Complexes of Glycolic Acid with Nitrogen Isolated in Argon Matrices. I : Structures and Thermal Effects

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Complexes of Glycolic Acid with Nitrogen Isolated in Argon Matrices. I : Structures and Thermal Effects © 2019 by the authors. Licensee MDPI, Basel, Switzerland. Published version Kosendiak, Iwona; Ahokas, Jussi M.E.; Krupa, Justyna; Lundell, Jan; Wierzejewska, Maria Kosendiak, I., Ahokas, J. M., Krupa, J., Lundell, J., & Wierzejewska, M. (2019). Complexes of Glycolic Acid with Nitrogen Isolated in Argon Matrices. I : Structures and Thermal Effects. Molecules, 24(18), Article 3262. https://doi.org/10.3390/molecules24183262 2019 molecules Article Complexes of Glycolic Acid with Nitrogen Isolated in Argon Matrices. I. Structures and Thermal Effects Iwona Kosendiak 1, Jussi M.E. Ahokas 2, Justyna Krupa 1, Jan Lundell 2,* and Maria Wierzejewska 1,* 1Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14, 50-383 Wroclaw, Poland 2Department of Chemistry, University of Jyvaskyla, P.O.Box 35, 40014 Jyvaskyla, Finland *Correspondence: [email protected] (J.L.); [email protected] (M.W.); Tel.: +358-40-744-5270 (J.L.); +48-71-375-7332 (M.W.) Received: 29 July 2019; Accepted: 5 September 2019; Published: 7 September 2019   Abstract: Molecular complexes between glycolic acid and nitrogen were studied in a low-temperature argon matrix with FTIR spectroscopy, and supported by MP2 and BLYPD3 calculations. The calculations indicate 11 and 10 stable complex structures at the MP2 and BLYPD3 levels of theories, respectively. However, only one hydrogen-bonded complex structure involving the most stable SSC conformer of glycolic acid was found experimentally, where the nitrogen molecule is bound with the carboxylic OH group of the SSC conformer. The complex shows a rich site structure variation upon deposition of the matrix in different temperatures and upon annealing experiments, which provide interesting prospects for site-selective chemistry. Keywords: hydrogen bond; matrix isolation; carboxylic acid; computational chemistry; vibrational spectroscopy 1. Introduction Hydrogen bonding is a significant non-covalent interaction that plays an important role in many areas of biology and chemistry [ 1 , 2 ]. Weakly bound molecular complexes with hydrogen bond or van der Waals interactions are frequently encountered in atmospheric chemistry and are known to affect both chemical and photochemical processes [ 3 – 5 ]. In complexes, the electronic, vibrational and rotational levels are disturbed as a result of interaction between complex subunits, and this leads to changes in spectral and photochemical characteristics of the complex components. On the other hand, weakly bound complexes frequently represent shallow energy minima on their potential energy surfaces, which lead to thermodynamic instability of such species at room temperature. From this point of view, the low temperature matrix isolation technique coupled with different spectroscopic methods is a very useful technique to trap and to study weakly bound complexes [ 6 , 7 ]. Recently, it was demonstrated that matrix isolation technique connected with computational studies is a powerful approach to study hydrogen bonds and other intermolecular interactions in complexes involving atmospheric constituents or species related to tropospheric IR-induced chemistry [8–14]. Glycolic acid (GA), the simplest α -hydroxycarboxylic acid with two OH groups, is capable of forming both intraand intermolecular hydrogen bonds. As such, GA is a model species to study in order to understand how competing OH-groups in the molecule affect the chemical reactivity and capability to form molecular complexes. Several reports on monomeric GA isolated in low-temperature matrices as well as its transformations have appeared in the literature [ 15 – 20 ]. The structures of the three most stable GA species are presented in Figure 1. Among the monomer structures, the most stable conformer is the SSC form, which is stabilized by the O–H · · · O intramolecular hydrogen bond. This conformer is present in the gas phase and in low-temperature matrices with an estimated population Molecules 2019,24, 3262; doi:10.3390/molecules24183262 www.mdpi.com/journal/molecules Molecules 2019,24, 3262 2 of 13 at room temperature of ca. 95%. A small amount of two less stable GAC and AAT conformers has also been detected [19,20]. Molecules 2019, 24, x FOR PEER REVIEW 2 of 13 intramolecular hydrogen bond. This conformer is present in the gas phase and in low-temperature matrices with an estimated population at room temperature of ca. 95%. A small amount of two less stable GAC and AAT conformers has also been detected [19,20]. Figure 1. Structures of the three most stable conformers of glycolic acid monomer. Contrary to the monomeric GA, only two studies on GA complexes in low-temperature matrices have appeared in literature. Recently, a study on GA dimers revealed for the first time information on three cyclic GA dimers identified in an argon matrix [21]. All of these dimers were formed between SSC conformers: one dimer with hydrogen bonds between the two carboxylic OH groups, one structure with hydrogen bonds between the two alcoholic OH groups and a mixed structure with one OH group from the carboxylic group acting as proton donor and the alcoholic OH group on the other subunit acting as proton acceptor. Molecular complexes between GA and molecular nitrogen were identified in a study applying Raman spectroscopy for argon-trapped species upon high vibrational excitation experiments [22]. In that study, nitrogen was found to form complexes with two GA conformers (SSC and AAT) in low temperature argon environment. However, this study did not scrutinise the actual molecular complex structures involved but only made conclusions of different GA…N2 systems involved based on their behaviour upon 532 nm irradiation. These Raman spectroscopy combined with visible light irradiation experiments acted as a prelude to our study presented here. In this paper, we present results of a combined study employing both theoretical methods and FTIR matrix isolation studies on complexes formed between glycolic acid and nitrogen molecule. The target is to study and to identify the 1:1 complexes GA forms with molecular nitrogen. This research is important in order to understand which type of complexes can be formed, if such species are experimentally detected, and, as these formed complexes are used as precursors for near-infrared irradiation experiments, if their photo-induced chemistry differ from isolated GA monomers. The study on the NIR irradiated GA-N2 complexes is reported separately in a following paper taking advantage of the results presented here. 2. Experimental and Computational Details The matrix samples were prepared by passing mixtures of high purity argon (Messer, 5.0) and nitrogen (Messer, 6.0) with the N2/Ar ratios of 1/4000 through the glass U-tube with glycolic acid (GA) situated outside the cryostat chamber. Optimizing the deposition temperature and matrix flow rate it was possible to obtain matrices containing nearly exclusively monomeric GA and GA…N2 complexes of 1:1 stoichiometry. The GA/N2/Ar gaseous mixtures were deposited onto a cold CsI window kept at 15 K or 18 K in an APD-Cryogenics (ARS-2HW) closed cycle cryostat. Annealing experiments were performed upon the deposited samples at 33 K. The sample temperature was maintained by a Scientific Instruments 9700 temperature controller equipped with a silicon diode and a resistive heater. FTIR spectra were collected at 10 K in a transmission mode with a 0.5 cm−1 resolution using a Bruker IFS 66 Fourier Transform spectrometer equipped with a liquid N2 cooled MCT detector. In order to support the experimental work, computational studies were carried out using the Gaussian16 program package [23]. The calculations were performed for GA:N2 1:1 complexes for the three most stable conformers (SSC, GAC, AAT) at the MP2 [24–27] and B3LYPD3 [28–32] levels of theory using the 6-311++G(2d,2p) basis set. All