DOI: 10.1002/cphc.201200840 Interactions of Aromatic Radicals with Water Rachel Crespo-Otero, [a] Kenny Bravo-Rodriguez, [a] Saonli Roy, [b] Tobias Benighaus, [a] Walter Thiel, [a] Wolfram Sander, [b] and Elsa SƁnchez-Garcȷa* [a] 1. Introduction While the formation of weakly bound complexes and aggre- gates of closed-shell molecules has been intensely studied during the last decades, non-covalent complexes involving rad- icals are less well known, mainly because the theoretical treat- ment of non-covalent complexes involving open-shell radical species is challenging, and experimental studies are hampered by the high reactivity of radicals that requires the use of low- temperature or time-resolved techniques. Given the impor- tance of radicals and radical reactions in such diverse aqueous environments as biological systems or tropospheric clouds, it is however highly desirable to understand the influence of spe- cific water interactions on the chemical and physical properties of radicals. Therefore, we address the interactions between water and three prototypical aromatic radicals in this article. Phenol–water complexes have been investigated in a number of theoretical studies. [1] Hobza et al. used ab initio methods to calculate several hydrogen-bonded structures and one hemibonded structure of the phenol–water cation radical. They found that the most stable structure has C s symmetry and features a linear hydrogen bond between the proton of the OH group of the phenol cation radical and the oxygen atom of water, with the water hydrogen atoms pointing away from the phenyl ring. [2] Experiments performed by Gor et al. [3] on phenol–water complexes could only identify the complex in which phenol acts as a proton donor, in agreement with previous theoretical results. In a matrix isolation and computational study of the phenox- yl radical complexes with water, Sander et al. [4] characterized a complex where water acts as hydrogen-bond donor. They found evidence for the formation of a stable complex with the p system of the phenoxyl radical. The aniline–water and toluene–water complexes have re- ceived less attention. [5] Spoerel and Stahl studied the aniline– water complex using ab initio computations and a pulsed mo- lecular beam FT microwave spectrometer. [5a] The aniline–water complexes were further investigated using REMPI spectroscopy as well as MP2 and CASSCF calculations on the ground and S 1 excited states. [5b] The effects of meta and para substituents at the aniline ring on the interaction between the p system and water were also addressed. [6] Toluene–water complexes were studied with up to four water molecules. [5c–e] Here we extend our previous theoretical work on the inter- actions of water with the methyl (CH 3 ), aminyl (NH 2 ), and hydroxyl (OH) radical [7] to the benzyl radical 1, anilinyl radical 2, and phenoxyl radical 3 (Scheme 1). The com- plexes of these radicals with water are compared to those of the cor- responding closed-shell molecules toluene 4, aniline 5, and phenol 6. 2. Results and Discussion Matrix Isolation of the Benzyl Radical–Water Complex The benzyl radical–water complex was generated by co-depo- sition of the benzyl radical 1 and a large excess of argon doped with 0.1 %–1 % H 2 O at 10 K and subsequent annealing of these matrices at temperatures up to 35 K. If the water con- The interactions of the benzyl radical (1), the anilinyl radical (2), and the phenoxyl radical (3) with water are investigated using density functional theory (DFT). In addition, we report dispersion-corrected DFT-D molecular dynamics simulations on these three systems and a matrix isolation study on 1–water. The radicals 1–3 form an interesting series with the number of lone pairs increasing from none to two. The anilinyl and benzyl radicals can act as Lewis base through their unpaired electrons, the lone pairs of the heteroatoms, or the doubly occupied p orbitals of the aromatic system. Matrix isolation experiments provide evidence for the formation of a p complex between 1 and water. By combining computational and experimental techniques we identify the possible interactions between the aromatic radicals 1–3 and water, predict the structure and vi- brational spectra of the resulting complexes, and analyze the effects of substitution and temperature. Scheme 1. Benzyl radical (1), anilinyl radical (2), and phe- noxyl radical (3). [a] Dr. R. Crespo-Otero, K. Bravo-Rodriguez, Dr. T. Benighaus, Prof. Dr. W. Thiel, Dr. E. SƁnchez-Garcȷa Department of Theory, Max-Planck-Institut fɒr Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mɒlheim an der Ruhr (Germany) Fax: (+ 49) (0)208/306-2980 E-mail : [email protected][b] S. Roy, Prof. Dr. W. Sander Lehrstuhl fɒr Organische Chemie II, Ruhr UniversitȨt Bochum UniversitȨtsstraße 150 44801 Bochum (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201200840. Special Section: Aggregation DFG 618 # 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2013, 14, 805 – 811 805 CHEMPHYSCHEM ARTICLES
