Formaldoxime hydrogen bonded complexes with ammonia and hydrogen chloride.

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx

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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 b

Faculty of Chemistry, University of Wrocław, Joliot Curie 14, 50-383 Wrocław, Poland Materials & Physics Research Centre, University of Salford, Salford M5 4WT, United Kingdom

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

1:1 and 1:2 CH2NOH complexes with

NH3 and HCl are trapped in the argon matrices. Formaldoxime complexes with ammonia are stabilized by strong OAH N bond. Formaldoxime complexes with HCl are stabilized by strong N HACl bond. Hydrogen bonds in the cyclic 1:2 complexes show strong cooperativity.

a r t i c l e

i n f o

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

a b s t r a c t An infrared spectroscopic and MP2/6–311++G(2d,2p) study of hydrogen bonded complexes of formaldoxime 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 CH2NOH/NH3/Ar, CH2NOH/HCl/Ar matrices, respectively, their structures were determined by comparison 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 CH2NOH to form seven membered cyclic structures stabilized 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. 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 ⇑ 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).

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 dilute gas phase which is a relatively rare phenomenon [3]. The selfassociation 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

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

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B. Golec et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx

methods [10]. The ability of oximes to form hetero-aggregates with various proton donors and acceptors was much less studied than oximes homo-aggregation in spite of the fact that the proton-donor and proton-acceptor properties of the >C@NOH group play an important role in structural motifs involving oximes. As a part of a more general study on the ability of oximes to engage in hydrogen bond formation we reported recently the results of studies on formaldoxime complexes with nitrogen [11], nitrous acid [12] and water [13]. In this paper we present an infrared matrix isolation and theoretical study of the complexes formed by formaldoxime with hydrogen chloride and ammonia. Ammonia and hydrogen chloride serve as an archetypal strong proton acceptor and strong proton donor, respectively, in studies of hydrogen bonding [14,15]. The study of the formaldoxime complexes with these two molecules should provide information on the proton acceptor and proton donor abilities of the >C@NOH group. Experimental and computational details Infrared matrix isolation studies Formaldoxime was generated from formaldoxime trimer hydrochloride (Aldrich, >98%) in the following way. A small amount of the salt was placed in a glass flask connected to the vacuum vessel of the cryostat. Upon heating to 323–338 K the salt decomposed, releasing gaseous formaldoxime. Formaldoxime and NH3/Ar or HCl/Ar mixtures with concentration varying from 1/100 to 1/800 were simultaneously deposited onto a gold-plated copper mirror held at 11 K by a closed cycle helium refrigerator (Air Products, Displex 202A). The concentration of formaldoxime in the CH2NOH/NH3(HCl)/Ar mixtures was varied by changing the flow rate of argon gas as well as the temperature of the hydrochloride salt. Infrared spectra with resolution 0.5 cm1 were recorded in a reflection 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 optimization and harmonic and anharmonic vibrational calculations. The structures of the monomers (CH2NOH, NH3, HCl) and the structures of the CH2NOH–NH3 and CH2NOH–HCl complexes were fully optimized at the MP2 level of theory with the 6–311++G(2d,2p) basis set. Vibrational wavenumbers were computed for both the monomers and the complexes. Interaction energies were corrected by the Boys–Bernardi full counterpoise procedure [17], and zeropoint vibrational energy corrections were also calculated. Nonadditivity is one of the most important characteristic of trimers [18–22]. This effect was quantitatively measured by the energy 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 denotes the monomers. Results Before the studies of the complexes were undertaken the infrared spectra of formaldoxime, ammonia and hydrogen chloride isolated in argon matrices were recorded; they were in accord with the literature spectra [23–26]. In the infrared spectra of matrices containing both formaldoxime and ammonia or hydrogen chloride new band sets were observed. They appeared in the vicinity of the