geometry optimizations were performed with the Figure 1. Structures of the three most stable conformers of glycolic acid monomer. Contrary to the monomeric GA, only two studies on GA complexes in low-temperature matrices have appeared in literature. Recently, a study on GA dimers revealed for the first time information on three cyclic GA dimers identified in an argon matrix [ 21 ]. All of these dimers were formed between SSC conformers: one dimer with hydrogen bonds between the two carboxylic OH groups, one structure with hydrogen bonds between the two alcoholic OH groups and a mixed structure with one OH group from the carboxylic group acting as proton donor and the alcoholic OH group on the other subunit acting as proton acceptor. Molecular complexes between GA and molecular nitrogen were identified in a study applying Raman spectroscopy for argon-trapped species upon high vibrational excitation experiments [ 22 ]. In that study, nitrogen was found to form complexes with two GA conformers (SSC and AAT) in low temperature argon environment. However, this study did not scrutinise the actual molecular complex structures involved but only made conclusions of different GA · · · N 2 systems involved based on their behaviour upon 532 nm irradiation. These Raman spectroscopy combined with visible light irradiation experiments acted as a prelude to our study presented here. In this paper, we present results of a combined study employing both theoretical methods and FTIR matrix isolation studies on complexes formed between glycolic acid and nitrogen molecule. The target is to study and to identify the 1:1 complexes GA forms with molecular nitrogen. This research is important in order to understand which type of complexes can be formed, if such species are experimentally detected, and, as these formed complexes are used as precursors for near-infrared irradiation experiments, if their photo-induced chemistry differ from isolated GA monomers. The study on the NIR irradiated GA-N 2 complexes is reported separately in a following paper taking advantage of the results presented here. 2. Experimental and Computational Details The matrix samples were prepared by passing mixtures of high purity argon (Messer, 5.0) and nitrogen (Messer, 6.0) with the N 2 /Ar ratios of 1/4000 through the glass U-tube with glycolic acid (GA) situated outside the cryostat chamber. Optimizing the deposition temperature and matrix flow rate it was possible to obtain matrices containing nearly exclusively monomeric GA and GA · · · N 2 complexes of 1:1 stoichiometry. The GA/N 2 /Ar gaseous mixtures were deposited onto a cold CsI window kept at 15 K or 18 K in an APD-Cryogenics (ARS-2HW) closed cycle cryostat. Annealing experiments were performed upon the deposited samples at 33 K. The sample temperature was maintained by a Scientific Instruments 9700 temperature controller equipped with a silicon diode and a resistive heater. FTIR spectra were collected at 10 K in a transmission mode with a 0.5 cm −1 resolution using a Bruker IFS 66 Fourier Transform spectrometer equipped with a liquid N2cooled MCT detector. In order to support the experimental work, computational studies were carried out using the Gaussian16 program package [ 23 ]. The calculations were performed for GA:N 2 1:1 complexes for the three most stable conformers (SSC, GAC, AAT) at the MP2 [ 24 – 27 ] and B3LYPD3 [ 28 – 32 ] levels of theory using the 6-311++G(2d,2p) basis set. All geometry optimizations were performed with the Boys-Bernardi full counterpoise method by Dannenberg [ 33 , 34 ]. The topological analysis of the electron density (Atoms-In-Molecules, AIM [ 35 ]) was performed at the MP2/6-311++G(2d,2p) level Molecules 2019,24, 3262 3 of 13 using AIM studio program (Version 12.09.23, Standard) [ 36 ]. The harmonic vibrational wavenumbers and infrared intensities were calculated both at MP2 and B3LYPD3 levels for the optimised structures to assist the analysis of the experimental spectra. The computed spectra were used to verify that computed complex structures