Transcript
DOI: 10.1002/cphc.201200840
Interactions of Aromatic Radicals with WaterRachel Crespo-Otero,[a] Kenny Bravo-Rodriguez,[a] Saonli Roy,[b] Tobias Benighaus,[a]
Walter Thiel,[a] Wolfram Sander,[b] and Elsa S�nchez-Garc�a*[a]
1. Introduction
While the formation of weakly bound complexes and aggre-gates of closed-shell molecules has been intensely studiedduring the last decades, non-covalent complexes involving rad-icals are less well known, mainly because the theoretical treat-ment of non-covalent complexes involving open-shell radicalspecies is challenging, and experimental studies are hamperedby the high reactivity of radicals that requires the use of low-temperature or time-resolved techniques. Given the impor-tance of radicals and radical reactions in such diverse aqueousenvironments as biological systems or tropospheric clouds, it ishowever highly desirable to understand the influence of spe-cific water interactions on the chemical and physical propertiesof radicals. Therefore, we address the interactions betweenwater and three prototypical aromatic radicals in this article.
Phenol–water complexes have been investigated ina number of theoretical studies.[1] Hobza et al. used ab initiomethods to calculate several hydrogen-bonded structures andone hemibonded structure of the phenol–water cation radical.They found that the most stable structure has Cs symmetryand features a linear hydrogen bond between the proton ofthe OH group of the phenol cation radical and the oxygenatom of water, with the water hydrogen atoms pointing awayfrom the phenyl ring.[2] Experiments performed by Gor et al.[3]
on phenol–water complexes could only identify the complex
in which phenol acts as a proton donor, in agreement withprevious theoretical results.
In a matrix isolation and computational study of the phenox-yl radical complexes with water, Sander et al.[4] characterizeda complex where water acts as hydrogen-bond donor. Theyfound evidence for the formation of a stable complex with thep system of the phenoxyl radical.
The aniline–water and toluene–water complexes have re-ceived less attention.[5] Spoerel and Stahl studied the aniline–water complex using ab initio computations and a pulsed mo-lecular beam FT microwave spectrometer.[5a] The aniline–watercomplexes were further investigated using REMPI spectroscopyas well as MP2 and CASSCF calculations on the ground and S1
excited states.[5b] The effects of meta and para substituents atthe aniline ring on the interaction between the p system andwater were also addressed.[6] Toluene–water complexes werestudied with up to four water molecules.[5c–e]
Here we extend our previous theoretical work on the inter-actions of water with the methyl(CH3), aminyl (NH2), and hydroxyl(OH) radical[7] to the benzyl radical1, anilinyl radical 2, and phenoxylradical 3 (Scheme 1). The com-plexes of these radicals with waterare compared to those of the cor-responding closed-shell moleculestoluene 4, aniline 5, and phenol 6.
2. Results and Discussion
Matrix Isolation of the Benzyl Radical–Water Complex
The benzyl radical–water complex was generated by co-depo-sition of the benzyl radical 1 and a large excess of argondoped with 0.1 %–1 % H2O at 10 K and subsequent annealingof these matrices at temperatures up to 35 K. If the water con-
The interactions of the benzyl radical (1), the anilinyl radical(2), and the phenoxyl radical (3) with water are investigatedusing density functional theory (DFT). In addition, we reportdispersion-corrected DFT-D molecular dynamics simulations onthese three systems and a matrix isolation study on 1–water.The radicals 1–3 form an interesting series with the number oflone pairs increasing from none to two. The anilinyl and benzylradicals can act as Lewis base through their unpaired electrons,
the lone pairs of the heteroatoms, or the doubly occupied p
orbitals of the aromatic system. Matrix isolation experimentsprovide evidence for the formation of a p complex between1 and water. By combining computational and experimentaltechniques we identify the possible interactions between thearomatic radicals 1–3 and water, predict the structure and vi-brational spectra of the resulting complexes, and analyze theeffects of substitution and temperature.
