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,* 1 Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14, 50-383 Wroclaw, Poland 2 Department of Chemistry, University of Jyvaskyla, P.O.Box 35, 40014 Jyvaskyla, Finland * Correspondence: jan.c.lundell@jyu.fi (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 intra- and 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 Figure1. 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, x FOR PEER REVIEW 2 of 13 Molecules 2019, 24, 3262 2 of 13 intramolecular hydrogen bond. This conformer is present in the gas phase and in low-temperature at roommatrices temperature with an ofestim ca.ated 95%. population A small amount at room of temperature two less stable of ca. GAC 95%. and A small AAT amount conformers of two has less also stable GAC and AAT conformers has also been detected [19,20]. been detected [19,20]. FigureFigure 1. Structures 1. Structures of theof the three three most most stable stable conformers conformers ofof glycolicglycolic acid monomer. monomer. ContraryContrary to the to the monomeric monomeric GA, GA, only only two two studies studies on on GA GA complexescomplexes in low low-temperature-temperature matrices matrices havehave appeared appeared in literature. in literature. Recently, Recently, a study a study on on GA GA dimers dimers revealed revealed for for the the first first time time information information on threeon cyclic three GA cyclic dimers GA dimers identified identified in an argonin an argon matrix matrix [21]. [21]. All of All these of these dimers dimers were were formed formed between between SSC conformers:SSC conformers: one dimer one with dimer hydrogen with hydrogen bonds between bonds between the two the carboxylic two carboxylic OH groups, OH onegroups, structure one withstructure hydrogen with bonds hydrogen between bonds the two between alcoholic the two OH alcoholic groups and OH a groups mixed structureand a mixed with structure one OH with group fromone the OH carboxylic group from group the actingcarboxylic as proton group acting donor as and proton the alcoholicdonor and OH the groupalcoholic on OH the group other subuniton the actingother as protonsubunit acceptor. acting as Molecularproton acceptor. complexes Molecular between complexes GA and between molecular GA nitrogenand molecular were nitrogen identified in a studywere identifi applyinged Ramanin a study spectroscopy applying Raman for argon-trapped spectroscopy species for argon upon-trapped high vibrational species upon excitation high experimentsvibrational [22 excitation]. In that experiments study, nitrogen [22]. In was that found study, to nitrogen form complexes was found with to form two complexes GA conformers with two GA conformers (SSC and AAT) in low temperature argon environment. However, this study did (SSC and AAT) in low temperature argon environment. However, this study did not scrutinise the not scrutinise the actual molecular complex structures involved but only made conclusions of actual molecular complex structures involved but only made conclusions of different GA N systems … ··· 2 involveddifferent based GA onN their2 systems behaviour involved upon based 532 on nm their irradiation. behaviour upon 532 nm irradiation. These Raman spectroscopy combined with visible light irradiation experiments acted as a These Raman spectroscopy combined with visible light irradiation experiments acted as a prelude prelude to our study presented here. In this paper, we present results of a combined study employing to our study presented here. In this paper, we present results of a combined study employing both both theoretical methods and FTIR matrix isolation studies on complexes formed between glycolic theoretical methods and FTIR matrix isolation studies on complexes formed between glycolic acid and acid and nitrogen molecule. The target is to study and to identify the 1:1 complexes GA forms with nitrogen molecule. The target is to study and to identify the 1:1 complexes GA forms with molecular molecular nitrogen. This research is important in order to understand which type of complexes can nitrogen.be formed, This researchif such species is important are experimentally in order to understanddetected, and which, as these type formed of complexes complexes can are be used formed, as if suchprecursors species arefor near experimentally-infrared irradiation detected, experiments and, as these, if their formed photo complexes-induced arechemistry used as differ precursors from for near-infraredisolated GA monomers. irradiation The experiments, study on the if NIR their irradiated photo-induced GA-N2 complexes chemistry is di reportedffer from separately isolated in GA monomers.a following The paper study taking on the advantage NIR irradiated of the results GA-N 2presentedcomplexes here. is reported separately in a following paper taking advantage of the results presented here. 2. Experimental and Computational Details 2. Experimental and Computational Details The matrix samples were prepared by passing mixtures of high purity argon (Messer, 5.0) and nitrogenThe matrix (Messer, samples 6.0) with were the prepared N2/Ar ratios by passing of 1/4000 mixtures through the of highglass purityU-tube argonwith glycolic (Messer, acid 5.0) (GA) and nitrogensituated (Messer, outside 6.0) the with cryostat the N chamber.2/Ar ratios Optimizing of 1/4000 throughthe deposition the glass temperature U-tube with and glycolicmatrix flow acid rate (GA) situatedit was outside possible the cryostatto obtain chamber. matrices Optimizing containing thenearly deposition exclusively temperature monomeric and GA matrix and flow GA… rateN2 it was possiblecomplexes to of obtain 1:1 stoichiometry. matrices containing The GA/N nearly2/Ar exclusively gaseous mixtures monomeric were GA deposited and GA ontoN a complexescold CsI ··· 2 of 1:1window stoichiometry. kept at 15 The K GAor 18/N 2K/Ar in gaseousan APD- mixturesCryogenics were (ARS deposited-2HW) closed onto acycle cold cryostat. CsI window Annealing kept at 15 Kexperiments or 18 K in an were APD-Cryogenics performed upon (ARS-2HW) the deposited closed samples cycle cryostat.at 33 K. The Annealing sample experimentstemperature was were performedmaintained upon by the a Scientific deposited Instruments samples at 9700 33 K. temperature The sample controller temperature equipped was maintained with a silicon by