were stable structures. Interpretation of the infrared spectra is based mainly on the MP2 results and this data is presented in the text. 3. Results and Discussion 3.1. Structure and Energetics of 1:1 GA· · · N2Complexes Out of seven theoretically predicted conformers of glycolic acid monomer (GA) the three most stable forms, SCC, GAC and AAT, were detected in low-temperature noble gas matrices [ 19 , 20 ]. The fourth SST conformer was identified only in nitrogen matrices upon near-IR excitation of the most stable SSC conformer [ 19 ]. Therefore, exclusive interaction of the SSC, GAC and AAT conformers with nitrogen are considered here since these conformers of monomeric GA are the most plausible to appear in a solid argon environment. At the MP2/6-311++G(2d,2p) level of theory 11 energy minima structures were found for 1:1 GA-N 2 complexes whereas the calculations performed at the B3LYPD3 level showed the presence of only ten energy minima structures. All optimized GA · · · N 2 complex structures are shown in Figure 2. The coordinates of all optimised complexes are presented in Table S1 (Supplementary Materials Information). Molecules 2019, 24, x FOR PEER REVIEW 3 of 13 Boys-Bernardi full counterpoise method by Dannenberg [33,34]. The topological analysis of the electron density (Atoms-In-Molecules, AIM [35]) was performed at the MP2/6-311++G(2d,2p) level using AIM studio program (Version 12.09.23, Standard) [36]. The harmonic vibrational wavenumbers and infrared intensities were calculated both at MP2 and B3LYPD3 levels for the optimised structures to assist the analysis of the experimental spectra. The computed spectra were used to verify that computed complex structures were stable structures. Interpretation of the infrared spectra is based mainly on the MP2 results and this data is presented in the text. 3. Results and Discussion 3.1. Structure and Energetics of 1:1 GA…N2 Complexes Out of seven theoretically predicted conformers of glycolic acid monomer (GA) the three most stable forms, SCC, GAC and AAT, were detected in low-temperature noble gas matrices [19,20]. The fourth SST conformer was identified only in nitrogen matrices upon near-IR excitation of the most stable SSC conformer [19]. Therefore, exclusive interaction of the SSC, GAC and AAT conformers with nitrogen are considered here since these conformers of monomeric GA are the most plausible to appear in a solid argon environment. At the MP2/6-311++G(2d,2p) level of theory 11 energy minima structures were found for 1:1 GAN2 complexes whereas the calculations performed at the B3LYPD3 level showed the presence of only ten energy minima structures. All optimized GA…N2 complex structures are shown in Figure 2. The coordinates of all optimised complexes are presented in Table S1 (Supplementary Materials Information). Figure 2. The MP2-optimized 1:1 structures of SSC, AAT and GAC complexes with molecular nitrogen. Figure 2. The MP2-optimized 1:1 structures of SSC, AAT and GAC complexes with molecular nitrogen. The AIM approach based on the topological analysis of electron density becomes nowadays almost routine method providing the characteristics of various types of interactions, among them hydrogen bonds and van der Waals interactions [ 37 – 39 ]. Two AIM parameters: the electron density Molecules 2019,24, 3262 4 of 13 ρ (r) and its Laplacian ∇2ρ (r) at bond critical points (BCP) obtained for all optimized 1:1 structures at MP2/6-311++G(2d,2p), using the MP2 computed density matrix, are collected in Table 1. Additionally, the positions of the bond (3, − 1) critical points derived from the AIM calculations are visualised together with the optimised complex structures in Figure 2. Table 1. Interatomic distances (Å), angles (degree) and electron density parameters of the bond (au) of the SSC, GAC and AAT complexes with N2(1:1) computed at the MP2/6-311++G(2d,2p) level. Complex Intermolecular Parameters AIM Parameters Interatomic Distances Angle BCP ρ(r) ∇2ρ(r) H···Y X···Y X–H···Y SSC1 2.257 3.226 179.0 H7· · · N10 0.014 0.053 SSC2 