Formaldoxime hydrogen bonded complexes with ammonia and hydrogen chloride Barbara Golec a , Małgorzata Mucha a , Magdalena Sałdyka a , Austin Barnes b,⇑ , Zofia Mielke a,⇑ a Faculty of Chemistry, University of Wrocław, Joliot Curie 14, 50-383 Wrocław, Poland b Materials & Physics Research Centre, University of Salford, Salford M5 4WT, United Kingdom highlights 1:1 and 1:2 CH 2 NOH complexes with NH 3 and HCl are trapped in the argon matrices. Formaldoxime complexes with ammonia are stabilized by strong OAHN bond. Formaldoxime complexes with HCl are stabilized by strong NHACl bond. Hydrogen bonds in the cyclic 1:2 complexes show strong cooperativity. graphical abstract article info Article history: Received 6 September 2013 Received in revised form 30 October 2013 Accepted 5 November 2013 Available online xxxx Keywords: Formaldoxime Ammonia Hydrogen bond Matrix isolation ab initio calculations Molecular complexes abstract An infrared spectroscopic and MP2/6–311++G(2d,2p) study of hydrogen bonded complexes of formal- doxime with ammonia and hydrogen chloride trapped in solid argon matrices is reported. Both 1:1 and 1:2 complexes between formaldoxime and ammonia, hydrogen chloride have been identified in the CH 2 NOH/NH 3 /Ar, CH 2 NOH/HCl/Ar matrices, respectively, their structures were determined by com- parison of the spectra with the results of calculations. In the 1:1 complexes present in the argon matrices the OH group of formaldoxime acts as a proton donor for ammonia and the nitrogen atom acts as a proton acceptor for hydrogen chloride. In the 1:2 complexes ammonia or hydrogen chloride dimers interact both with the OH group and the nitrogen atom of CH 2 NOH to form seven membered cyclic structures stabi- lized by three hydrogen bonds. The theoretical spectra generally agree well with the experimental ones, but they seriously underestimate the shift of the OH stretch for the 1:1 CH 2 NOHNH 3 complex. Ó 2013 Elsevier B.V. All rights reserved. Introduction Oximes with the characteristic >C@NOH group are important biological and chemical systems. The >C@NOH group involves the OH hydrogen bond donor and two hydrogen bond acceptor sites, namely the C@N nitrogen and the OAH oxygen, thus oximes may form a variety of hydrogen bonds. The intermolecular hydrogen bond motifs involving oximes play an important role in molecular design [1,2]. The probability of formation of the cyclic heterodimer between carboxylic acid and oxime is as high as 90% when both molecules are present in a crystal. Oximes exhibit significant molecular association even in the di- lute gas phase which is a relatively rare phenomenon [3]. The self- association of the simplest ketoxime, namely acetone oxime, have been extensively studied in solution [3–5], in the solid state [6–8] and, more recently, in the gas phase [9]. The dimerisation of the simplest aldoxime, namely formaldoxime has been studied with the help of the matrix isolation technique and quantum chemistry 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.017 ⇑ Corresponding authors. Tel.: +48 71 3757475; fax: +48 71 3282348 (Z. Mielke). E-mail addresses: [email protected](A. Barnes), zofi[email protected]. wroc.pl (Z. Mielke). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa Please cite this article in press as: B. Golec et al., Formaldoxime hydrogen bonded complexes with ammonia and hydrogen chloride, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2013), http://dx.doi.org/10.1016/j.saa.2013.11.017
Transcript
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy
journal homepage: www.elsevier .com/locate /saa
Formaldoxime hydrogen bonded complexes with ammoniaand hydrogen chloride
1386-1425/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.saa.2013.11.017
Please cite this article in press as: B. Golec et al., Formaldoxime hydrogen bonded complexes with ammonia and hydrogen chloride, SpectrochimiPart A: Molecular and Biomolecular Spectroscopy (2013), http://dx.doi.org/10.1016/j.saa.2013.11.017
Barbara Golec a, Małgorzata Mucha a, Magdalena Sałdyka a, Austin Barnes b,⇑, Zofia Mielke a,⇑a Faculty of Chemistry, University of Wrocław, Joliot Curie 14, 50-383 Wrocław, Polandb Materials & Physics Research Centre, University of Salford, Salford M5 4WT, United Kingdom
h i g h l i g h t s
� 1:1 and 1:2 CH2NOH complexes withNH3 and HCl are trapped in the argonmatrices.� Formaldoxime complexes with
ammonia are stabilized by strongOAH� � �N bond.� Formaldoxime complexes with HCl
are stabilized by strong N� � �HAClbond.� Hydrogen bonds in the cyclic 1:2
complexes show strong cooperativity.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:Received 6 September 2013Received in revised form 30 October 2013Accepted 5 November 2013Available online xxxx
Keywords:FormaldoximeAmmoniaHydrogen bondMatrix isolationab initio calculationsMolecular complexes
a b s t r a c t
An infrared spectroscopic and MP2/6–311++G(2d,2p) study of hydrogen bonded complexes of formal-doxime with ammonia and hydrogen chloride trapped in solid argon matrices is reported. Both 1:1and 1:2 complexes between formaldoxime and ammonia, hydrogen chloride have been identified inthe CH2NOH/NH3/Ar, CH2NOH/HCl/Ar matrices, respectively, their structures were determined by com-parison of the spectra with the results of calculations. In the 1:1 complexes present in the argon matricesthe OH group of formaldoxime acts as a proton donor for ammonia and the nitrogen atom acts as a protonacceptor for hydrogen chloride. In the 1:2 complexes ammonia or hydrogen chloride dimers interact bothwith the OH group and the nitrogen atom of CH2NOH to form seven membered cyclic structures stabi-lized by three hydrogen bonds. The theoretical spectra generally agree well with the experimental ones,but they seriously underestimate the shift of the OH stretch for the 1:1 CH2NOH� � �NH3 complex.
