Matrix-isolation and ab initio studies of oxalic acid

Infrared-Induced Rotamerization of Oxalic Acid Monomer in Argon Matrix

“…The assignments of these bands were discussed in detail elsewhere. 3,4 As expected, apart from the bands due to the conformational ground state (cTc), some minor bands ascribable to other conformers are also present in the spectrum. As mentioned in the Introduction, by irradiating the matrix in the OH stretching fundamental region, Nieminen et al were able to observe conformational isomerization, the reaction product correspond-ing to the lowest-energy conformer, whereas this conformer behaved as the reactive species when the sample was irradiated in the UV region.…”

“…As mentioned in the Introduction, by irradiating the matrix in the OH stretching fundamental region, Nieminen et al were able to observe conformational isomerization, the reaction product correspond-ing to the lowest-energy conformer, whereas this conformer behaved as the reactive species when the sample was irradiated in the UV region. 3 The IR-reactive species was identified as corresponding to form cTt and a tentative assignment of the bands due to this conformer was also proposed in this previous study. However, the relatively low efficiency of the photoisomerization process did not allow the unequivocal assignment of all the observed features.…”

“…4 However, only the second most stable conformer predicted by the calculations (form cTt, in Figure 1) could be unequivocally identified experimentally. 3 A recent Stark-modulated free jet microwave spectroscopic study 8 has confirmed the structure of this conformer. The other theoretically predicted conformers of oxalic acid have not been observed until now, though at least the third-and fourth-lowestenergy conformers (forms tTt and tCt) were calculated to have relatively accessible excess energies to the conformational ground state.…”

“…1,2 However, in both gaseous phase and in inert gas matrices at low temperatures, monomeric oxalic acid has been found to exist predominantly in a different conformation, where the two H-O-C-C dihedrals adopt the cis configuration and are involved in intramolecular hydrogen bonding (form cTc). [3][4][5] High-level ab initio and density functional theory (DFT) calculations on monomeric oxalic acid have also been reported previously. [6][7][8] In general, the highest-level theoretical calculations predict the existence of at least five conformers of oxalic acid.…”

“…Irradiation above 2000 cm -1 was found to be active at initiating the reverse process (cTtfcTc isomerization). 3 It has repeatedly been reported for other carboxylic acid monomers that selective excitation of the OH stretching vibrations are of great utility to study conformational isomerization processes in this type of compound. [10][11][12] Furthermore, contrary to UV irradiation, IR irradiation does not usually lead to decomposition of the matrix-isolated molecules.…”