monomer absorptions which facilitated their assignment to the perturbed 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 bands belonging to group I (2936.2, 1494.3, 1370.4, 1173.8, 1049.2, 936.5 and 926.5 cm1) strongly decreased whereas those belonging to group II (2834.1, 1501.2, 1381.2, 1176.1, 1098.5, 1082.4, 933.9 and 929.3 cm1) increased after matrix annealing. Moreover the relative intensities of bands II grew with respect to bands I when the NH3 concentration in the matrix was increased. The above experimental data indicate that bands I can be assigned with confidence to the 1:1 CH2NOH NH3 complex whereas bands II can be attributed to a CH2NOH (NH3)2 complex. All the wavenumbers identified for the bands belonging to the groups I and II are collected in Table 1. In Fig. 1 the most representative regions of the spectra of the CH2NOH/NH3/Ar matrix recorded directly after matrix deposition and after annealing are presented. The bands identified at 1049.2 cm1 (I) and at 1098.5, 1082.4 cm1 (II) are attributed to the perturbed ammonia bending vibrations, dNH3; all other absorptions are assigned to perturbed formaldoxime modes. The mOH stretching vibration of the CH2NOH NH3 complex appears at 2936.2 cm1 and shows a very large, ca. 685 cm1, red shift with respect to the corresponding vibration of CH2NOH. The perturbation of the formaldoxime mOH vibration in the complex with ammonia is much larger than in the complex with the water molecule (138.9 cm1) [13]. The large red shift of the OH stretch in the CH2NOH NH3 complex is accompanied by relatively strong blue shifts of the dNOH and mNO vibrations (+58.4 cm1, +43.2 cm1, respectively). The vibrations of ammonia are also strongly perturbed as evidenced by the relatively large blue shift of dNH3 (+74.9 cm1). Such a pattern of bands indicates that formaldoxime forms with ammonia a strong complex stabilized by an OAH N hydrogen bond. The comparison of the perturbations of dNH3 vibrations in the ammonia complexes with hydroxylamine [27], water [28] and formaldoxime (+56, +62, +74.9 cm1, respectively) indicates that CH2NOH NH3 is the strongest among the three complexes. The ab initio calculations at the MP2/6–311++G(2d,2p) level indicated three stationary points for the CH2NOH NH3 system that are shown in Fig. 2. The selected bond distances (in Å) and interaction energies (in kJ mol1) are also presented. In Table 1S, Supporting material, the geometrical parameters for the three structures are collected and in Table 2S the calculated harmonic and anharmonic wavenumbers are presented. Structure IA 1 (DECP ZPE ¼ 26:22 kJ mol ) stabilized by the OAH N bond is much 1 more stable than structures IB (DECP ZPE ¼ 7:46 kJ mol ) and IC 1 CP (DEZPE ¼ 7:30 kJ mol ) in which ammonia acts as a proton donor forming an NAH O or NAH N hydrogen bond. There is probably an additional weak interaction between the CH group of CH2NOH and a nitrogen atom in the IB and IC structures. The formation of the OAH N hydrogen bond in structure IA is reflected in a strong decrease of the calculated mOH stretching wavenumber and an increase of the dNH3 and mNO wavenumbers (Dmcalc = 407, +82, +28 cm1 respectively). The comparison of the calculated wavenumbers for the three structures with those identified for the 1:1 complex trapped in the matrix clearly shows that the complex has the IA structure. However, one should notice the relatively large difference between the observed (Dmexp = 684.5 cm1) and calculated (Dmcalc = 407 cm1) wavenumber shifts for the mOH stretching vibration. The formation of the NAH O bond in the IB complex is reflected in the noticeable red shift of the calculated NAO stretching wavenumber (Dmcalc = 19 cm1) whereas the perturbations of the OH stretch and NOH bend are very small. In turn,


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B. Golec et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx Table 1 The identified wavenumbers for the CH2NOH NH3 and CH2NOH (NH3)2 complexes and comparison of the observed and calculated anharmonic wavenumber shifts, Dma. Assignment

CH2NOH mOH dCH2 dHON qCH2 xCH2 mNO NH3 dHNH a

Monomers

CH2NOH NH3

CH2NOH (NH3)2

mexp.

mexp. group I

Dmexp.

Dmcalc. IA

mexp. group II

Dmexp.