2.475 3.363 152.8 H9···N10 0.009 0.037 SSC3 3.358 C1···N10 0.006 0.024 3.280 C4···N10 0.006 0.024 Ring critical point 0.006 0.025 GAC1 2.262 3.230 178.1 H7· · · N10 0.014 0.053 GAC2 2.363 3.304 166.4 H9· · · N10 0.012 0.045 3.410 O6···N10 0.004 0.016 Ring critical point 0.004 0.016 GAC3 3.249 O8···N10 0.006 0.022 3.252 C4···N10 0.006 0.026 Ring critical point 0.005 0.025 GAC4 3.323 O5···N10 0.006 0.023 2.971 3.684 123.1 H2···N10 0.005 0.017 Ring critical point 0.005 0.018 GAC5 2.832 3.521 121.5 H3· · · N10 0.004 0.016 3.188 O8···N10 0.005 0.018 Ring critical point 0.004 0.018 AAT1 2.459 3.212 134.2 H7· · · N10 0.010 0.040 1.950 2.584 120.7 H7···O8 0.027 0.117 Ring critical point 0.025 0.143 AAT2 2.547 3.287 133.9 H9· · · N10 0.008 0.030 1.985 2.640 122.7 H7···O8 0.028 0.108 Ring critical point 0.026 0.137 AAT3 1.922 2.575 122.2 H7···O8 0.029 0.122 3.184 C4···N10 0.006 0.027 Ring critical point 0.026 0.151 For interaction of SSC with nitrogen three minima were found and two of them contain the O–H · · · N hydrogen bond formed between carboxylic or alcoholic OH group and N 2 molecule (species denoted SSC1 and SSC2, respectively). In both of these configurations, the N 2 molecule lies in the SSC plane. The intermolecular H · · · N distance equals 2.257 and 2.475 Å in SSC1 and SSC2, respectively. Interaction with the carboxylic OH group results in an almost linear hydrogen bond with the Θ (O6H7N10) angle of 179.0 ◦ whereas in SSC2 the O–H · · · N bridge is more bent to Θ (O8H9N10) = 152.8 ◦ . In the third SSC-N 2 complex (SSC3) the N 2 molecule is located above the SSC molecular plane with the N10 atom directed to the centre of the C–C bond. The SSC3 complex also exhibit relatively short N10 · · · C1 and N10 · · · C4 atomic distances of 3.358 and 3.280 Å, respectively. Based on the AIM analysis two van der Waals interaction paths are found between N10 nitrogen atom and C1 or C4 carbon atoms. Two hydrogen bonded complexes, analogous to those optimized for SSC · · · N 2 were found for both GAC · · · N 2 and AAT · · · N 2 interactions. The intermolecular H · · · N distances are in the range Molecules 2019,24, 3262 5 of 13 of 2.262–2.547 Å. Both GAC1 and GAC2 contain almost linear O–H · · · N bridges (178 and 166 ◦ , respectively) whereas in AAT1 and AAT2 the hydrogen bond bridges are not linear (in both cases the O–H · · · N angle equals 134 ◦ ). Similarly to the SSC conformer one non-hydrogen bonded structure (AAT3) was found for AAT. It is characterised by one van der Waals contact between N10 and C1 atoms. For GAC · · · N 2 complexes three non-hydrogen bonded minima were found (GAC3, GAC4 and GAC5) but none of them corresponds strictly to the structural configurations found for SSC3 or AAT3. Two of these species are characterised by the C–H · · · N contact and the N10 · · · O5 (GAC4) or N10 · · · O8 (GAC5) interaction. In the third structure (GAC3) two van der Waals contacts are present between N10 · · · O8 and N10 · · · C4. We do recognise here that the AIM method does not always result to bond paths with chemical meaning [ 40 , 41 ]. However, here we use the method to give a brief indication how the complex is built up, and how the nitrogen is connected with the GA subunit, even though some of the AIM bond path charts are very complex and can induce multiple interpretation of the nature of the interaction. The calculated interaction and relative energies of these species optimised at the MP2 and B3LYPD3 levels are summarised in Table 2. Table 2. Computed interaction energies and relative energies compared to global minimum SSC1 (in kJ mol−1). Structure Interaction Energy Relative Energy MP2 B3LYPD3 MP2 B3LYPD3 SSC1 −7.70 −8.41 0.00 0.00 SSC2 −4.48 −5.15 3.23 3.27 SSC3 −5.40 −5.77 2.35 2.63 GAC1 −7.53 −8.28 10.70 10.49 GAC2 −5.61 −6.23 12.80 12.68 GAC3 −5.02 −5.31 13.28 13.46 GAC4 −4.48 −4.94 13.75 13.78 GAC5 −4.02 - 14.21 - AAT1 −4.85 −5.56 15.71 16.23 AAT2 −9.12 −9.71 12.19 12.34 AAT3 −5.40 −5.73 15.08 15.91 The most stable structure overall in energy is the SSC1 complex. The other two SSC