Infrared-induced rotamerization of oxalic acid monomer in argon matrix

Infrared-induced conformational isomerization of oxalic acid monomer isolat ed in an argon matrix at 7.5 K was studied by infrared spectroscopy. For th e first time, three conformational states of this molecule were identified experimentally and their vibrational spectra are assigned. In good agreemen t with density functional theory predictions, all the observed conformers e xhibit a trans O=C-C=O axis, differing in the relative conformation of thei r O-C-O-H axes. In the most stable conformer (belonging to the C-2h symmetr y point group), two intramolecular OH ... O= hydrogen bonds are present, Th e second (C-s) most stable conformer shows a single OH ... O= bond, and the third one (C-2h) does not exhibit any intramolecular hydrogen bond. Using narrowband tunable irradiation in the near-infrared region it was possible to promote very efficiently conformer interconversions, which was followed spectroscopically.