Scheme 1. Benzyl radical (1),anilinyl radical (2), and phe-noxyl radical (3).[a] Dr. R. Crespo-Otero, K. Bravo-Rodriguez, Dr. T. Benighaus, Prof. Dr. W. Thiel,
Dr. E. S�nchez-Garc�aDepartment of Theory, Max-Planck-Institut f�r KohlenforschungKaiser-Wilhelm-Platz 1, 45470 M�lheim an der Ruhr (Germany)Fax: (+ 49) (0)208/306-2980E-mail : [email protected]
[b] S. Roy, Prof. Dr. W. SanderLehrstuhl f�r Organische Chemie II, Ruhr Universit�t BochumUniversit�tsstraße 150 44801 Bochum (Germany)
Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cphc.201200840.
centration in argon was kept as low as 0.1 %, the deposition ofmatrices also containing 1 did not result in new IR absorptionsthat could be attributed to intermolecular complexes. Howev-er, annealing these matrices at temperatures up to 35 K forseveral minutes resulted in the appearance of new absorptionsat 885.8, 765.3, and 671.4 cm�1 (Figure 1, Table 1). If the argonis doped with 1 % water, these new bands are already presentafter deposition of the matrix at 10 K. This indicates the forma-
tion of a bimolecular complex between 1 and water. In a similarway, a complex between 1 and D2O was observed if D2O wasused in the experiments. The IR absorptions of the 1–watercomplex were assigned by comparison with results from DFTcalculations (vide infra).
Structures and Energetics of Benzyl Radical–WaterComplexes
The benzyl radical 1 (Figure 2) lacks lone pairs and acidic hy-drogen atoms that could act as strong hydrogen-bond accept-ors and donors, respectively. Therefore, only weak intermolecu-lar complexes with water are expected. The only minima thatwere found both at the UM05-2X and the UB3LYP-D level arethe OH···p complexes C6 and C7 with one of the water hydro-gen atoms interacting with the p system of the benzyl radical(unless otherwise specified, we refer in the following to theUM05-2X results and give the corresponding UB3LYP-D valuesin parenthesis). Analogous p complexes were not found in thecase of the radicals 2 and 3. The basis set superposition error(BSSE)-corrected DEBSSE energies of C6 and C7 are very similar,�3.57 (�3.85) and �3.47 (�3.80) kcal mol�1, respectively(Table 2). In C6, one OH group of water is placed at a distanceof 2.93 (2.74) � from the substituted carbon atom of thebenzyl radical, while the other water hydrogen atom is inter-acting with the aromatic p system at a distance of 2.49(2.45) � and an OH–p angle of 1378 (1328).
The H···p distances and bond angles are defined with re-spect to a dummy atom located at the center of the aromaticring (defined by the average of the atomic coordinates of thering atoms).
Figure 1. IR spectra showing the formation of dimers between 1 and water.a) IR spectrum obtained after trapping the FVP products of 7 in argon at10 K. b) IR spectrum obtained after trapping the FVP products of 7 in 1 %water doped argon at 10 K. c) IR spectrum of matrix (b) after annealing at35 K and subsequently cooling down to 10 K. d) IR spectrum obtained aftertrapping the FVP products of 7 in 1 % D2O-doped argon at 10 K. e) IR spec-trum of matrix (d) after annealing at 35 K and cooling down to 10 K. Peakdenoted with a star belongs to precursor 7 (see Materials in the Experimen-tal Section).
Table 1. Calculated [UM052X/6-311 + + G(2d,2p) and UB3LYP-D/6-311 + + G(2d,2p)] and experimental vibrational frequencies [cm�1] and relative intensities of thebenzyl radical 1 and the benzyl radical–water dimer. Values for the corresponding experimental and theoretical Dn are shown in parenthesis.