� 2013 Elsevier B.V. All rights reserved.
Introduction
Oximes with the characteristic >C@NOH group are importantbiological and chemical systems. The >C@NOH group involvesthe OH hydrogen bond donor and two hydrogen bond acceptorsites, namely the C@N nitrogen and the OAH oxygen, thus oximesmay form a variety of hydrogen bonds. The intermolecular
hydrogen bond motifs involving oximes play an important role inmolecular design [1,2]. The probability of formation of the cyclicheterodimer between carboxylic acid and oxime is as high as 90%when both molecules are present in a crystal.
Oximes exhibit significant molecular association even in the di-lute gas phase which is a relatively rare phenomenon [3]. The self-association of the simplest ketoxime, namely acetone oxime, havebeen extensively studied in solution [3–5], in the solid state [6–8]and, more recently, in the gas phase [9]. The dimerisation of thesimplest aldoxime, namely formaldoxime has been studied withthe help of the matrix isolation technique and quantum chemistry
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methods [10]. The ability of oximes to form hetero-aggregates withvarious proton donors and acceptors was much less studied thanoximes homo-aggregation in spite of the fact that the proton-donorand proton-acceptor properties of the >C@NOH group play animportant role in structural motifs involving oximes. As a part ofa more general study on the ability of oximes to engage in hydro-gen bond formation we reported recently the results of studies onformaldoxime complexes with nitrogen [11], nitrous acid [12] andwater [13]. In this paper we present an infrared matrix isolationand theoretical study of the complexes formed by formaldoximewith hydrogen chloride and ammonia. Ammonia and hydrogenchloride serve as an archetypal strong proton acceptor and strongproton donor, respectively, in studies of hydrogen bonding [14,15].The study of the formaldoxime complexes with these two mole-cules should provide information on the proton acceptor and pro-ton donor abilities of the >C@NOH group.
Experimental and computational details
Infrared matrix isolation studies
Formaldoxime was generated from formaldoxime trimer hydro-chloride (Aldrich, >98%) in the following way. A small amount ofthe salt was placed in a glass flask connected to the vacuum vesselof the cryostat. Upon heating to 323–338 K the salt decomposed,releasing gaseous formaldoxime. Formaldoxime and NH3/Ar orHCl/Ar mixtures with concentration varying from 1/100 to 1/800were simultaneously deposited onto a gold-plated copper mirrorheld at 11 K by a closed cycle helium refrigerator (Air Products,Displex 202A). The concentration of formaldoxime in the CH2-
NOH/NH3(HCl)/Ar mixtures was varied by changing the flow rateof argon gas as well as the temperature of the hydrochloride salt.Infrared spectra with resolution 0.5 cm�1 were recorded in a reflec-tion mode with a Bruker 113v spectrometer using a liquid N2
cooled MCT detector.
Computational details
The Gaussian 09 program [16] was used for the geometry opti-mization and harmonic and anharmonic vibrational calculations.The structures of the monomers (CH2NOH, NH3, HCl) and the struc-tures of the CH2NOH–NH3 and CH2NOH–HCl complexes were fullyoptimized at the MP2 level of theory with the 6–311++G(2d,2p) ba-sis set. Vibrational wavenumbers were computed for both themonomers and the complexes. Interaction energies were correctedby the Boys–Bernardi full counterpoise procedure [17], and zero-point vibrational energy corrections were also calculated.