Dmcalc. IIA

3620.7 1408.3 1314.2 1153.7 953.1 886.4 880.2

2936.2 1494.3 1370.4 1173.8 936.5 926.5

684.5 +86.0 +58.4 +20.1 16.6 +43.2

407 +79 +62 +13 18 +28

2834.1 1501.2 1381.2 1176.1 933.9 929.3

786.6 +92.9 +67.0 +22.4 19.2 +46.0

732 +154 +68 +18 26 +45

974.3

1049.2

+74.9

+82

1098.5 1082.4

+124.2 +108.1

+132 +77

Dm = mcomp mmon.

the formation of the IC complex only slightly affects the NOH group vibrations (see Table 2S). Neither of these two complexes was identified in the studied matrices. All bands II assigned to the 1:2 formaldoxime–ammonia complex are shifted in the same direction as bands I (toward lower or higher wavenumbers) with respect to the corresponding bands of ammonia or formaldoxime monomers, however they show larger shifts than the bands I (see Table 1). The band II attributed to the OH stretch is 786.6 cm1 red shifted with respect to mOH of CH2NOH monomer and the bands II due to the NH3 bending vibrations are 124.2, 108.1 cm1 blue shifted as compared to the band of NH3 monomer. The MP2/6–311++G(2d,2p) calculations performed for the CH2NOH (NH3)2 system identified four stationary points that are presented in Fig. 2. The selected bond distances (in Å) and interaction energies (in kJ mol1) are also presented. In Table 1S, Supporting material, the selected geometrical parameters for the four structures are collected and in Table 3S the calculated harmonic and anharmonic wavenumbers are shown. The most stable is the se1 ven-membered cyclic structure IIA (DECP ZPE ¼ 45:75 kJ mol ) in which ammonia dimer is interacting with the OH group and with the nitrogen atom of formaldoxime. The complex is stabilized by two hydrogen bonds between formaldoxime and ammonia: OAH N, NAH N and one NAH N bond between the two 1 ammonia molecules. The structures IIB (DECP ZPE ¼ 34:52 kJ mol ) 1 and IIC (DECP ¼ 32:60 kJ mol ) have comparable stabilities. In ZPE both structures one ammonia molecule is attached to the OH group of CH2NOH forming an OAH N bond whereas the second ammonia molecule serves as a proton donor toward the oxygen atom in IIB and toward the nitrogen atom in IIC. In the weakest IID complex 1 (DECP ZPE ¼ 13:13 kJ mol ) the ammonia dimer is attached to the oxygen atom of formaldoxime. The comparison of the calculated wavenumbers and wavenumbers shifts for the IIA, IIB, IIC and IID structures with those observed for the bands II shows that the CH2NOH (NH3)2 complex trapped in 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/Ar matrices can also be separated into two groups. The group I involves the bands at 3564.6, 1422.0, 1342.0, 1171.7, 963.7, 966.0, 910.7 cm1 which are relatively intense in the spectra of dilute matrices 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 cm1. They are very weak or are not observed in the spectra of dilute matrices but their intensities increase with HCl concentration and after matrix annealing. The relative intensities

of the bands belonging to groups I or II are constant within experimental error in all performed experiments. The above experimental data allowed us to assign the bands I and II to the CH2NOH HCl and CH2NOH (HCl)2 complexes, respectively. Moreover, in the spectra of matrices recorded after annealing weak bands at 3485.5, 1354.0 and 969.2 cm1 appeared which also grew after annealing but their relative intensities with respect to the bands II varied in the performed experiments. The bands are also tentatively assigned to CH2NOH (HCl)2 complexes of different structure than that producing the bands II. All wavenumbers identified for the bands belonging to groups I and II are collected in Table 2. In Fig. 3 the most representative regions of the spectra of the CH2NOH/HCl/Ar matrix recorded directly after matrix deposition and after annealing are presented; the region of HCl stretching vibrations is presented in Fig. 1S, Supporting material. All bands belonging to group I appear in the vicinity of the formaldoxime monomer absorptions and can be assigned with confidence to the perturbed CH2NOH vibrations. The largest perturbations are observed for the NOH group vibrations; mOH, dNOH and mNO are shifted by 56.1 cm1 and +37.8, +27.4 cm1, respectively, (see Table 2) with respect to the CH2NOH monomer absorptions. However, the shifts are much smaller than those observed for the corresponding CH2NOH vibrations in the CH2NOH NH3 complex, particularly for the OH stretch. Unfortunately, no absorption due to the hydrogen bonded HCl molecule was identified which may due to the band being broad and diffuse. The calculations performed at the MP2/6–311++G(2d,2p) level predict the stability of three structures, IA, IB and IC for the CH2NOH HCl complex. All optimized structures are shown in Fig. 4; their interaction energies and selected geometrical parameters are also presented. In Table 4S, Supplementary data, all calculated parameters for the optimized structures are listed and in Table 5S the calculated harmonic and anharmonic wavenumbers are collected. 1 In the most stable IA structure (DECP ZPE ¼ 15:85 kJ mol ) hydrogen chloride serves as a proton donor toward the nitrogen atom of CH2NOH forming N HACl bond. The N H distance is equal to 1.82 Å and the h(NHCl) angle is 159°. The non-linearity of the h(NHCl) angle suggests an additional very weak interaction between the chlorine atom of HCl and the OH group of formaldoxime. 1 In the IB structure (DECP ZPE ¼ 11:01 kJ mol ) hydrogen chloride is bonded to the oxygen atom of formaldoxime forming an O HACl hydrogen bond. In turn, in the less stable IC complex 1 (DECP ZPE ¼ 3:07 kJ mol ), the OH group of formaldoxime serves as a proton donor toward the chlorine atom of HCl and an OAH Cl bond is formed. The comparison of the observed wavenumbers and wavenumbers shifts with the calculated ones for the three structures (see


