properties of the coordination polymer network. Forexample, it was found that 209Pb CSA NMR parameters can bedirectly correlated to the activity of the ‘‘lone pair’’ of the Pb2+,and related to optical properties of the coordination networks[595,598].
15N NMRA B
Fig. 20. (A) The 60.83 MHz 15N NMR spectra of the cyanide region for different diam[Au(CN)4], 6: trans-[CoCl2(en)2] [Au(CN)4], 8: [Co (NH3)6] [Au(CN)4]3.4H2O, 9: [Hddiazabicyclo[2.2.2]octane); (C) approximate 15N NMR chemical shift regions for cyanChemical Society).
By looking at the different 13C and 15N NMR studies carried outso far on crystalline coordination polymers involving cyanides, itappears that solid state NMR can be used to inform on (i) the num-ber of non-equivalent cyanide ligands in the crystal structure[590,592,600,601]; (ii) the nature and oxidation state of the metalion to which the cyanide ligand is coordinated (for examplewhether it is diamagnetic or paramagnetic) [591,592]; (iii) themode of binding to the metal ion (whether it is bound by C– orN–) [589,591]; (iv) the C–N bond distance (in the case of diamag-netic coordination polymer networks) [592]; (v) the ‘‘linearity’’ ofthe C–M–N bond (through the measurement of 1J couplings be-tween the metal and 13C or 15N) [591], and (vi) the electron spindensities on the C and N atoms [588]. Moreover, it was observedthat the presence of additional weak interactions within the crystalstructures can affect 13C and 15N NMR parameters, such as metall-ophilic interactions (e.g. Au� � �Au or Au� � �Tl interactions) [592], orinteractions of terminal cyanide ligands with a more distant metalion [591] or with neighbouring molecules or ions through H-bond-ing [589]. Based on all these considerations, structural modelswere proposed for materials for which structure determinationbased on X-ray diffraction was initially unavailable [591], or forwhich disorder was suspected [591,592]. In the latter case, a de-tailed analysis of the AuCN coordination polymer was carried out[592]. By combining 13C dipolar recoupling experiments and a de-tailed analysis of the 13C and 15N isotropic chemical shifts, thepresence of ‘‘chain slipping’’ defects between the different Au–C–N chains was observed, the extent of ‘‘slipping’’ depending on thesynthetic procedure used for the preparation of the coordinationpolymer [592].
3.4.3. Metal Organic Frameworks (MOFs)MOFs are a family of nanoporous O/I hybrid materials, which
has drawn increasing attention over the past few years[602–604], notably for applications in gas storage and separation,drug delivery, catalysis Although the structure determination ofMOFs still mainly relies on XRD, NMR has started to emerge as akey complementary tool for their characterization, as it can, for
C
agnetic [Au(CN)4]�-salts, 1: [nBu4N] [Au(CN)4], 2: [AsPh4] [Au(CN)4], 3: [N(PPh3)2]abco] [Au(CN)4]. (B) The asymmetric unit of [Hdabco][Au(CN)4] (dabco = 1,4-ide groups. Adapted from Ref. [589] with permission (Copyright 2011 American
C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48 31
example, provide information on the pore content, or on thedynamics of the organic ligands at different temperatures. Below,recent examples of applications of solid state NMR techniques tocharacterization of MOF frameworks are first presented, followedby applications of NMR to the analysis of the guest species trappedinside the pores and to the dynamical phenomena taking place inMOF structures. It should be noted that complementary referencesand examples can also be found in the recent reviews by Brunner[50] and Sutrisno and Huang [605].
3.4.3.1. Analysis of the framework structure. Both the ‘‘metal’’ and‘‘organic’’ components of MOFs have been analyzed by solid stateNMR, most characterizations having been done so far on diamag-netic frameworks.