Nonadditivity is one of the most important characteristic of tri-mers [18–22]. This effect was quantitatively measured by the en-ergy defined as:
ENA ¼ DEint;ABC � DEint;AB � DEint;BC � DEint;AC
where DEint,ABC is the interaction energy of the ABC trimer, DEint,AB,DEint,BC, DEint,AC are energies of respective dimers, A, B, C denotesthe monomers.
Results
Before the studies of the complexes were undertaken the infra-red spectra of formaldoxime, ammonia and hydrogen chloride iso-lated in argon matrices were recorded; they were in accord withthe literature spectra [23–26]. In the infrared spectra of matricescontaining both formaldoxime and ammonia or hydrogen chloridenew band sets were observed. They appeared in the vicinity of the
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monomer absorptions which facilitated their assignment to theperturbed vibrations of the CH2NOH and NH3 or HCl molecules.
Formaldoxime–ammonia complexes
The new bands that appeared in the spectra of the CH2NOH/NH3/Ar matrices can be classified into two groups. The bandsbelonging to group I (2936.2, 1494.3, 1370.4, 1173.8, 1049.2,936.5 and 926.5 cm�1) strongly decreased whereas those belong-ing to group II (2834.1, 1501.2, 1381.2, 1176.1, 1098.5, 1082.4,933.9 and 929.3 cm�1) increased after matrix annealing. Moreoverthe relative intensities of bands II grew with respect to bands Iwhen the NH3 concentration in the matrix was increased. Theabove experimental data indicate that bands I can be assigned withconfidence to the 1:1 CH2NOH� � �NH3 complex whereas bands IIcan be attributed to a CH2NOH� � �(NH3)2 complex. All the wave-numbers identified for the bands belonging to the groups I and IIare collected in Table 1. In Fig. 1 the most representative regionsof the spectra of the CH2NOH/NH3/Ar matrix recorded directly aftermatrix deposition and after annealing are presented.
The bands identified at 1049.2 cm�1 (I) and at 1098.5,1082.4 cm�1 (II) are attributed to the perturbed ammonia bendingvibrations, dNH3; all other absorptions are assigned to perturbedformaldoxime modes. The mOH stretching vibration of the CH2-
NOH� � �NH3 complex appears at 2936.2 cm�1 and shows a verylarge, ca. 685 cm�1, red shift with respect to the correspondingvibration of CH2NOH. The perturbation of the formaldoxime mOHvibration in the complex with ammonia is much larger than inthe complex with the water molecule (�138.9 cm�1) [13]. Thelarge red shift of the OH stretch in the CH2NOH� � �NH3 complex isaccompanied by relatively strong blue shifts of the dNOH andmNO vibrations (+58.4 cm�1, +43.2 cm�1, respectively). The vibra-tions of ammonia are also strongly perturbed as evidenced bythe relatively large blue shift of dNH3 (+74.9 cm�1). Such a patternof bands indicates that formaldoxime forms with ammonia astrong complex stabilized by an OAH� � �N hydrogen bond. Thecomparison of the perturbations of dNH3 vibrations in the ammo-nia complexes with hydroxylamine [27], water [28] and formal-doxime (+56, +62, +74.9 cm�1, respectively) indicates thatCH2NOH� � �NH3 is the strongest among the three complexes.
The ab initio calculations at the MP2/6–311++G(2d,2p) levelindicated three stationary points for the CH2NOH� � �NH3 systemthat are shown in Fig. 2. The selected bond distances (in Å) andinteraction energies (in kJ mol�1) are also presented. In Table 1S,Supporting material, the geometrical parameters for the threestructures are collected and in Table 2S the calculated harmonicand anharmonic wavenumbers are presented. Structure IA
(DECPZPE ¼ �26:22 kJ mol�1) stabilized by the OAH� � �N bond is much
more stable than structures IB (DECPZPE ¼ �7:46 kJ mol�1) and IC
(DECPZPE ¼ �7:30 kJ mol�1) in which ammonia acts as a proton donor
forming an NAH� � �O or NAH� � �N hydrogen bond. There is probablyan additional weak interaction between the CH group of CH2NOHand a nitrogen atom in the IB and IC structures. The formation ofthe OAH� � �N hydrogen bond in structure IA is reflected in a strongdecrease of the calculated mOH stretching wavenumber and an in-crease of the dNH3 and mNO wavenumbers (Dmcalc = �407, +82,+28 cm�1 respectively). The comparison of the calculated wave-numbers for the three structures with those identified for the 1:1complex trapped in the matrix clearly shows that the complexhas the IA structure. However, one should notice the relativelylarge difference between the observed (Dmexp = �684.5 cm�1) andcalculated (Dmcalc = �407 cm�1) wavenumber shifts for the mOHstretching vibration. The formation of the NAH� � �O bond in the IB
complex is reflected in the noticeable red shift of the calculatedNAO stretching wavenumber (Dmcalc = �19 cm�1) whereas the per-turbations of the OH stretch and NOH bend are very small. In turn,
ed complexes with ammonia and hydrogen chloride, Spectrochimica Acta1016/j.saa.2013.11.017
Table 1The identified wavenumbers for the CH2NOH� � �NH3 and CH2NOH� � �(NH3)2 complexes and comparison of the observed and calculated anharmonic wavenumber shifts, Dma.