In structure C7 the water molecule also interacts with the ar-omatic p system, but in a different manner. The minimum ge-ometry of this dimer depends on the chosen density function-al. At the UB3LYP-D level, C7 has an almost symmetrical struc-ture in which the hydrogen atoms of the water molecule areboth positioned at 2.59 � and 1398 or 1378 relative to thecenter of the bond between the ortho and meta carbon atomsof the benzyl radical. In the UM05-2X minimum, the water mol-ecule is rotated, and there is only one O�H···p interaction(2.51 �, 1608, Figure 2).
Additionally, we considered other geometrical rearrange-ments for the complex 1–water (see the Supporting Informa-tion). At the UB3LYP-D level, the complexes C1 and C2 corre-spond to transition states with imaginary vibrations of 10 and17 cm�1 (Figure 1S and Table 1S in the Supporting Informa-tion). Using the M05-2X functional, C1 is found to be a mini-mum, while the optimization of C2 converges to the C6 geom-etry. We do not discuss these weak complexes further, becausetheir binding energies are smaller than those of C6 and C7.
The IR spectra calculated for the dimers C6 and C7 are verysimilar (Tables 1 and 2S), but the calculated shifts for complexC6 are in better agreement with the experimental shifts thanthose calculated for C7 (Table 1).
Structures and Energetics of Anilinyl Radical–WaterComplexes
For the complexes between the anilinyl radical 2 and water,three main interaction motifs N1, N2, and N5 can be identified(Figure 2). These structures are found both with the M05-2Xand B3LYP-D density functionals. In the most stable complexN1, with a BSSE-corrected DEBSSE binding energy of �5.77(�8.54) kcal mol�1 (Table 2), the radical acts as hydrogen-bondacceptor, and water as hydrogen-bond donor. One OH groupof the water molecule is located in the symmetry plane of 2 toform a hydrogen bond with the lone pair at the nitrogenatom. The non-bonding NH distance is 1.97 (1.90) �, and theOHN angle is 1638 (1658). In addition, there is a secondary in-teraction between the O atom of water and one ortho hydro-gen atom of the radical at 2.45 (2.43) �.
In N2, the NH hydrogen atom of the radical serves as hydro-gen-bond donor and forms a NHrad···Ow hydrogen bond with
the oxygen atom of water. Thenon-bonding OH distance is 2.26(2.27) �, and the NHradOw angleis 1738 (1718). Similar to N1, anadditional CHrad···Ow interactionat 2.61 (2.57) � contributes tothe stabilization of the complex.The smaller interaction energy of�2.44 (�3.54) kcal mol�1 and thelarger hydrogen-bond distances(compared to N1) indicate thatthe anilinyl radical 2 formsa stronger complex with waterwhen acting as hydrogen-bondacceptor.
In N5, with an interaction energy of �1.40 (�2.26) kcalmol�1, the heteroatom of the radical is not directly involved inthe complexation. Instead, the oxygen atom of water formsa CHrad···Ow hydrogen bond with the meta and para hydrogenatoms of 2 at 2.61 (2.57) and 2.66 (2.61) �, respectively.
Structures and Energetics of Phenoxyl Radical–WaterComplexes
The phenoxyl radical 3 interacts with water in a similar way asthe anilinyl radical 2. The most stable complex O1 [�6.06(�7.92) kcal mol�1][8] is stabilized by the Orad···OHw interaction at1.92 (1.88) � between the hydrogen atom of water and theradical heteroatom. The OradOHw angle is 1608 (1648). In addi-tion, one ortho hydrogen atom of radical 3 interacts with thewater oxygen atom at 2.37 (2.38) �. The phenoxyl oxygenatom is not involved in the weak complex O3 [�2.84(�2.77) kcal mol�1] which has a geometry reminiscent of N5with CHrad···Ow distances of 2.65 (2.60) and 2.57 (2.55) �
Figure 2. Radical–water complexes. Distances for relevant interactions fromM05-2X (a) and B3LYP-D (b) are given in Angstrom [�]. See text for discus-sion
Table 2. Interaction energies (DE) of radical–water complexes that are minima at both levels. Energies correct-ed for basis set superpositon error (DEBSSE) and including zero-point vibrational energies (DEBSSE + ZPE). All valuesin kcal mol�1. See Figure 2 for the structure of the complexes.