4


Fig. 1. Selected regions of the spectra of the NH3/Ar = 1/300 (a); CH2NOH/Ar (b) and CH2NOH/NH3/Ar matrices recorded directly after deposition and after annealing to 33 K for 10 min (c and d). The arrows indicate the bands due to the 1:1 (I) and 1:2 (II) complexes.

Tables 2 and 5S) indicates that the most stable IA complex is trapped in the matrix. The identified mOH wavenumber shows a distinct (but not large) red shift whereas the dNOH and mNO wavenumbers show blue shifts with respect to the corresponding CH2NOH vibrations, in accordance with the calculated shifts for the IA structure. In the IB complex the calculated dNOH and mNO wavenumbers exhibit red shifts whereas the perturbations of the IC complex vibrations are smaller than those observed for the complex trapped in the matrix. The calculations show, in accord with the experimental spectra, the distinct red shift of the OH stretch and blue shifts of the NOH bend and NO stretch. Such a spectral pattern suggests that in addition to a strong N HACl bond the

complex is stabilized by a very weak OAH Cl bond forming a cyclic structure. A similar structure was observed for the NH2OH HAF complex that was stabilized by a strong N HAF and much weaker OAH F bonds [29]. The bands II exhibit larger shifts with respect to the formaldoxime monomer bands than the absorptions I but the corresponding bands within the two groups are shifted in the same direction (toward higher or lower wavenumbers with respect to formaldoxime vibrations). This fact suggests that in the CH2NOH (HCl)2 complex, like in CH2NOH HCl, N HACl and OAH Cl hydrogen bonds are formed. The ab initio calculations at the MP2/6–311++G(2d,2p) level indicated four stationary points for the CH2NOH (HCl)2 system that are shown in Fig. 4. In the most stable seven-membered cyclic 1 structure IIA (DECP ZPE ¼ 30:05 kJ mol ) the nitrogen atom of formaldoxime serves as a proton acceptor for one hydrogen chloride subunit of the HCl dimer and the OH group acts as a proton donor toward a chlorine atom of the second HCl molecule. The N HACl and OAH Cl bonds in the 1:2 complex are stronger than in the 1:1 one as indicated by the shortening of the N H and H Cl distances in IIA as compared to IA (1.68 Å versus 1.82 Å for N H and 2.32 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 1 (DECP ZPE ¼ 24:95 kJ mol ) one HCl molecule acts as a proton donor for the nitrogen atom and as a weak proton acceptor for the OH group of formaldoxime. The second HCl molecule serves as a proton donor for the oxygen atom of formaldoxime. So, the complex is stabilized by two strong hydrogen bonds N HACl, O HACl and a very weak OAH Cl bond. In turn, in the IIC structure 1 (DECP ZPE ¼ 20:30 kJ mol ) one HCl molecule is attached to the nitrogen atom and a second one to the OH group which serves as a proton donor for the chlorine atom. In the least stable IID complex 1 (DECP ZPE ¼ 19:13 kJ mol ) two HCl molecules are bonded to an oxygen atom which serves as a double acceptor. The comparison of the bands II wavenumbers and wavenumber shifts with those calculated for the four structures leads to the conclusion that the bands II correspond to the IIA structure. The structures IIB and IID could be immediately rejected as their formation should lead to a red shift of the NO vibration whereas a blue shift is observed for the complex present in the matrix (see Table 6S). The patterns of wavenumber shifts for the IIA and IIC complexes are very similar to each other except for the HCl vibrations which are much more perturbed in IIA than in IIC. The more intense of the two perturbed HCl vibrations is calculated to exhibit a 927 cm1 red shift in IIA and a 547 cm1 red shift in IIC. Only one perturbed HCl vibration was tentatively identified for the 1:2 complex as a broad diffuse band at ca. 2080 cm1; the absorption exhibits a ca. 808 cm1 red shift with respect to the HCl monomer absorption. The position of this band and its shift with respect to the HCl monomer absorption do not give a definitive answer as to whether it corresponds to a complex of the IIA or IIC structure. However, the 808 cm1 red shift is much larger than that calculated for IIC and is close to the shift calculated for IIA. Taking this into account as well as the fact that IIA is the most stable structure we assigned bands II to a complex of IIA structure. The additional bands that were observed in the spectra of the studied matrices at 3485.5, 1354.0, 1190.7 and 969.2 cm1 most probably belong to the IIB complex.