B. Golec et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx 3
the formation of the IC complex only slightly affects the NOH groupvibrations (see Table 2S). Neither of these two complexes wasidentified in the studied matrices.
All bands II assigned to the 1:2 formaldoxime–ammonia com-plex are shifted in the same direction as bands I (toward loweror higher wavenumbers) with respect to the corresponding bandsof ammonia or formaldoxime monomers, however they show lar-ger shifts than the bands I (see Table 1). The band II attributed tothe OH stretch is 786.6 cm�1 red shifted with respect to mOH ofCH2NOH monomer and the bands II due to the NH3 bending vibra-tions are 124.2, 108.1 cm�1 blue shifted as compared to the band ofNH3 monomer.
The MP2/6–311++G(2d,2p) calculations performed for the CH2-
NOH� � �(NH3)2 system identified four stationary points that are pre-sented in Fig. 2. The selected bond distances (in Å) and interactionenergies (in kJ mol�1) are also presented. In Table 1S, Supportingmaterial, the selected geometrical parameters for the four struc-tures are collected and in Table 3S the calculated harmonic andanharmonic wavenumbers are shown. The most stable is the se-ven-membered cyclic structure IIA (DECP
ZPE ¼ �45:75 kJ mol�1) inwhich ammonia dimer is interacting with the OH group and withthe nitrogen atom of formaldoxime. The complex is stabilized bytwo hydrogen bonds between formaldoxime and ammonia:OAH� � �N, NAH� � �N and one NAH� � �N bond between the twoammonia molecules. The structures IIB (DECP
ZPE ¼ �34:52 kJ mol�1)and IIC (DECP
ZPE ¼ �32:60 kJ mol�1) have comparable stabilities. Inboth structures one ammonia molecule is attached to the OH groupof CH2NOH forming an OAH� � �N bond whereas the second ammo-nia molecule serves as a proton donor toward the oxygen atom inIIB and toward the nitrogen atom in IIC. In the weakest IID complex(DECP
ZPE ¼ �13:13 kJ mol�1) the ammonia dimer is attached to theoxygen atom of formaldoxime.
The comparison of the calculated wavenumbers and wavenum-bers shifts for the IIA, IIB, IIC and IID structures with those observedfor the bands II shows that the CH2NOH� � �(NH3)2 complex trappedin the matrix has the IIA structure (see Tables 1 and 3S).
Formaldoxime–hydrogen chloride complexes
The new bands appearing in the spectra of the CH2NOH/HCl/Armatrices can also be separated into two groups. The group I in-volves the bands at 3564.6, 1422.0, 1342.0, 1171.7, 963.7, 966.0,910.7 cm�1 which are relatively intense in the spectra of dilutematrices and strongly diminish after matrix annealing to 33 K.The bands belonging to group II appear at 3497.6, 1357.9, 1184.7,959.3, 911.5 cm�1. They are very weak or are not observed in thespectra of dilute matrices but their intensities increase with HClconcentration and after matrix annealing. The relative intensities
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of the bands belonging to groups I or II are constant withinexperimental error in all performed experiments. The aboveexperimental data allowed us to assign the bands I and II to theCH2NOH� � �HCl and CH2NOH� � �(HCl)2 complexes, respectively.Moreover, in the spectra of matrices recorded after annealing weakbands at 3485.5, 1354.0 and 969.2 cm�1 appeared which also grewafter annealing but their relative intensities with respect to thebands II varied in the performed experiments. The bands are alsotentatively assigned to CH2NOH� � �(HCl)2 complexes of differentstructure than that producing the bands II. All wavenumbers iden-tified for the bands belonging to groups I and II are collected in Ta-ble 2. In Fig. 3 the most representative regions of the spectra of theCH2NOH/HCl/Ar matrix recorded directly after matrix depositionand after annealing are presented; the region of HCl stretchingvibrations is presented in Fig. 1S, Supporting material.
All bands belonging to group I appear in the vicinity of theformaldoxime monomer absorptions and can be assigned withconfidence to the perturbed CH2NOH vibrations. The largest per-turbations are observed for the NOH group vibrations; mOH, dNOHand mNO are shifted by �56.1 cm�1 and +37.8, +27.4 cm�1, respec-tively, (see Table 2) with respect to the CH2NOH monomer absorp-tions. However, the shifts are much smaller than those observedfor the corresponding CH2NOH vibrations in the CH2NOH� � �NH3
complex, particularly for the OH stretch. Unfortunately, no absorp-tion due to the hydrogen bonded HCl molecule was identifiedwhich may due to the band being broad and diffuse.