complexes are about 3 kJ mol −1 higher in energy. The GAC and AAT complexes with nitrogen molecule are about 10–14 and 12–15 kJ mol −1 higher in energy than the complexes of the most stable SSC conformer. These energy differences correspond to the energy differences found for the isolated GA conformers without zero-point nor entropy corrections. The largest interaction energy among all considered GA · · · N 2 structures is found for the AAT2 complex, which equals to − 9.12 kJ mol −1 . The interaction in AAT2 is characterised by the alcoholic O–H group acting as a proton donor to nitrogen molecule. As can be seen in Table 2the interaction energies of all other GA complexes are in the range between − 4.02 to − 9.12 kJ mol −1 . Interestingly, both GAC2 and AAT2 complexes with alcoholic OH group interacting with N 2 are characterised by larger interaction energy than that found for the SSC2 form. The GAC and AAT conformers of the GA monomer were found to be about 9.5 and 12.0 kJ mol −1 less stable than SSC whereas, for interaction of N 2 with the OH alcoholic group, the situation is reversed. Such an observation was described for glyoxylic acid–water complexes [10] as well. Looking at the AIM results performed for GAC2 the above situation can be related to the presence of a second weak van der Waals interaction between nitrogen molecule and oxygen atom of the carboxylic OH group. In turn, according to the AIM calculations, the intramolecular hydrogen bond, existing already in monomeric AAT, formed between the carboxylic OH and oxygen of the alcoholic OH becomes stronger in AAT2 complex contributing to its overall stability. In general, the formation of Molecules 2019,24, 3262 6 of 13 an additional van der Waals contact with either of the two OH groups of the GA subunit strengthens the intermolecular interaction. When no such additional interaction appears, a typical hydrogen-bonded structure as found in SSC1 and GAC1 complexes is prevalent in analogy with previous carboxylic acid complexes with nitrogen molecules [42,43]. 3.2. Computed Spectra Infrared and Raman spectra were calculated for all the computationally identified complex structures (see Figure 2). The infrared spectra were used to verify that the optimised structures were stable minima on the potential energy surfaces. Additionally, comparison of complex and monomer spectra is useful for understanding the experimental findings. Raman spectra were also computed, and Raman intensities (I Raman ) were derived from calculated scattering activities values according to the procedure described by Michalska and co-workers [ 44 , 45 ] employing T =14 K for the Boltzmann distribution factor and excitation frequency of 532 nm (18 797 cm −1 ). These values correspond to the experimental conditions used in our previous work on GA · · · N 2 [ 22 ]. The computational results obtained at the MP2-level of theory are shown in Table 3for the most stable SSC1 complex. For comparison, the observed Raman bands have been included in Table 3. Table 3. Computed band positions for the most stable SSC · · · N 2 complex (SSC1) compared to the experimental Raman band positions (cm −1 ) together with computed infrared (km mol −1 ) and Raman intensities. Mode Band Position IIR IRaman Raman Exp [22] ν13781 79 477 ν23754 302 1880 3562, 3554, 3545 ν33130 7 1202 ν43087 25 2536 ν52175 1 674 ν61789 246 739 1777, 1775 ν71518 12 509 ν81492 2 106 ν91372 124 202 ν10 1315 33 134 ν11 1271 0 355 ν12 1190 161 175 ν13 1112 226 168 ν14 1053 1 13 ν15 878 28 1107 ν16 689 108 16 ν17 653 18 518 ν18 540 1 204 ν19 480 18 528 ν20 340 77 7 ν21 287 12 32 ν22 102 0 1504 ν23 92 8 1787 ν24 80 5 757 ν25 79 2 224 ν26 23 0 387 ν27 18 1 2940 The two OH vibrational bands for SSC1 are the alcoholic and carboxylic OH stretching vibrations at 3781 and 3754 cm −1 , respectively. Another significant vibrational mode is the carbonyl stretching mode ( ν6 ) that was computed to be at 1789 cm −1 . Both of these spectral regions have been probed in the Raman experiments, and GA · · · N 2 complexes have been identified. In order to make assignment of the complex structures appearing in the experiments, infrared and computational data is needed. Here, we Molecules 2019,24, 3262 7 of 13 consider all three SSC complexes found computationally in connection with annealing experiments in solid argon. These results are discussed below that shed light on the complex structures also observed in the Raman experiments [22]. 