Discussion The CH2NOH NH3 complex present in the matrix is stabilized by a strong OAH N(H3) hydrogen bond as evidenced by the large red shift, Dmexp = 684.5 cm1, of the OH stretch of the bonded formaldoxime molecule. The calculations indicate that the OAH N bond is almost linear (h(OHN) ffi 165°), the calculated


5


1 Fig. 2. The MP2/6–311++G(2d,2p) optimized structures of the CH2NOH NH3 and CH2NOH (NH3)2 complexes. The energy values, DECP ], are also presented. ZPE [kJ mol

Table 2 The identified wavenumbers for the CH2NOH HCl and CH2NOH (HCl)2 complexes and comparison of the observed and calculated anharmonic wavenumber shifts, Dma. Assignment

CH2NOH mOH dCH2 dHON qCH2 xCH2

mNO HCl mHCl a b c

Monomers

CH2NOH HCl

mexp.

mexp. group I

Dmexp.

Dmcalc. IA

mexp. group II

Dmexp.

Dmcalc. IIA

3620.7 1408.3 1314.2 1153.7 953.1

3564.6 1422.0 1342.0 1171.7 963.7 966.0 910.7

56.1 +11.7 +37.8 +18.0 +11.8

64 +11 +53 +13 2

3497.6 – 1357.9 1184.7 959.3

–123.1 – +43.7 +31.0 +6.2

162 +21 +52 +29 4

+27.4

+23

911.5

+28.2

+49

500 (1318)c

2080b

808

250 (626)c 927 (2241)c

886.4 880.2

CH2NOH (HCl)2

2888.0

Dm = mcomp mmon. Tentative assignment. The intensities (km mol1) are given in parentheses.

H N bond length equals 1.83 Å and the interaction energy, 1 DECP ZPE ¼ 26:22 kJ mol . The theoretical spectra generally agree well with the experimental ones, however they underestimate the shift of the OH stretch which is predicted as Dmcalc = 407 cm1 (for anharmonic wavenumbers). The discrepancy between the observed and calculated wavenumber shifts for the OH stretch may be due to limited accuracy of calculated anharmonic wavenumbers, however the effect of argon environment on the structural properties of the complex cannot be excluded. Earlier matrix isola-

tion studies performed for the ammonia complex with hydrogen halides showed extreme sensitivity of the proton stretching wavenumber of the H3N HX (X = Cl, Br, I) complexes to the matrix environment [14,30–36]. The CH2NOH (NH3)2 complex present in the matrix has a seven-membered cyclic structure; the OH group serves as a proton donor and the nitrogen atom of CH2NOH as an acceptor for the ammonia dimer. So, in addition to the NAH N bond between the two ammonia molecules, the complex is stabilized by the


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B. Golec et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2013) xxx–xxx 1

Fig. 3. Selected regions of the spectra of the HCl/Ar = 1/300 (a); CH2NOH/Ar (b) and CH2NOH/HCl/Ar matrices recorded directly after deposition and after annealing to 28 K and 33 K for 10 min (c–e). The arrows indicate the bands due to the 1:1 (I) and 1:2 (II) complexes. The dots indicate bands that are also tentatively assigned to the 1:2 complexes and the star denotes the band due to the HCl complex with water.