Concerning the organic ligands, 13C (as well as 15N and 1H) NMRcharacterizations can be used to assist in the determination of thecrystal structure, by providing information on the number of non-equivalent ligands [606,607], and/or to validate the structuralmodel using a combined experimental-computational approach[220,608], as illustrated recently on a series of functionalizedUiO-66(Zr) MOFs. Valuable insight into the protonation state oforganic ligands [426], and in post-functionalization processes canalso be accessed [609]. For example, in an investigation by Stockand co-workers on the post-functionalization of an Al-amin-oterephthalate MOF using formic acid, 15N NMR provided directevidence of the success of the post-synthetic modification, and alsorevealed the presence of residual unreacted amine functions [609].The same group carried out a detailed study on the methylation ofAl-aminoterephthalate MOFs [610]. Using 2D 13C-MAS-J-HMQCand 1H–1H homonuclear correlation experiments, demonstrationof the functionalization by the methyl group was confirmed. Morerecently, it has been shown that (i) 17O solid state NMR can also bevery useful for characterizing the organic ligands in MOFs, as itmay help detect phase transitions [611a], and that (ii) the combi-nation of a selection of high resolution NMR experiments and com-putational modeling can help determine the mode of distributionof different (1,4-benzenedicarboxylate) linkers (bearing differentfunctional groups) within MOF structures (i.e. random, alternatedor clustered distribution) [611b].
The local environment around the metal ions present in theinorganic framework can also be studied. This is particularly truefor MOF structures based on Al building blocks, as 27Al is a veryNMR-sensitive isotope [606,612–614]. 27Al solid state NMR charac-terizations of MOF structures (especially those of the ‘‘MIL’’ family)were thus reported early on, using for example MAS and MQ-MASexperiments, in order to allow the number of inequivalent Al sitesto be determined. Gallium is also a sensitive isotope and 71Ga NMRexperiments have been reported for MOF structures [615]. A seriesof Sc-terephthalate MOF structures have been synthesized byWright and co-workers for CO2-uptake applications, and it wasshown that 45Sc MAS NMR can be used to provide informationabout the number of Sc sites and the local disorder around the cat-ions [218,607,616]. Finally, recent studies have demonstrated thatMOFs containing Mg, Zn and Ca can also be characterized using25Mg, 67Zn and 43Ca natural abundance MAS NMR experiments[211,215,216,617]. Indeed, by working at ultra-high magnetic field(21.1 T), 67Zn MAS NMR spectra of several MOFs (including fromthe ‘‘ZIF’’ family) were recorded, showing the great sensitivity ofthis technique to the Zn local environment. Another important fea-ture related to all the examples mentioned here is that the NMRspectra of MOF metal ions were systematically found to be sensi-tive to the pore content, as attested by the changes in the quadru-polar lineshapes observed when emptying the pores or filling themwith various guest molecules [211,215,216,218,609,618], or whenperforming post-functionalizations on the MOF framework [609].
To date, only a few NMR characterizations of paramagneticMOFs have been reported [56,70,72,427,619–622]. Studies of theframework structure have been performed using mainly 1H and13C NMR, and the most complete investigation to date is the workby Ashbrook and co-workers on the characterization of two Cu(II)-based MOFs, HKUST-1 and STAM-1 [619]. 13C spin-echo MAS NMRspectra were recorded mainly at high magnetic fields (14.1 and20.0 T) under fast-MAS conditions (�60 kHz spinning speed). Thewide range of 13C chemical shifts in these phases (>800 ppm) re-quired stepped frequency acquisition to be used to ensure properdetection of all resonances. Challenges related to the interpretationof 13C NMR spectra in paramagnetic systems were discussed. It wasshown that an unambiguous assignment of all the different 13C res-onances cannot be reached by analyzing only the peak positionsand linewidths, the T1 relaxation values and the CP build-up curvesof the C resonances. However, using organic ligands selectively la-beled at some 13C positions, it was possible to assign all 13C peaksof the two MOFs (for HKUST-1, this allowed previous erroneousassignments to be corrected).
3.4.3.2. Analysis of the guests. As porous structures, MOFs can incor-porate a variety of guests, including small molecules, ions, metalcomplexes and nanoparticles. Some of these entrapped specieshave been detected or purposely analyzed by NMR, as detailedbelow.
Many different species can remain trapped inside the pores ofMOFs during their synthesis. For example, as-synthesized MOFstructures very often contain residual solvent inside the pores,including organic molecules like DMF, which can easily be detectedby 13C NMR spectroscopy [607,609,619,623]. Water is also com-monly found inside the pores, and differences in the NMR spectraof hydrated and dehydrated MOFs have often been compared,either by looking at the signals of the ligands [607,620], or thoseof the metal ions forming the framework [215,218,614,618]. Itwas also observed that organic ligands can remain entrapped in-side the porous network during the synthesis. A detailed analysisof the protonation state and location of such ‘‘extra-framework’’ li-gands was performed for example in the case of the MIL-110 struc-ture using a combination of 1H DQ–SQ and 1H–27Al HETCORexperiments [614]. Salt-inclusion and anion templating effectswere also studied by NMR in the case of MOF architectures pre-pared by ion- and liquid-assisted grinding techniques [624]. Forexample, nitrate inclusion was demonstrated by preparing MOFsusing 15N-enriched KNO3 or NH4NO3. The 15N resonances in the fi-nal products were found to be different in comparison with thestarting salts, thereby confirming the inclusion of the ions in theporous network.