3.3. Experimental Results At first, blank experiments were performed for GA isolated in solid argon. The deposition of GA at 15 K yielded matrices containing almost exclusively monomers of the acid. The strong bands of the O–H and the C=O stretching vibrations of the SSC conformer were observed at 3561 and 1773 cm −1 . Much weaker absorptions of the two less abundant AAT and GAC conformers were barely observed at 3671, 3473 and 1806 cm −1 and 3648, 3561 and 1784 cm −1 for AAT and GAC, respectively, in accordance with the predicted gas phase abundances. When the deposition temperature was higher than 15 K or when the matrix with GA monomers was annealed a number of new weak bands appeared due to the GA dimers [21]. Complex formation between glycolic acid and nitrogen was observed when GA was deposited together with the N 2 doped argon with the N 2 :Ar ratio of 1:4000 or higher. New bands due to the N 2 complexes with GA appeared in several spectral regions in the vicinity of the GA monomer absorptions. Table 4contains the most characteristic wavenumber shifts ( ∆ν = νcomplex −νmonomer ) calculated for the 1:1 species of interest compared to the experimental values. From now on the notation OH C and OHAdenotes carboxylic and alcoholic OH group, respectively. Table 4. MP2/6-311++G(2d,2p) calculated wavenumber shifts ∆ν (cm −1 ) and intensities (km mol −1 ) of the SSC complexes awith nitrogen compared to the corresponding experimental shifts. MP2/6-311++G(2d,2p) Experimental Shifts Assignment SSC1 SSC2 SSC3 * ∆νI∆νI∆νI∆ν −5 79 3 119 0 32 −4 (ν1)νOHA −31 302 1 117 0 140 −11.5, −14.5, −19.0/−21.0 (ν2)νOHC −5 246 1 274 −1 249 2, −3 (ν6)νC=O 10 124 −4 126 1 123 12, 6 (ν9)δOHA+δOHC+νC–OC 15 161 −2 126 0 139 14, 13, 8, 5, 3 (ν12)δOHC+νC–OC+ωCH2 4 28 −1 30 1 28 6, 4, 2 (ν15)νC–C +νC–OC a CorrespondingpositionsfortheSSCmonomerare: 3786,3785, 1794,1362, 1175and874cm −1 (MP2/6-311++G(2d,2p)) and 3561, 3561, 1773, 1332, 1143 and 854 cm −1 (Ar matrix [ 19 ]). * In SSC3 the ν OH C and ν OH A vibrations are coupled. Figure 3shows the stretching ν OH, ν C=O and in-plane-deformation δ OH C regions of the infrared spectra of GA/N 2 /Ar matrices deposited at three different temperatures and compared to the GA/Ar spectrum. In the ν OH stretching region of the spectrum of GA co-deposited at 15 K with the N 2 /Ar =1/4000 mixture several new maxima due to the GA · · · N 2 complexes are distinguished. They are situated below the ν OH of the SSC conformer at 3549.5, 3546.5 and 3542.0/3540.0 (a doublet) cm −1 . Additionally, a broadening on the lower wavenumber side of the ν OH of the SSC monomer was found at 3556 cm −1 (at 15 K seen as a weak shoulder and better shaped at 18 and 25 K). The same set of bands (being much weaker) was observed in the ν OH region when GA was deposited with an extremely diluted N 2 /Ar (in the case when small leak in the vacuum system was present). This indicates that all mentioned spectral features are due to the 1:1 GA· · · N2species. Molecules 2019,24, 3262 8 of 13 Molecules 2019, 24, x FOR PEER REVIEW 8 of 13 found at 3556 cm−1 (at 15 K seen as a weak shoulder and better shaped at 18 and 25 K). The same set of bands (being much weaker) was observed in the νOH region when GA was deposited with an extremely diluted N2/Ar (in the case when small leak in the vacuum system was present). This indicates that all mentioned spectral features are due to the 1:1 GA…N2 species. Figure 3. The νOH, δOHC and νC=O regions of the infrared spectra of GA co-deposited with N2/Ar = 1/4000 at 15 K, 18 K and 25 K (measurement at 10 K) (traces (b–d), respectively, compared with GA/Ar spectrum (a). As shown