M05-2X/6-311 + + G(2d,2p) B3LYP-D/6-311 + + G(2d,2p)Complex DE DEBSSE DEBSSE + ZPE DE DEBSSE DEBSSE + ZPE
(Figure 2, Table 2). There is another complex O4 (Figure 1S)that corresponds to a transition state (imaginary vibration of32 cm�1) at both levels of theory.
Comparison with Closed-Shell Systems
In the toluene–water complex reported in the literature (MP2/aug-cc-pVDZ) the water molecule interacts with the p systemof toluene.[5c,d] Its geometry is similar to that of the benzyl radi-cal–water complex C6. However, the water molecule in C6adopts a different orientation, with one water hydrogen atomlocated nearly in the C2v plane of the benzyl radical (Figures 2and 2S).
In their study of the aniline–water system, Spoerel and Stahlassumed that the hydrogen Naniline···HOw bond is linear and thatthe water molecule is located in the plane of the aniline mole-cule.[5a] The structure in which the free water hydrogen atom isdirected toward the aniline ring was supported by experimen-tal data. The structure of this anti conformer was reexaminedtogether with the syn conformer using REMPI spectroscopy aswell as MP2 and CASSCF calculations of the S1 excited state ofaniline complexes.[5b] It was found that the anti conformer hasa non-linear Naniline···HOw hydrogen bond between the aminonitrogen atom and the hydrogen atom of water. By contrast, inthe syn conformer, the water molecule is located above the ar-omatic ring, and there is no hydrogen bond between the ani-line nitrogen atom and a hydrogen atom of water (Figure 2S).Other local minima were identified, in which the water mole-cule is located outside the plane of the aromatic ring and di-rectly interacts with the amino group, but these were not dis-cussed further.[5b] As expected, our most stable anilinyl radical–water dimer showed the water molecule forming anNaniline···HOw hydrogen bond in the molecular plane of the radi-cal.
The most stable phenol–water complex studied by Guedeset al.[1a] has an in-plane interaction between the phenolic hy-drogen atom and the oxygen atom of water. In the samestudy, a phenoxyl radical–water complex was presented witha structure very similar to our calculated complex O1(Orad···HOw distance of 1.909 � at the B3PW91/D95V(d,p) levelof theory). In a later comprehensive study of phenol–water in-teractions, Bandyopadhyay et al.[1b] calculated OHO and p···HOcomplexes between phenol and water, and found that theOHphenol···OHw complex is the most stable (Figure 2S).
For further analysis, we computed the closed-shell com-plexes between toluene 4, aniline 5, phenol 6 and water at theM05-2X/6-311 + + G(2d,2p) level (Figure 3, Table 3) to allow fordirect comparisons with the corresponding radical–water com-plexes. In all three cases, the OHw···p complex was found asa minimum. Similar to the benzyl radical–water system, onlythe p complex was identified for toluene–water. In the case ofaniline–water, we could not locate a complex in which the rad-ical and the water molecule are in the same plane; the anilinenitrogen atom does not act as hydrogen donor.
Unlike the non-aromatic radicals previously studied by us,[7]
the closed-shell complexes of the aromatic radicals are gener-ally weaker than their open-shell counterparts, as evidenced
by the relative interactions energies and hydrogen bond dis-tances. For example, in both complexes NA (closed-shell) andN1 (open-shell), water acts as hydrogen donor, but the interac-tion energies and distances are different: �3.83 versus�5.77 kcal mol�1 and 2.09 versus 1.97 � for NA versus N1. Like-wise, OB (closed-shell) and O1 (open-shell) are similar in shapeoverall, with water acting as hydrogen donor, but the interac-tion energies are again smaller for OB than for O1 (�2.89 vs�6.06 kcal mol�1), with O···HOw distances of 2.02 and 1.92 �,OHOw hydrogen bond angles of 1638 and 1608, and CH···Ow
distances of 2.59 and 2.37 �, respectively.To investigate the effects of the unpaired electron on the
stabilization of these complexes, we calculated the spin densi-ties. In our previous studies on the non-aromatic radicals CH3,NH2, and OH, we found spin densities at the formal radicalcenter of 1.08, 1.04, and 1.02, respectively [natural bond orbital(NBO) partition, B3LYP/6-311 + + G(2d,2p)] .[7] The phenyl sub-stitution results in a delocalization of the spin density over thearomatic ring (Figure 4), which increases from the benzyl radi-cal 1 to the anilinyl radical 2 and is largest in the phenoxyl rad-ical 3. The M05-2X spin densities at the C, N, and O radical cen-
Figure 3. Closed-shell complexes [M05-2X/6-311 + + G(2d,2p)] . Distances forrelevant interactions are given in Angstrom [�]. See text for discussion.