Once the MOF has been synthesized and activated (i.e. ‘‘de-sol-vated’’), many different small molecules can be incorporated insideits pores. In particular, for gas separation applications, experimen-tal methodologies allowing controlled investigations of gas-loadedMOFs have been developed [50]. The incorporation of CO2 insideMOF architectures has been studied by NMR [68,625]. Particularcare was taken in trying to establish the dynamics of the entrappedCO2 molecules in a Mg-dihydroxyterephthalate MOF as a functionof temperature, as mentioned in Section 2.1.2 [68]. In the case ofthe paramagnetic Cu(II)-MOF HKUST-1, the influence of the hydra-tion level on the long-term stability of the MOF structure was fol-lowed by 1H and 13C NMR over a 60-day period, and it wassuggested that decomposition occurs for higher water contents[70]. Concerning the entrapment of small organic solvent mole-cules, it was shown in the case of a Cu(II)-MOF structure thatNMR can be a particularly useful technique in helping to establishthe flexibility of MOFs upon adsorption–desorption of CH3OH andCH3CN [56]. Furthermore, in the case of Mg-MOFs, it was foundthat after exposure of ‘‘desolvated’’ MOFs to DMF, benzene, aceto-
Fig. 21. Static 25Mg NMR spectra at 55.1 MHz of CPO-27-Mg MOF as a function ofthe rehydration degree. ⁄ indicates a small amount of impurity likely due tomagnesium oxide. Reproduced from Ref. [215] with permission (Copyright 2013American Chemical Society).
32 C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
nitrile or acetone, changes are noticeable in the 25Mg NMR spectra,indicating that this technique is sensitive to the nature of the mol-ecules inside the pores (Fig. 21) [215,216]. Advanced 1H–14N,1H–13C and 1H–27Al 2D HETCOR experiments were more recentlycarried out to characterize the entrapment of small molecules likeacetone into functionalized Al-MIL-53 MOFs [612].
The incorporation of larger-size molecules inside the pores ofMOF frameworks has also been studied, and in this context, NMRcan be useful in proving the presence of the molecule in the poresand/or helping to understand the properties of the resulting mate-rial. For example, MOFs bearing macrocyclic polyether rings alongthe bridging units were synthesized, and their capacity to complexa paraquat dication through supramolecular interactions wasdemonstrated using 15N MAS NMR [626]. On a different front,the incorporation of enantiomerically pure (R)/(S)-1-phenyl-2,2,2-trifluoroethanol into a chiral MOF was studied using 13C solidstate NMR, and very slight shifts in 13C resonances of the chirallinkers were observed, because of diastereoisomers in the solidstate [627]. In the field of lithium-ion battery applications, theMIL-53(Fe) framework were loaded with 1,4-benzoquinone mole-cules, in order to enhance the electrochemical performance[622]. 13C MAS NMR experiments were carried out as a functionof the Li content, with the aim of helping to understand the differ-ent steps of the electrochemical process, by looking at the changesin the signals of the framework and of the quinone/quinolate spe-cies. 7Li MAS NMR studies were also carried out to complementthese studies, and confirmed the important role played by the qui-none molecules in the redox processes.
The formation of metal and metal-oxide nanoparticles (NPs) in-side the pores of MOFs has also drawn particular attention,
especially in the field of catalysis. Solid state NMR has been usedto characterize these materials [628–632]. One of the proceduresdeveloped in the case of metal NPs consists in incorporating anorganometallic precursor inside the MOF pores by sublimation,and then reducing it by hydrogenation to form the correspondingNPs. Before reduction, the 13C MAS NMR spectra demonstrate thepresence of the organometallic precursor inside the pores[628,632]. After complete hydrogenation, the precursor 13C signalsdisappear, while those coming from the MOF framework either re-main intact or can vary, as a proof of the reduction of the MOF li-gands under these conditions [628,629,632]. One of the mostadvanced NMR studies to date in this field is the study by Fischerand co-workers on Cu/ZnO NPs incorporated inside the MOF-5structure [631]. Using a gas phase impregnation procedure, Cuand Zn organometallic precursors were incorporated inside MOF-5 pores, and then decomposed into the corresponding NPs. In thecase of ZnO, extensive 17O solid state NMR studies were carriedout to ensure that the formation of ZnO NPs did not induce anydegradation of the MOF framework.