in Figure 3, when matrices were deposited at 15 K and 18 K the most intense of all new bands present in the νOHC region was a doublet at 3542.0/3540.0 cm−1. Each of the bands observed in the νOHC region has its counterpart in the in-plane-bending δOHC region. Here, in the region of 1140–1160 cm−1, where we find the SSC monomer band at 1143.5 cm−1, a new set of bands due to the GA…N2 complex appeared at higher wavenumbers. A doublet in the νOHC region at 3542.0/3540.0 cm−1 and a weak band at 3557.0 cm−1 (shifted by −19/−21 cm−1 and −4 cm−1) fit well to the calculated positions of the stretching vibrations of carboxylic and alcoholic groups, respectively, in the most stable SSC1 complex. The calculated ΔνOH shifts in SSC1 form equal to −31 and −5 cm−1 for the hydrogen bonded carboxylic hydroxyl group and for non-hydrogen bonded alcoholic OH, respectively. Similar agreement was also found in other spectral regions as presented in Table 4. For the in-plane-deformation δOHC mode the most intense doublet in the spectra of matrices deposited at 15 and 18K at 1157.0/1155.5 cm−1 is blue-shifted by 13.5 and 12 cm−1 relative to the corresponding monomer absorption. This is also in a very good agreement with the predicted shift of 15 cm−1. Both in νOHC and δOHC regions additional bands are present, and they are slightly shifted compared with those already assigned above. There are several possible reasons for the observed additional absorptions in the spectra. One of them is that there exists different structures of SSC…N2 Figure 3. The ν OH, δ OH C and ν C=O regions of the infrared spectra of GA co-deposited with N 2 /Ar = 1/4000 at 15 K, 18 K and 25 K (measurement at 10 K) (traces ( b – d ), respectively, compared with GA/Ar spectrum (a). As shown in Figure 3, when matrices were deposited at 15 K and 18 K the most intense of all new bands present in the ν OH C region was a doublet at 3542.0/3540.0 cm −1 . Each of the bands observed in the ν OH C region has its counterpart in the in-plane-bending δ OH C region. Here, in the region of 1140–1160 cm −1 , where we find the SSC monomer band at 1143.5 cm −1 , a new set of bands due to the GA · · · N 2 complex appeared at higher wavenumbers. A doublet in the ν OH C region at 3542.0/3540.0 cm −1 and a weak band at 3557.0 cm −1 (shifted by − 19/ − 21 cm −1 and − 4 cm −1 ) fit well to the calculated positions of the stretching vibrations of carboxylic and alcoholic groups, respectively, in the most stable SSC1 complex. The calculated ∆ν OH shifts in SSC1 form equal to − 31 and − 5 cm −1 for the hydrogen bonded carboxylic hydroxyl group and for non-hydrogen bonded alcoholic OH, respectively. Similar agreement was also found in other spectral regions as presented in Table 4. For the in-plane-deformation δ OH C mode the most intense doublet in the spectra of matrices deposited at 15 and 18K at 1157.0/1155.5 cm −1 is blue-shifted by 13.5 and 12 cm −1 relative to the corresponding monomer absorption. This is also in a very good agreement with the predicted shift of 15 cm−1. Both in ν OH C and δ OH C regions additional bands are present, and they are slightly shifted compared with those already assigned above. There are several possible reasons for the observed additional absorptions in the spectra. One of them is that there exists different structures of SSC · · · N 2 complexes. As shown in Table 4the ν OH shifts predicted for SSC2 and SSC3 complexes do not fit to any of the observed bands. This indicates that there is only one type of complex in the experimental conditions used instead of three different SSC complexes identified computationally. Consequently, only the SSC1 structure is to be formed in the argon matrix, and another explanation should be found for the additional bands in the ν OH C and δ OH C regions. The most obvious explanation is the presence of multiple trapping sites in the matrix, which can slightly perturb the structures of the complexes. For the SSC1 structure, the position of the N 2 molecule relative to the OH group of the carboxylic moiety