Table 3. Interaction energies (DE) of the closed-shell complexes. Energiescorrected for basis set superposition errors (DEBSSE) and including zero-point vibrational energies (DEBSSE +ZPE). All values in kcal mol�1. SeeFigure 3 for the structure of the complexes.
M05-2X/6-311 + + G(2d,2p)Complex DE DEBSSE DEBSSE + ZPE
ters of 1, 2, and 3 are 0.75, 0.64, and 0.41, respectively (B3LYPresults : 0.71, 0.61, and 0.42, Figure 4). Overall, there is an in-crease of the spin densities in the aromatic rings when goingfrom 1 to 3. For radicals 2 and 3 no p complexes could be lo-calized. Apparently, the unpaired electron does not have animportant role in the stabilization of the complexes (as previ-ously found for the CH3, NH2 and OH radicals). There is onlyvery little spin density on the water molecule in the currentlystudied complexes.
In the sequence 1–3, the electron densities in the p orbitalsperpendicular to the molecular plane decrease (total of theNBO densities considering the six perpendicular p orbitals : 6.0,5.8, and 5.6). In aniline (5) and phenol (6) these p electron den-sities are 0.3 and 0.5 e higher than in 2 and 3. In the case oftoluene (4) the electron density is almost the same. Therefore,the p systems of 2 and 3 are weaker donors than the corre-sponding p systems of closed-shell molecules 5 and 6.
The different stabilities of the various water complexes ofthe radicals and closed-shell molecules can be qualitatively un-derstood by comparing the electrostatic potential maps(Figure 5). Those for phenol 6 and the phenoxyl radical 3 havebeen discussed elsewhere by us[4] and are thus shown hereonly for comparison.
The negative electrostatic potential at the nitrogen atom ishigher and more localized in radical 2 compared with aniline5. This indicates that 2 is a better hydrogen-bond acceptorthan 5. The electrostatic maps also reveal that the p system isa better acceptor in 5 than in 2 and that the nitrogen atom of2 is a better acceptor than the p system of 2. This explainswhy, unlike for the aniline–water system, no p complex of theanilinyl radical 2 with water could be located. The electrostaticpotential also rationalizes why the anilinyl radical 2 is a muchbetter hydrogen-bond donor than aniline 5, and why com-plexes N1, N2, and N5 are formed.
The electrostatic potential maps of toluene 4 and the benzylradical 1 corroborate that both molecules interact with waterpreferentially through their p system, which is more delocal-ized in 1. This is consistent with the structure of complex C6,in which one hydrogen atom of water points towards the sub-stituted carbon atom of 1. The low negative electrostatic po-tential at one CH hydrogen atom of the benzyl group and anortho hydrogen atom of the aromatic ring also help to rational-ize the finding of a floppy and weak benzyl–water complexC1.
QM/MD Simulations
The B3LYP-D/6-311 + + G(2d,2p)-optimized structures of N1,O1, and C1 (Figures 2 and 1S) were taken as starting points for25 ps quantum mechanics/molecular dynamics (QM/MD) simu-lations at temperatures of 100 and 298 K using the same levelof theory. In all cases, the complexes dissociated rapidly at298 K (Figure 6). However, they were stable at 100 K. The initialgeometry of the complexes was basically retained during thewhole simulation in the case of N1 and O1 (Figures 6, 3S, 4S,and 5S). The average distance between the heteroatom of theradical and the water oxygen atom at 100 K amounted to2.89�0.08 and 2.87�0.10 �, respectively. This confirms theproximity between water and the radical heteroatom duringthe entire simulation.