Finally, two last examples worth mentioning here concernMOFs which were prepared for hydrogen-storage applications.NMR spectroscopy was used here either to determine Li-environ-ments in Li-doped MOFs (using 7Li NMR experiments) [633], orto prove that NaAlH4 is not decomposed upon incorporation insidethe pores of the Ti-doped MOF-74(Mg) and to follow the behaviourof the material after desorption/re-adsorption of H2 (using 23Naand 27Al NMR experiments) [634].
3.4.3.3. Dynamics in MOF frameworks. Several different dynamicphenomena can take place within MOF structures, which either in-volve the ligands of the MOF framework, or the guest moleculestrapped inside the pores. Both types of investigations are of inter-est, for example to help understand the diffusivity of gases withinMOF structures and the flexibility of MOF structures. Movementsof guest molecules have been discussed elsewhere (see Sections2.1.3 and 2.1.4) [68,635], and the main focus here will be to lookinto the studies on the MOF framework dynamics.
The dynamics of terephthalate ligands in different MOF struc-tures have been investigated by NMR [218,616,636]. In all cases,per-deuterated terephthalate ligands were used in the analyses,and 2H wideline spectra were recorded at different temperatures,to gain insight into the dynamics and the barrier to rotation ofthe p-flips. In the case of a Sc-terephthalate MOF, unexpected 2Hlineshapes were observed above 353 K, and it was proposed thatthe two crystallographically nonequivalent terephthalate ligandsin the structure may exhibit a different motional behaviour[616]. It should be noted that in addition to NMR, other techniqueshave also been tested to probe the dynamics of such ligands, suchas dielectric relaxation spectroscopy [220], and neutron scattering[636a,637]. Phenyl dynamics were also probed by Griffin and co-workers in the case of a tetraphenylene MOF structure whichwas synthesized purposely to understand the origin of aggrega-tion-induced emission phenomena [638]. 2H NMR spectra were re-corded between �300 and 420 K, and the phenyl flipping energybarrier was estimated to be �43 kJ/mol. This value is �twice largerthan the one expected in a ‘‘free’’ ligand in the gas phase (accordingto DFT), and it was suggested that this is the reason for the partic-ular fluorescence properties of the material. The origin of this high-er activation barrier was also analyzed in detail by DFT, in order tobe able to propose means of tuning this activation energy in futuresyntheses.
A novel type of MOF has recently been reported, which has beenpurposely designed to exhibit dynamic properties. Indeed, Loeb,Schurko, and co-workers demonstrated that it is possible tosynthesize MOFs bearing rotaxane-type linkers [639]. It was shownthat the MOF needs to be activated (i.e. ‘‘dehydrated’’), so that the
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crown-ether unit can start moving. 13C and 2H solid state NMRexperiments were carried out at various temperatures, and fourdistinct motional regimes were revealed by 2H NMR, with the com-plete rotation of the crown-ether within the MOF occurring onlyabove 373 K.
3.4.4. Functionalized metal nanoparticlesMetal NPs are of great interest for a variety of applications,
including catalysis [640,641] and medicine [642,643]. Compositematerials involving metal NPs is another active field of research.For example, the formation of well-dispersed metal NPs at the sur-face of mesoporous silica matrices is a common approach in heter-ogeneous catalysis, while the dispersion of metal NPs at the surfaceof nanotubes or inside p-conjugated polymer matrices is of interestin electronics, and notably for the elaboration of more efficientdye-sensitized solar cells.