The calculations performed at the MP2/6–311++G(2d,2p) levelpredict the stability of three structures, IA, IB and IC for the CH2-
NOH� � �HCl complex. All optimized structures are shown in Fig. 4;their interaction energies and selected geometrical parametersare also presented. In Table 4S, Supplementary data, all calculatedparameters for the optimized structures are listed and in Table 5Sthe calculated harmonic and anharmonic wavenumbers arecollected.
In the most stable IA structure (DECPZPE ¼ �15:85 kJ mol�1) hydro-
gen chloride serves as a proton donor toward the nitrogen atom ofCH2NOH forming N� � �HACl bond. The N� � �H distance is equal to1.82 Å and the h(NHCl) angle is 159�. The non-linearity of theh(NHCl) angle suggests an additional very weak interaction be-tween the chlorine atom of HCl and the OH group of formaldoxime.In the IB structure (DECP
ZPE ¼ �11:01 kJ mol�1) hydrogen chloride isbonded to the oxygen atom of formaldoxime forming an O� � �HAClhydrogen bond. In turn, in the less stable IC complex(DECP
ZPE ¼ �3:07 kJ mol�1), the OH group of formaldoxime servesas a proton donor toward the chlorine atom of HCl and anOAH� � �Cl bond is formed.
The comparison of the observed wavenumbers and wavenum-bers shifts with the calculated ones for the three structures (see
ed complexes with ammonia and hydrogen chloride, Spectrochimica Acta1016/j.saa.2013.11.017
Fig. 1. Selected regions of the spectra of the NH3/Ar = 1/300 (a); CH2NOH/Ar (b) andCH2NOH/NH3/Ar matrices recorded directly after deposition and after annealing to33 K for 10 min (c and d). The arrows indicate the bands due to the 1:1 (I) and 1:2(II) complexes.
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Tables 2 and 5S) indicates that the most stable IA complex istrapped in the matrix. The identified mOH wavenumber shows adistinct (but not large) red shift whereas the dNOH and mNO wave-numbers show blue shifts with respect to the corresponding CH2-
NOH vibrations, in accordance with the calculated shifts for theIA structure. In the IB complex the calculated dNOH and mNO wave-numbers exhibit red shifts whereas the perturbations of the IC
complex vibrations are smaller than those observed for the com-plex trapped in the matrix. The calculations show, in accord withthe experimental spectra, the distinct red shift of the OH stretchand blue shifts of the NOH bend and NO stretch. Such a spectralpattern suggests that in addition to a strong N� � �HACl bond the
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complex is stabilized by a very weak OAH� � �Cl bond forming a cyc-lic structure. A similar structure was observed for the NH2-
OH� � �HAF complex that was stabilized by a strong N� � �HAF andmuch weaker OAH� � �F bonds [29].
The bands II exhibit larger shifts with respect to the formal-doxime monomer bands than the absorptions I but the correspond-ing bands within the two groups are shifted in the same direction(toward higher or lower wavenumbers with respect to formal-doxime vibrations). This fact suggests that in the CH2NOH� � �(HCl)2
complex, like in CH2NOH� � �HCl, N� � �HACl and OAH� � �Cl hydrogenbonds are formed.
The ab initio calculations at the MP2/6–311++G(2d,2p) levelindicated four stationary points for the CH2NOH� � �(HCl)2 systemthat are shown in Fig. 4. In the most stable seven-membered cyclicstructure IIA (DECP
ZPE ¼ �30:05 kJ mol�1) the nitrogen atom of form-aldoxime serves as a proton acceptor for one hydrogen chloridesubunit of the HCl dimer and the OH group acts as a proton donortoward a chlorine atom of the second HCl molecule. The N� � �HACland OAH� � �Cl bonds in the 1:2 complex are stronger than in the1:1 one as indicated by the shortening of the N� � �H and H� � �Cl dis-tances in IIA as compared to IA (1.68 Å versus 1.82 Å for N� � �H and2.32 Å versus 2.84 Å for H� � �Cl). The angles h(NHCl) and h(OHCl)are close to linear (178.2�, 170.9�, respectively). In the IIB structure(DECP
ZPE ¼ �24:95 kJ mol�1) one HCl molecule acts as a proton donorfor the nitrogen atom and as a weak proton acceptor for the OHgroup of formaldoxime. The second HCl molecule serves as a pro-ton donor for the oxygen atom of formaldoxime. So, the complexis stabilized by two strong hydrogen bonds N� � �HACl, O� � �HACland a very weak OAH� � �Cl bond. In turn, in the IIC structure(DECP
ZPE ¼ �20:30 kJ mol�1) one HCl molecule is attached to thenitrogen atom and a second one to the OH group which serves asa proton donor for the chlorine atom. In the least stable IID complex(DECP
ZPE ¼ �19:13 kJ mol�1) two HCl molecules are bonded to anoxygen atom which serves as a double acceptor.