On the other hand, in the case of the benzyl radical, thewater molecule quickly left its original position from C1 to in-teract with the p system of the benzyl radical forming firstcomplex C7 and later C6. This again corroborates that thebenzyl radical interacts with water most strongly through its p
system. Because of the flatness of the potential energy surfaceand the resulting mobility of water, both benzyl–water com-plexes C6 and C7 are expected to be present in a mixture. Inthe QM/MD simulations, the average distance between the Catom of the CH2 group and the oxygen atom of water wasrather large (4.37�0.51 �), which is a consequence of visitingdifferent structures (C1, C7, C6) during the dynamics (Figure 6).
3. Conclusions
The calculated structures of the most stable complexes of thearchetypical aromatic radicals benzyl 1, anilinyl 2, and phenox-yl 3 with water (C6/C7, N1, and O1, respectively) can be quali-tatively understood in terms of electrostatic potential maps(Figure 5). The benzyl radical is neither a good hydrogen-bond
Figure 4. Calculated NBO spin densities in radicals 1, 2, and 3 at the M05-2X/6-311 + + G(2d,2p) level.
Figure 5. Electrostatic potential (red, �4 � 10�2 a.u. ; blue, + 4 � 10�2 a.u.)mapped on a surface of constant electron density (0.02 electrons ��3) for theclosed-shell molecules (left) and open-shell radicals (right) calculated at theM05-2X/6-311 + + G(2d,2p) level.
acceptor nor a good hydrogen-bond donor, and the only com-plexes found (apart from very weak van der Waals complexes)are the OH···p complexes C6/C7, which only differ in the orien-tation of the water molecule on top of the p system. The low-temperature experiments revealed that a complex between1 and water is indeed formed, and the observed spectra are inreasonable agreement with the calculated spectrum of theslightly more stable complex C6. The QM/MD simulations indi-cated that the surface separating C6 and C7 is very flat so thata rapid conversion is expected even at low temperatures. Thecomputed binding energies [M05-2X/6-311 + + G(2d,2p) +
BSSE, see Table 2] of these complexes are roughly 3.5 kcalmol�1 indicating a weak hydrogen bond. The OH···p complexesof 1 are clearly more strongly bound than that of toluene 4(Table 3).
The most stable complexes of the anilinyl radical 2 and phe-noxyl radical 3 with water (N1 and O1) are similar in structureand energy. In both cases, the lone pair at the heteroatom isby far the dominant hydrogen-bond acceptor (compared withthe p system). The resulting Nrad···HOw and Orad···HOw com-plexes with water contain strong hydrogen bonds, while the p
complexes are no minima at all. The anilinyl radical 2 differsfrom the two other radicals studied presently in that it also hasan acidic hydrogen atom that can form NHrad···Ow hydrogenbonds with water as hydrogen-bond acceptor (N2). However,
this complex was calculated to be much less stable than N1and is expected to readily rearrange to N1. So far, due to thelack of suitable precursors, no experiments have been reportedon complexes of the anilinyl radical 2. Interestingly, aniline 5only acts as hydrogen-bond acceptor in the complexes withwater (NA and NB, Figure 3); no complexes were found inwhich aniline would act as hydrogen-bond donor.
Experimental Section
Materials
1-Iodo-2-methylbenzene 7 was purchased from Sigma Aldrich(98 % purity) and directly used for the matrix experiments withoutfurther purification. Benzyl iodide 8 was synthesized according toa literature procedure (Scheme 2).[9]
Matrix Isolation
Matrix isolation experiments were performed by standard tech-niques using an APD HC-4 closed cycle helium compressor forcooling to 10 K. For studies in the mid IR range (400 and4000 cm�1), matrices were deposited on CsI spectroscopic win-dows, and the spectra recorded on FTIR spectrometers with a stan-dard resolution of 0.5 cm�1. Flash vacuum pyrolysis (FVP) was car-ried out by slow sublimation of the precursors 7 or 8 througha quartz tube electrically heated to 650 8C and 460 8C, respectively.Both precursors produced the benzyl radical 1 in reasonable yields.Complexes between 1 and water were generated by co-depositionof the FVP products of 7 or 8 and with a large excess of argon(99.99 %) doped with 0.1–1 % water. After deposition at 10 K, thematrices were annealed at temperatures between 25 and 35 K byheating the matrix at a fixed temperature rate of 1 K min�1. Afterannealing, the matrices were cooled back to 10 K.
