FTIR matrix isolation and theoretical studies of glycolic acid dimers

Glycolic acid (GA) dimers were studied in low temp erature argon matrices by means of FTIR spectroscopy. Experimentally, the dimers we re produced when monomeric glycolic acid molecules were thermally mobilized upon anneal i g of argon matrices at 25-35 K. The experimental spectra observed upon annealing indica te the presence of three different dimer structures. Computationally, MP2 and DFT calculatio ns were used to study the potential dimer species in order to scrutinize the possible d imer structures, their energetics and their spectral features. Altogether 27 local minima were found for dimer structures for the three lowest conformers of glycolic acid considered based on previous studies on glycolic monomer in argon matrices. Comparing the computational and the experimental spectra especially in the O-H and C=O stretching regions it was possible to assign the experimental observations to the three most stable dimer species. *corresponding authors: e-mails addresses: jussi. m.e.ahokas@jyu.fi (J. A.) maria.wierzejewska@chem.uni.wroc.pl (M.W.) M AN US CR IP T AC CE PT ED ACCEPTED MANUSCRIPT 2


Introduction
Hydrogen bonding as one of the strongest intermolecular interactions plays an important role in many chemical and biological processes. Hydrogen bonded complexes have also a significant influence on the atmospheric chemistry and are considered to contribute to chemistry of interstellar media and planetary atmospheres [1][2][3]. Among many important hydrogen bonded species carboxylic acid dimers have been extensively studied both theoretically and experimentally [4][5][6][7][8][9]. Carboxylic acids form in the gas phase very stable cyclic dimers with two equivalent O-H … O bridges. In the solid state both cyclic structures (dimers) and infinite chains with monomers joined by hydrogen bonds are observed [10][11][12][13][14][15][16].
Monomeric glycolic acid (GA), the smallest of the α-hydroxy carboxylic acids, has been studied in the gas phase [17][18][19][20], low temperature matrices [21][22][23][24][25] and in supersonic expansion [26]. The crystal structure of GA is made up of chains and no isolated cyclic dimers are found. The GA molecules are linked by hydrogen bonds formed between carboxylic groups as proton donors and hydroxylic oxygen atoms as proton acceptors. The geometry of the individual GA molecules in the crystal resembles that of the SSC (syn-syncis) conformer observed in the gas phase with the alcohol OH group slightly deviated from the molecular plane.
According to theoretical predictions [20,21,24] the SSC conformer (see Fig. 1) is a global minimum on the potential energy surface of GA. All available experimental data confirm that SSC is the most populated form of the compound. The relative abundancy of the most stable monomeric form SSC is ca. 95% at room temperature [23]. In addition to SSC only small amounts of two less stable isomers: AAT (anti-anti-trans) and GAC (gauche-anticis) were found in jets [26] and in low temperature matrices [21][22][23][24][25].

M A N U S C R I P T
A C C E P T E D ACCEPTED MANUSCRIPT Fig. 1. Schematic representation of three most stable conformers of glycolic acid monomer. Relative energies ∆Ε obtained at MP2/aug-cc-pVDZ are given in kJ mol -1 [24].
In this paper we present results of experimental and theoretical studies of glycolic acid dimers. Matrix isolation (MI) coupled with FTIR spectroscopy was applied which is an established technique for studying small hydrogen bonded systems [27][28][29]. When supported with computational methods the isolated hydrogen bonded complex structures are well resolved and distinguishable.