Metal NPs often can be considered as hybrid nano-objects inthemselves, because of the presence of organic ligands at their sur-face. Given the importance of surface stabilization/reactivity whenit comes to studying such nano-objects, many different character-ization techniques including NMR have been proposed to study thesurface functionalities. Solution NMR has traditionally been usedto characterize different organic ligands at the surface of NPs[119,644,645], for example by performing 2D NMR experimentsto gain insight into the ligand arrangement [646]. However, insome cases, signals corresponding to the atoms closest to the metalNP surface are not visible in solution, due to significant linebroa-dening. Recent studies have demonstrated the advantage of apply-ing solid state NMR techniques to the characterization of surfacefunctions in Ru [647–649], Au [273,650–658], Pd [659] and Ru/Pt[660] NPs, in order to understand their surface structure and/orreactivity.
Early on, Chaudret, Limbach and co-workers advanced theinvestigation of functionalized metal nanoparticles using solidstate NMR. Most of the initial studies were performed with theprospect of establishing the surface functionalities of rutheniumNPs, because of their interest in hydrogenation catalysis. A seriesof model Ru-complexes and Ru-clusters of known structure wereinvestigated both experimentally [661,662] and computationally[661,663,664], and a recent review brings together the differentfindings [665]. The purpose of these studies was to establish thebackground necessary for the interpretation of the spectra ofgrafted Ru-NPs, such as the link between 2H quadrupolar parame-ters CQ and gQ and the mode of binding of the deuterium to the NPsurface (i.e. Ru–D, Ru� � �D� � �Ru or Ru� � �D2, for example). Such mod-eling studies also revealed that discrepancies between experimen-tal and calculated values can be observed caused by rotations/vibrations within the experimental complexes, which are difficultto take into account in the calculations [664]. The surface structureand reactivity of Ru-NPs bearing different functionalities at theirsurface (phosphines, amines. . .) have been compared [649]. In-deed, after reaction with isotopically-enriched ethylene, the2Hand 13C solid state NMR spectra of the nanoparticles were re-corded, revealing the formation of Ru–CH3 surface groups and thusthe breaking of C@C bonds. Such observations are crucial in help-ing to understand hydrogenation catalytic processes occurring atthe surface of Ru-NPs. More recently, 13C solid state NMR was usedto characterize Ru-NPs functionalized by N-heterocyclic carbenes(NHCs) [648]. In this case, 13C MAS and 1H–13C CP MAS experi-ments were carried out to probe the NHC environment at the nano-particle surface, the NHC ligands having been purposely enrichedin 13C at the carbene position. The remaining free surface siteswere then analyzed by adding 13C-labeled CO: the additional sig-nals observed by 13C NMR were ascribed to bridging and terminalCO groups. From these NMR analyses, models describing the modeof binding of the NHC ligands at the NP surfaces were then
proposed. A similar methodology involving 13CO as a surface probemolecule was later used for the characterization of other Ru-NPsand of bimetallic core–shell Ru–Pt NPs [660,666].
In the case of Au NPs, surface functionalization by thiols[657,658] (notably cysteine-or thiol terminated amino acids andpeptides) [273,650,656], phosphines [652,654], phosphinines[651], dimethylaminopyridine/polystyrenesulfonate [655], andalso peptide-biphenyl hybrid ligands [653] has been studied by so-lid state NMR. Reven and co-workers reported a series of NMRstudies of solid Au-NPs capped by alkane-thiol derivatives, inwhich the length of the organic carbon chain and the nature ofthe end group (–CH3, –SO3H, –PO3H2) was varied [657,658]. Abroadening and shift of the 13C signals closest to the NP surfacewas observed, and the arrangement and mobility of the grafted or-ganic chains was discussed, notably on the basis of VT NMR exper-iments. Three different aspects of the grafting oftriphenylphosphine at the surface of �1.8 nm Au-NPs were lookedinto by Sharma et al. [652,654]: (i) the local environment of thePPh3 ligand at the NP surface, (ii) the ligand dynamics once graftedat the surface of the NP, and (iii) the ligand-exchange reactionsoccurring at the NP surface. Concerning the first point, 31P solidstate NMR experiments were carried out, and the signal linewidthwas interpreted based on 31P hole-burning experiments, showingthat it is mainly caused by heterogeneities in the local environ-ment of the ligand at the NP surface. Using perdeuterated ligands,it was then shown by 2H NMR spectroscopy that when grafted atthe surface of Au-NP, these PPh3 ligands undergo a fast rotationof the aromatic rings around the P–C bonds. It was concluded thatsuch a rapid rotation may imply the presence of low-coordinationAu sites at the nanoparticle surface, a potentially important obser-vation in view of catalytic applications. Another recent exampleworth mentioning here is the combined NMR/IR study of the sec-ondary structure of two ‘‘small’’ cysteine-terminated peptides(CALNN and CFGAILSS) at the surface of Au-NPs of different diam-eter [650]. The influence of the Au-NP diameter on the conforma-tion of grafted b-sheets of CFGAILSS was discussed, and byperforming 13C solid state NMR rotational resonance experimentson selectively 13C-enriched CFGAILSS fragments, the distance be-tween specific C atoms belonging to two different peptide chainswas estimated.