Computational Details
The multiple minima hypersurface (MMH) approach[10] was used forsearching the minima of the C6H5�R···H2O complexes (R = CH2, NH,and O). 500 randomly generated geometries were optimized andanalyzed using the PM3 and AM1[11] semiempirical quantum me-chanical Hamiltonians. The results from these semiempirical calcu-lations provided a preliminary overview over the interactions be-tween these radicals and water. The relevant configurations werefurther refined at the unrestricted DFT level of theory using theM05-2X[12] density functional (which has been successfully used byus to investigate other open-shell complexes with water[7]) as im-plemented in Gaussian 09[13] and the B3LYP functional[14] with em-
Figure 6. Time evolution of the distance [�] between the radical heteroatom(or CH2 carbon atom of the benzyl radical) and the oxygen atom of waterfrom MD simulations at 100 K (top) and 298 K (bottom). Time is given infemtoseconds [fs] .
pirical dispersion corrections (UB3LYP-D)[15] using Turbomole (ver-sion 5.10).[16] The 6-311 + + G(2d,2p) basis set[17] was used through-out for geometry optimization and the computation of vibrationalspectra. The closed-shell complexes of toluene, aniline, and phenolwith water were calculated at the M05-2X/6–311 + + G(2d,2p)level.
Interaction energies were determined by subtracting the energiesof the monomers from those of the complexes. They were correct-ed for BSSEs by applying the counterpoise (CP) scheme of Boysand Bernardi.[18] Zero-point energies (ZPE) were computed basedon harmonic frequency calculations.
Molecular dynamics (MD) simulations provide a dynamic picture ofthe behavior of chemical systems and processes. One limitation ofclassical MD is the lack of parameters to adequately describe theproperties of open-shell systems. Although computationally moreexpensive, quantum mechanics/molecular dynamics approaches(QM/MD) solve this problem and provide a more accurate descrip-tion of systems including open-shell molecules. Thus, QM/MD sim-ulations were performed using the program ChemShell[19] as an in-terface to Turbomole (version 5.10). The B3LYP hybrid functionalwith empirical dispersion corrections (UB3LYP-D) and the 6-311 +
+ G(2d,2p) basis set were used. The simulations were run for 25 pswith a time step of 1 fs under NVT (canonical) conditions at tem-peratures of 100 and 298 K. A Nos�–Hoover chain (NHC) thermo-stat[20] was used together with a reversible noniterative leapfrog-type integrator. The UB3LYP-D/6-311 + + G(2d,2p)-optimized geo-metries of the C1, O1, and N1 complexes were taken as startingpoints.
To understand the nature of the complexes and the role of the un-paired orbitals in the radical–water complexes, we applied naturalbond orbital (NBO)[21] analysis, which provides a simple descriptionof the chemical bond based on orbital interaction concepts.[22] TheNBOs are a set of localized orbitals that fulfill the requirements oforthonormality and maximum occupancy. Their form is normallyconsistent with the Lewis model of electronic structure, since thetransformation from canonical molecular orbitals (MOs) to NBOsusually produces highly occupied and nearly empty localized orbi-tals. The former can be classified as core orbitals, lone pairs (n), orbonding (s, p) orbitals, while the latter normally represent anti-bonding (s*, p*) orbitals that can be used to describe charge trans-fer or non-covalent interactions.
Acknowledgements
This work was financially supported by the Deutsche Forschungs-gemeinschaft (DFG) in the framework of the Special ResearchUnit 618 (FOR 618). E.S.-G. and K.B.-R. acknowledge a Liebig sti-pend and doctoral scholarship, respectively, from the Fonds derChemischen Industrie.
Keywords: density functional theory · matrix isolation · QMmolecular dynamics · radicals · water
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Received: October 8, 2012Published online on January 18, 2013