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 expected from the cooperative effect; the infrared spectra demonstrate a 786.6 cm1 shift for the OH stretching vibration in this complex. The calculations indicate that the OAH N(H3) bond is practically linear (h(OHN) ffi 176°), the calculated H N bond length, 1.77 Å, is 0.06 Å shorter than in the 1:1 complex. The H2NAH N bond, in which CH2NOH acts as a proton acceptor, deviates more from linearity (h(NHN) ffi 162°). The calculated H N distance, 2.16 Å, can be compared with that in the ammonia dimer, 2.14 Å, indicating comparable strength of the two NAH N hydrogen bonds in the complex. The interaction energy of IIA

equals DECP ZPE ¼ 45:75 kJ mol . In the IIB, IIC complexes one NH3 molecule is attached to the OH group as in IA and the second one to the acceptor site of CH2NOH, namely to the oxygen (IIB) or nitrogen atoms (IIC) like in the IB or IC, respectively. The interaction energies of IIB and IIC are equal DECP ZPE ¼ 34:52; 1 32:60 kJ mol , respectively. In the least stable complex, IID 1 (DECP ZPE ¼ 13:13 kJ mol ), the ammonia dimer is attached to an oxygen atom of formaldoxime. The calculated three-body cooperative effect equals ENA = 8.23, 0.21, 0.50 and 0.38 kJ mol1 for IIA, IIB, IIC and IID, respectively. The cooperativity leads to relatively strong stabilization of the trimer energy in IIA structure while for IIB, IIC and IID the effect is small. In the CH2NOH HCl complex present in the matrix the hydrogen chloride molecule acts as a proton donor toward the nitrogen atom and, simultaneously, as a weak proton acceptor for the OH group of formaldoxime, forming a strong N HACl and a weak OAH Cl bonds. Unfortunately, the mHCl vibration of the 1:1 complex was not identified; the calculations predict a 500 cm1 wavenumber shift of the HCl stretch. A weak OAH Cl interaction is reflected in a distinct perturbation of the NOH group vibrations of formaldoxime. The calculated interaction energy equals 15.85 kJ mol1. In the CH2NOH (HCl)2 complex the formaldoxime molecule interacts with the hydrogen chloride dimer forming a seven-membered cyclic ring. The complex is stabilised by N HACl, OAH Cl bonds between formaldoxime and hydrogen chloride, similar to those formed in the IA, IC 1:1 complexes, and by the Cl HACl bond of the HCl dimer. Due to the cooperative effect the N HACl and OAH Cl bonds become stronger in the 1:2 complex than in the 1:1 ones as indicated by larger perturbations of the formaldoxime vibrations. The calculations show a shortening of the N H distance from 1.82 Å in the 1:1 complex to 1.68 Å in the 1:2 complex and a shortening of the H Cl distance from 2.47 Å to 2.32 Å. The calculated interaction energy equals 30.05 kJ mol1. The calculated shift for mHCl of the hydrogen chloride molecule bonded to a nitrogen atom (927 cm1 for anharmonic wavenumbers) is slightly larger than the experimental shift for the tentatively identified perturbed hydrogen chloride stretching wavenumber (808 cm1). In the IIB, IIC complexes one of the two HCl molecules acts 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 it is bonded to the oxygen atom) and the role of proton acceptor in IIC (in which the chlorine atom acts as a proton acceptor for the OH group). In IID structure an oxygen atom of formaldoxime serves as a proton acceptor for two HCl molecules, for this complex 1 DECP ZPE ¼ 19:13 kJ mol . The ENA values for the hydrogen chloride complexes equal to 10.98, +1.63, 1.50 and +6.64 kJ mol1 for IIA, IIB, IIC and IID, respectively. They confirm a large percentage contribution of an attractive three body interaction energy to the cyclic IIA structure and small contribution to the IIC one. In contrast, in IIB and IID structures anti-cooperative effect occurs which is particularly strong in IID. Two hydrogen bonds donated to the same double acceptor, as is the case of IID structure, are known to be anticooperative [19]. In Table 3 the observed and calculated wavenumbers shifts (DmOHexp, DmOHcalc) for the OH stretching vibration in the CH2NOH NH3 complex are compared with the corresponding shifts for the ammonia complexes with various OH proton donors. With the exception of two complexes for which anharmonic wavenumbers were calculated (see Table 3) all other values correspond to harmonic calculations. The H2O NH3 and CH3OH NH3 complexes involve OAH N bonds of medium strength as indicated by the experimental and calculated DmOH values lying within the range 200–300 cm1 [37–40]. As illustrated in Table 3, the calculated DmOHcalc values are very close to the experimental ones for these two complexes. In turn, for CH2NOH NH3, OOH NH3 and