It should be noted that most NMR characterizations have fo-cused so far on the characterization of the surface-grafted speciesrather than the metallic core. One reason to this is that the coreis most often composed of metals like Ru, Au, Ag, and Pd, whichdo not have any ‘‘NMR-friendly’’ isotope suitable for the study oflimited quantities of sample (i.e. similar to those produced whenpreparing metal NPs). One of the most striking examples is gold,as the resonance frequency of 197Au (I = 3/2, 100% natural abun-dance) is only �10.6 MHz on a 14.1 T magnet, making it one ofthe ‘‘lowest-gamma’’ isotopes of the whole periodic table. Anexception worth mentioning is the case of Pt-NPs, as it has beenshown using 195Pt NMR that it is possible to differentiate Pt atomsbelonging to the core or surface of the NPs [330].
4. Summary and outlook
This review article has presented the most recent advances insolid state NMR applied to the description of hybrid materials,showing how instrumental and methodological advances in NMRhave opened new avenues for detailed spectroscopic characteriza-tion of these materials. In particular, HP 129Xe NMR, MAS PFG,DOSY, MACS, PHIP, and DNP approaches give clearly new insightsinto the structure, texture and dynamics of O/I materials at differ-ent length scales. The most important classes of hybrids have beenreviewed including silica-based materials, ionic solid derived
34 C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
architectures, biomaterials, metal complexes, coordination net-works (including MOFs), and functionalized metal nanoparticles.Specific NMR studies have been described in order to highlightthe structure/properties of these families of compounds.
From the chemical point of view, the synthesis of novel hybridderivatives combining new O/I components for tailored applica-tions is limited only by the imagination of chemists. We anticipatethat future advances in the field of solid state NMR will be of par-amount importance for a better understanding of complex nano-composites. In particular, methods for improving NMR sensitivitywill be of the highest interest for materials scientists.
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List of abbreviations and acronyms
1D: one-dimensional2D: two-dimensional2,6-ndc: 2,6-naphthalenedicarboxylateACC: amorphous calcium carbonateAlPO: aluminophosphate porous materialAP-HH CP: adiabatic passage Hartmann–Hahn cross polarizationAPTES: aminopropyltriethoxysilaneASR: absolute sensitivity ratioBABA: back-to-backbCTbK: bis-cyclohexyl-TEMPO-bisketalBOMD: Born–Oppenheimer molecular dynamicsbTbK: bis-TEMPO-bisketalBTC: benzene 1,3,5-tricarboxylateCALNN: cysteine terminated peptideCE: cross effectCFGAILSS: cysteine terminated peptideCP: cross-polarisationCP-MAS: cross-polarisation magic angle spinningCPMG: Carr-Purcell-Meiboom-GillCPO: coordination polymer of OsloCSA: chemical shift anisotropyCT: contact timeCTA: cetyltrimethylammoniumCTAB: cetyltrimethylammonium bromided: chemical shiftD: dipolar interactionDAB: 1,4-diazabutadienedabco: 1,4-diazabicyclo[2.2.2]octaneDFPT: density functional perturbation theoryDFS: double frequency sweepsDFT: density functional theoryDFT-D: density functional theory augmented with an empirical dispersion termDNP: dynamic nuclear polarizationDOSY: diffusion ordered spectroscopyDPPC: dipalmitoylphosphatidylcholineDQ: double quantumDQ–SQ: double quantum–single quantumDRAWS: dipolar recovery with a windowless sequenceEDTA: ethylenediaminetetraacetic acidEFG: electric field gradientEPR: electronic paramagnetic resonanceEXAFS: extended X-ray absorption fine structureEXSY: exchange spectroscopyFSG: frequency switch GaussianFSLG: frequency switched Lee-GoldburgGIAO: gauge including atomic orbitalsGIPAW: gauge-including projector augmented waveGSH: glutathioneH4dobdc: 2,5-dihydroxyterephthalic acidHA: hydroxyapatiteHAmTAZ: 3-amino-1,2,4-triazoleHARDSHIP: heteronuclear recoupling with dephasing by strong homonuclear