Experimental and computational details
The matrix samples were prepared by passing high purity argon (Messer, 5.0) through the glass U-tube with glycolic acid (GA) situated outside the cryostat chamber. The matrix-tosample ratio could not be precisely determined, but matrices containing dimeric GA species could be obtained by optimizing the deposition temperature and the gas flow rate (2 mbar per minute). The GA/Ar gaseous mixtures were deposited onto a cold CsI window kept at 15 K, 18 K or 25 K by a closed cycle cryostat APD-Cryogenics (ARS-2HW). Annealing experiments were performed upon the deposited samples at 25, 27, 30, 33, and 35 K. Sample temperature was maintained by a Scientific Instruments 9700 temperature controller equipped with a silicon diode and a resistive heater. Infrared spectra were recorded in a transmission mode with a 0.5 cm -1 resolution using a purged Bruker IFS 66 Fourier Transform spectrometer equipped with a liquid N 2 cooled MCT detector.
All calculations were carried out using the Gaussian09 program package [30]. For all types of dimers (SSC-SSC, SSC-GAC, SSC-AAT, GAC-GAC, AAT-AAT) MP2/6- [31][32][33][34][35] calculations were performed. The SSC-SSC dimers were additionally studied at the B3LYP, B3LYPD3 and B2PLYPD3 levels [36][37][38][39][40][41][42] of theory in order to assess their performance compared to the MP2 calculations. The obtained energies of the dimers were corrected for basis set superposition error (BSSE) by the Boys-Bernardi full counterpoise method [43]. The topological analysis of the electron density (AIM) [44] has been performed at the MP2 level using AIM program as implemented in Gaussian09. The harmonic vibrational wavenumbers and intensities were calculated at all four levels for the optimized dimer structures to assist the analysis of the experimental spectra. In the next sections the MP2 energies will be used, unless otherwise indicated. During the study it was found that MP2 and DFT calculations yield very similar structural parameters and identical energy orders between complex structures. On the other hand, the B3LYP wavenumbers were used for the purpose of discussion due to their best fit to the experimental findings, as the MP2 computed vibrational wavenumbers showed irregular performance for different complex structures. The actual origin of this observation is still unclear and more elaborate computational studies are underway beyond harmonic approach to investigate this issue further. The B3LYP/6-311++G(2d,2p) wavenumbers of the SSC-SSC dimers scaled by 0.9506, above 2500 cm −1 , and by 0.9845, in the 2500−500 cm −1 spectral region, were used to simulate their infrared spectra. The scaling factors were obtained from fitting the computed vibrational wavenumbers with respect to the observed vibrational band positions.

Computational results
Since three monomeric GA isomers SSC, GAC and AAT (see Fig. 1) were detected in argon matrices [21,22,24]    The computed interaction energies are presented in Table 2, whereas the relative energies are given in Table S2. For the four most stable dimer species the interaction energy order is identical at all four levels of theory and the same is true for the overall geometry of these structures.  Table 1).    The Type 3 absorbers are the best candidates for the dimers due to their monotonic population increase. The validity of this assumption is supported by a linear dependence obtained for a plot of the normalized integrated absorbance of Type 1 bands versus the normalized integrated absorbance of bands of Type 3 (see Fig. 6). According to this linearity between the Types 1 and 3, the role of the Type 2 absorbers is negligible in the dimerization process. These assumptions were supported by experiments when the deposition temperature was varied. At higher deposition temperatures, the highest intensity bands of the Type 3 absorbers were already found after deposition. Unfortunately, the higher deposition temperature also favored the formation of higher aggregates and accordingly broad bands were present in the spectra. Some absorption bands of unstable dimer structures or unstable trapping sites of the dimer were also observed after the deposition at 25 K. These bands Determination of the structures of the glycolic acid dimers present in the studied matrices was not an easy task. For proper choice of the dimer species for the spectra interpretation two criteria were taken into account. The first criterion was the interaction energy of the dimers and the second, equally important factor, was the calculated spectral characteristics of the dimeric forms. The best fit between theoretical and experimental spectra was obtained for B3LYP calculations and these data were used for the analysis. On the basis of the mentioned criteria the three most stable dimer structures (dSS1, dSS2, dSS3) have been chosen for the interpretation of the spectra after annealing (see Figs 3 and 4).
The most informative for the analysis is the OH stretching region of the spectra (Fig.   3). One can readily notice that there are two separate regions where the dimer bands appear. and dSS3, respectively. In Table 3, the calculated and the experimental ∆νOH and ∆νC=O shifts are compared for the most stable dimer structures. The remaining data are shown in Table S3 (Supplementary data). shifted νC=O bands as compared to the monomer band (see Table S3) but this is not what is observed experimentally. Therefore, the only possible structure to explain the appearance of the νOH bands in question is dSS2 dimer. Possible explanation of the observed multiple structure of the absorption at ca. 3440 cm -1 could be matrix site effect.
Further analysis of the dimer structures present in argon matrices was performed based on the νC=O stretching region (Fig. 4). Two dimer bands with the pronounced shoulders are observed below the νC=O monomer absorption at 1757/1761sh and 1719/1717sh cm -1 . There is also an additional broad band with a maximum at 1733 cm -1 (see Fig. 4). The corresponding

Highlights
Structure of glycolic acid dimers present in solid Ar was studied by FTIR spectroscopy MP2 and DFT calculations revealed 27 stable dimer structures Comparison of the experiment and theory allowed to identify the most stable species