The comparison of the bands II wavenumbers and wavenumbershifts with those calculated for the four structures leads to the con-clusion that the bands II correspond to the IIA structure. The struc-tures IIB and IID could be immediately rejected as their formationshould lead to a red shift of the NO vibration whereas a blue shiftis observed for the complex present in the matrix (see Table 6S).The patterns of wavenumber shifts for the IIA and IIC complexesare very similar to each other except for the HCl vibrations whichare much more perturbed in IIA than in IIC. The more intense of thetwo perturbed HCl vibrations is calculated to exhibit a 927 cm�1
red shift in IIA and a 547 cm�1 red shift in IIC. Only one perturbedHCl vibration was tentatively identified for the 1:2 complex as abroad diffuse band at ca. 2080 cm�1; the absorption exhibits a ca.808 cm�1 red shift with respect to the HCl monomer absorption.The position of this band and its shift with respect to the HClmonomer absorption do not give a definitive answer as to whetherit corresponds to a complex of the IIA or IIC structure. However, the808 cm�1 red shift is much larger than that calculated for IIC and isclose to the shift calculated for IIA. Taking this into account as wellas the fact that IIA is the most stable structure we assigned bands IIto a complex of IIA structure. The additional bands that were ob-served in the spectra of the studied matrices at 3485.5, 1354.0,1190.7 and 969.2 cm�1 most probably belong to the IIB complex.
Discussion
The CH2NOH� � �NH3 complex present in the matrix is stabilizedby a strong OAH� � �N(H3) hydrogen bond as evidenced by the largered shift, Dmexp = �684.5 cm�1, of the OH stretch of the bondedformaldoxime molecule. The calculations indicate that theOAH� � �N bond is almost linear (h(OHN) ffi 165�), the calculated
ed complexes with ammonia and hydrogen chloride, Spectrochimica Acta1016/j.saa.2013.11.017
Fig. 2. The MP2/6–311++G(2d,2p) optimized structures of the CH2NOH� � �NH3 and CH2NOH� � �(NH3)2 complexes. The energy values, DECPZPE [kJ mol�1], are also presented.
Table 2The identified wavenumbers for the CH2NOH� � �HCl and CH2NOH� � �(HCl)2 complexes and comparison of the observed and calculated anharmonic wavenumber shifts, Dma.
a Dm = mcomp � mmon.b Tentative assignment.c The intensities (km mol�1) are given in parentheses.
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H� � �N bond length equals 1.83 Å and the interaction energy,DECP
ZPE ¼ �26:22 kJ mol�1. The theoretical spectra generally agreewell with the experimental ones, however they underestimatethe shift of the OH stretch which is predicted as Dmcalc = �407 cm�1
(for anharmonic wavenumbers). The discrepancy between the ob-served and calculated wavenumber shifts for the OH stretch maybe due to limited accuracy of calculated anharmonic wavenum-bers, however the effect of argon environment on the structuralproperties of the complex cannot be excluded. Earlier matrix isola-
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tion studies performed for the ammonia complex with hydrogenhalides showed extreme sensitivity of the proton stretching wave-number of the H3N� � �HX (X = Cl, Br, I) complexes to the matrixenvironment [14,30–36].
The CH2NOH� � �(NH3)2 complex present in the matrix has a se-ven-membered cyclic structure; the OH group serves as a protondonor and the nitrogen atom of CH2NOH as an acceptor for theammonia dimer. So, in addition to the NAH� � �N bond betweenthe two ammonia molecules, the complex is stabilized by the
ed complexes with ammonia and hydrogen chloride, Spectrochimica Acta1016/j.saa.2013.11.017
Fig. 3. Selected regions of the spectra of the HCl/Ar = 1/300 (a); CH2NOH/Ar (b) andCH2NOH/HCl/Ar matrices recorded directly after deposition and after annealing to28 K and 33 K for 10 min (c–e). The arrows indicate the bands due to the 1:1 (I) and1:2 (II) complexes. The dots indicate bands that are also tentatively assigned to the1:2 complexes and the star denotes the band due to the HCl complex with water.