7

1 Fig. 4. The MP2/6–311++G(2d,2p) optimized structures of the CH2NOH HCl and CH2NOH (HCl)2 complexes. The energy values, DECP ], are also presented. ZPE [kJ mol

Table 3 Comparison of the observed wavenumbers and the observed and calculated wavenumber shifts for the OH stretching vibration in ammonia complexes with various OH group proton donors. Experimental

m

D m

Ref.

D m

Ref.

H2O

3434.9

203.1

[37]

CH3OH H2NOH CH2NOH HOO

3400.7 3290 2936.2 2654.4

[40] [27] This work [42]

[38] [39] [40] [41] This work [44]

HONO

2765.2 2737.5 1870 1400

266.1 345 684.5 758.6 705.3 818.7b

190.6 238a 273.4 279 407a 559.1

[43]

613.5

[43]

1650 2100

[45] [47]

925.3 1170a 1398.8 1447

[46] [47] [46] [48]

HNO3 H2SO4

a b

Calculated

Calculated anharmonic wavenumbers. Calculated with respect to the centre of the m(OH) doublet.

ONOH NH3, the observed wavenumbers shifts are in the range 650–850 cm1 indicating the presence of strong OAH N bonds in these complexes [42,43]. For all of the three complexes the calculated value, DmOHcalc, is much lower than the experimental one, DmOHexp (407, 559.1, 613.5 cm1 versus 684.5, 705.3, 818.7 cm1 for the CH2NOH, HOO and HONO complexes, respectively) [42– 44]. The difference between DmOHexp and DmOHcalc becomes even larger for the strongest ammonia complexes with nitric and sulfuric acids. The experiments demonstrated a ca. 1650 cm1 red shift for the ammonia complex with nitric acid [45] whereas the calculations result in a 925.3 cm1 shift [46]. In turn, the experimental DmOHexp value for the complex with sulfuric acid is as large as

2100 cm1 [47] whereas the calculations predicted only ca. 1170, 1398.8 or 1447 cm1 shifts [46–48]. This comparison raises the question whether the discrepancies between the experimental and calculated wavenumbers are due to inaccurate calculations of anharmonic effects, to the effect of matrix environment on the structural and vibrational properties of the complex or to both factors. Further studies are needed to explain this phenomenon. In the context of the above discussion it is worth to note that, in contrast with the CH2NOH NH3 complex, for the CH2NOH (NH3)2 one the calculated DmOHcalc value (732 cm1) is quite close to the experimental one (786.6 cm1). The wavenumbers of the OH stretching and NOH bending frequencies (2834.1, 1381.2 cm1, respectively) of CH2NOH (NH3)2 indicate that in this case a Fermi resonance interaction may occur between OH stretching fundamental (2834.1 cm1) and a first overtone of the NOH bending vibration whose wavenumber is close to the OH stretching fundamental (21381.2-j, j – anharmonic correction). Fermi resonance will affect the position of the band due to the OH stretching vibration. In the case of the CH2NOH NH3 complex the Fermi resonance interaction is very weak or does not exists as the m(OH) fundamental and 2d(NOH) overtone lie quite apart from each other (2936.2, 21370.4-j). Conclusions 1:1 and 1:2 complexes between formaldoxime and ammonia or hydrogen chloride have been identified in CH2NOH/NH3/Ar, CH2NOH/HCl/Ar matrices, respectively; their structures were determined on the basis of MP2/6–311++G(2d,2p) calculations. In the 1:1 complexes ammonia is attached to the OH group and hydrogen chloride to the nitrogen atom of formaldoxime. In the 1:2 complexes the ammonia or hydrogen chloride dimers interact both with the OH group as a proton donor and the nitrogen atom as


8


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