interactions of protonsHETCOR: heteronuclear correlationHKUST: Hong Kong university of science and technologyHMBC: heteronuclear multiple bond correlationHMQC: heteronuclear multiple quantum correlationHOMCOR: homonuclear correlationHOMO: highest occupied molecular orbitalHP: hyper-polarizedHRMAS: high resolution magic angle spinning
48 C. Bonhomme et al. / Progress in Nuclear Magnetic Resonance Spectroscopy 77 (2014) 1–48
IMC: indomethacinINADEQUATE: incredible natural abundance double quantum transfer experimentINEPT: insensitive nuclei enhanced by polarization transferIPC: internal post curingIRCP: inversion-recovery cross polarizationIRMOF: isoreticular metal organic frameworkIST: microporous aluminophosphateJ: indirect (or scalar) couplingLDH: layered double hydroxideL-Dopa: L-3,4-dihydroxyphenylalanineLG: Lee-GoldburgLUMO: lowest unoccupied molecular orbitalMACS: magic angle coil spinningMAS: magic angle spinningMAS PFG: magic angle spinning pulsed field gradientMCF: mesocellular silica foamMCM: Mobil Composition of MatterMD: molecular dynamicsMFI: mordenite framework invertedMIL: matériau institut LavoisierMOF: metal organic frameworkMQ: multiple quantumMQ-MAS: multiple quantum magic angle spinningNBB: nano-building blockNMR: nuclear magnetic resonanceNDC: naphthalenedicarboxylateNHC: N-heterocyclic carbineNOE: nuclear Overhauser effectNOESY: nuclear Overhauser effect spectroscopyNP: nanoparticleNQR: nuclear quadrupole resonanceO/I: organic/inorganicONIOM: Our own N-layered Integrated molecular Orbital and molecular MechanicsPBS: phosphate buffer salinepd2�: 1,3-propanediolatePEMFC: proton exchange membrane fuel cellPEO: polyethylene oxidePFG: pulsed field gradientPCH: porous clay heterostructuresPHIP: para-hydrogen induced polarizationPISEMA: polarization inversion spin exchange at the magic anglePMO: periodic mesoporous organosilicaPOSS: polyhedral silsesquioxanePPh3: triphenylphosphinepyz: pyrazineQ: quadrupolar interaction
Qn: Si(OSi)n(OX)4�n unitQCP: quantitative cross polarizationQCPMG: quadrupolar Carr-Purcell-Meiboom-GillQCPMG-MAS: combination of QCPMG and MAS experimentsQM/MM: quantum mechanics/molecular mechanicsRAPT: rotor-assisted population transferREDOR: rotational echo double resonanceRFDR: radiofrequency driven dipolar recouplingRHF: restricted Hartree FockRNA: ribonucleic acidRT: room temperatureSBA: Santa BarbaraSDS: sodium dodecyl sulfateSDV: silica deposition vesicleSVD: singular value decompositionSENS: surface enhanced NMR spectroscopySIT: silicon transporterSMARTER: structural elucidation by nuclear magnetic resonance computational
modeling and X-ray diffractionS/N: signal-to-noiseSPINOE: spin polarization-induced nuclear Overhauser effectSQ: single quantumSSNMR: solid state nuclear magnetic resonanceSTA: St Andrews microporous solidSTAM: St Andrews MOFT 02: transverse dephasing time (obtained via an echo)TEM: transmission electron microscopytfd: S2C2(CF3)2
Tn: RSi(OSi)n(OX)(3�n) unittCP: contact time (in CP experiment)TEMPO: (2,2,6,6-tetramethylpiperidin-1-yl)oxidanylterpy: terpyridineTOTAPOL: 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-olTPA: tetrapropylammoniumTPP: tris(o-phenyldioxy)cyclophosphazeneTRAPDOR: transfer of population double resonanceTQ: triple quantumUiO: university of OsloVT: variable temperatureWURST: wideband uniform rate and smooth truncationXANES: X-ray absorption near edge structureXAS: X-ray absorption spectroscopyXRD: X-ray diffractionZIF: zeolitic imidazolate frameworkZORA: zeroth-order regular approximationZSM: Zeolite Socony Mobil