6 B. Golec et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx
OAH� � �N(H3) and H2NAH� � �N hydrogen bonds. The OAH� � �N(H3)bond is stronger in the 1:2 complex than in the 1:1 as would be ex-pected from the cooperative effect; the infrared spectra demon-strate a �786.6 cm�1 shift for the OH stretching vibration in thiscomplex. The calculations indicate that the OAH� � �N(H3) bond ispractically linear (h(OHN) ffi 176�), the calculated H� � �N bondlength, 1.77 Å, is 0.06 Å shorter than in the 1:1 complex. TheH2NAH� � �N bond, in which CH2NOH acts as a proton acceptor,deviates more from linearity (h(NHN) ffi 162�). The calculatedH� � �N distance, 2.16 Å, can be compared with that in the ammoniadimer, 2.14 Å, indicating comparable strength of the two NAH� � �Nhydrogen bonds in the complex. The interaction energy of IIA
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equals DECPZPE ¼ �45:75 kJ mol�1. In the IIB, IIC complexes one NH3
molecule is attached to the OH group as in IA and the second oneto the acceptor site of CH2NOH, namely to the oxygen (IIB) ornitrogen atoms (IIC) like in the IB or IC, respectively. The interac-tion energies of IIB and IIC are equal DECP
ZPE ¼ �34:52;
�32:60 kJ mol�1, respectively. In the least stable complex, IID
(DECPZPE ¼ �13:13 kJ mol�1), the ammonia dimer is attached to an
oxygen atom of formaldoxime. The calculated three-body coopera-tive effect equals ENA = �8.23, �0.21, �0.50 and �0.38 kJ mol�1 forIIA, IIB, IIC and IID, respectively. The cooperativity leads to relativelystrong stabilization of the trimer energy in IIA structure while forIIB, IIC and IID the effect is small.
In the CH2NOH� � �HCl complex present in the matrix the hydro-gen chloride molecule acts as a proton donor toward the nitrogenatom and, simultaneously, as a weak proton acceptor for the OHgroup of formaldoxime, forming a strong N� � �HACl and a weakOAH� � �Cl bonds. Unfortunately, the mHCl vibration of the 1:1 com-plex was not identified; the calculations predict a �500 cm�1
wavenumber shift of the HCl stretch. A weak OAH� � �Cl interactionis reflected in a distinct perturbation of the NOH group vibrationsof formaldoxime. The calculated interaction energy equals�15.85 kJ mol�1.
In the CH2NOH� � �(HCl)2 complex the formaldoxime moleculeinteracts with the hydrogen chloride dimer forming a seven-mem-bered cyclic ring. The complex is stabilised by N� � �HACl, OAH� � �Clbonds between formaldoxime and hydrogen chloride, similar tothose formed in the IA, IC 1:1 complexes, and by the Cl� � �HACl bondof the HCl dimer. Due to the cooperative effect the N� � �HACl andOAH� � �Cl bonds become stronger in the 1:2 complex than in the1:1 ones as indicated by larger perturbations of the formaldoximevibrations. The calculations show a shortening of the N� � �H dis-tance from 1.82 Å in the 1:1 complex to 1.68 Å in the 1:2 complexand a shortening of the H� � �Cl distance from 2.47 Å to 2.32 Å. Thecalculated interaction energy equals �30.05 kJ mol�1. The calcu-lated shift for mHCl of the hydrogen chloride molecule bonded toa nitrogen atom (�927 cm�1 for anharmonic wavenumbers) isslightly larger than the experimental shift for the tentatively iden-tified perturbed hydrogen chloride stretching wavenumber(�808 cm�1). In the IIB, IIC complexes one of the two HCl moleculesacts as a proton donor being attached to a nitrogen atom (IIB, IIC)and the second HCl plays the role of proton donor in IIB (where itis bonded to the oxygen atom) and the role of proton acceptor inIIC (in which the chlorine atom acts as a proton acceptor for theOH group). In IID structure an oxygen atom of formaldoxime servesas a proton acceptor for two HCl molecules, for this complexDECP
ZPE ¼ �19:13 kJ mol�1. The ENA values for the hydrogen chloridecomplexes equal to �10.98, +1.63, �1.50 and +6.64 kJ mol�1 for IIA,IIB, IIC and IID, respectively. They confirm a large percentage contri-bution of an attractive three body interaction energy to the cyclicIIA structure and small contribution to the IIC one. In contrast, inIIB and IID structures anti-cooperative effect occurs which is partic-ularly strong in IID. Two hydrogen bonds donated to the same dou-ble acceptor, as is the case of IID structure, are known to beanticooperative [19].
In Table 3 the observed and calculated wavenumbers shifts(DmOHexp, DmOHcalc) for the OH stretching vibration in theCH2NOH� � �NH3 complex are compared with the correspondingshifts for the ammonia complexes with various OH proton donors.With the exception of two complexes for which anharmonic wave-numbers were calculated (see Table 3) all other values correspondto harmonic calculations. The H2O� � �NH3 and CH3OH� � �NH3 com-plexes involve OAH� � �N bonds of medium strength as indicatedby the experimental and calculated DmOH values lying within therange 200–300 cm�1 [37–40]. As illustrated in Table 3, the calcu-lated DmOHcalc values are very close to the experimental ones forthese two complexes. In turn, for CH2NOH� � �NH3, OOH� � �NH3 and
ed complexes with ammonia and hydrogen chloride, Spectrochimica Acta1016/j.saa.2013.11.017
Fig. 4. The MP2/6–311++G(2d,2p) optimized structures of the CH2NOH� � �HCl and CH2NOH� � �(HCl)2 complexes. The energy values, DECPZPE [kJ mol�1], are also presented.
Table 3Comparison of the observed wavenumbers and the observed and calculatedwavenumber shifts for the OH stretching vibration in ammonia complexes withvarious OH group proton donors.
Experimental Calculated
m �Dm Ref. �Dm Ref.
H2O 3434.9 203.1 [37] 190.6 [38]238a [39]
CH3OH 3400.7 266.1 [40] 273.4 [40]H2NOH 3290 345 [27] 279 [41]CH2NOH 2936.2 684.5 This work 407a This workHOO 2654.4 758.6 [42] 559.1 [44]
a Calculated anharmonic wavenumbers.b Calculated with respect to the centre of the m(OH) doublet.
B. Golec et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx 7
ONOH� � �NH3, the observed wavenumbers shifts are in the range650–850 cm�1 indicating the presence of strong OAH� � �N bondsin these complexes [42,43]. For all of the three complexes the cal-culated value, DmOHcalc, is much lower than the experimental one,DmOHexp (407, 559.1, 613.5 cm�1 versus 684.5, 705.3, 818.7 cm�1
for the CH2NOH, HOO and HONO complexes, respectively) [42–44]. The difference between DmOHexp and DmOHcalc becomes evenlarger for the strongest ammonia complexes with nitric and sulfu-ric acids. The experiments demonstrated a ca. 1650 cm�1 red shiftfor the ammonia complex with nitric acid [45] whereas the calcu-lations result in a 925.3 cm�1 shift [46]. In turn, the experimentalDmOHexp value for the complex with sulfuric acid is as large as
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2100 cm�1 [47] whereas the calculations predicted only ca. 1170,1398.8 or 1447 cm�1 shifts [46–48]. This comparison raises thequestion whether the discrepancies between the experimentaland calculated wavenumbers are due to inaccurate calculationsof anharmonic effects, to the effect of matrix environment on thestructural and vibrational properties of the complex or to both fac-tors. Further studies are needed to explain this phenomenon.
In the context of the above discussion it is worth to note that, incontrast with the CH2NOH� � �NH3 complex, for the CH2NOH� � �(NH3)2 one the calculated DmOHcalc value (�732 cm�1) is quiteclose to the experimental one (�786.6 cm�1). The wavenumbersof the OH stretching and NOH bending frequencies (2834.1,1381.2 cm�1, respectively) of CH2NOH� � �(NH3)2 indicate that inthis case a Fermi resonance interaction may occur between OHstretching fundamental (2834.1 cm�1) and a first overtone of theNOH bending vibration whose wavenumber is close to the OHstretching fundamental (2�1381.2-j, j – anharmonic correction).Fermi resonance will affect the position of the band due to theOH stretching vibration. In the case of the CH2NOH� � �NH3 complexthe Fermi resonance interaction is very weak or does not exists asthe m(OH) fundamental and 2d(NOH) overtone lie quite apart fromeach other (2936.2, 2�1370.4-j).
Conclusions
1:1 and 1:2 complexes between formaldoxime and ammonia orhydrogen chloride have been identified in CH2NOH/NH3/Ar,CH2NOH/HCl/Ar matrices, respectively; their structures weredetermined on the basis of MP2/6–311++G(2d,2p) calculations. Inthe 1:1 complexes ammonia is attached to the OH group andhydrogen chloride to the nitrogen atom of formaldoxime. In the1:2 complexes the ammonia or hydrogen chloride dimers interactboth with the OH group as a proton donor and the nitrogen atom as
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8 B. Golec et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx
a proton acceptor to form seven-membered rings. The three hydro-gen bonds within the ring exhibit a substantial amount of cooper-ativity. The theoretical spectra generally agree well with theexperimental ones, however they seriously underestimate the shiftof the OH stretch for the 1:1 CH2NOH� � �NH3 complex.
Acknowledgement
The authors acknowledge the Wrocław Centre for Networkingand Supercomputing (WCSS) for providing computer time andfacilities.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2013.11.017.
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