Anharmonic treatment of the lowest-energy conformers of glycine: A theoretical study

The structure and energetics of the four lowest-energy conformers of glycin e were determined at the MP2/aug-cc-pVDZ level of theory. The optimized str uctural parameters for these conformers agree with previous theoretical res ults obtained by highly correlated ab initio methods and with available exp erimental data. The only structure with planar heavy atom arrangement is co nformer I (global minimum), the other conformers have nonplanar heavy atom arrangements. In accordance with temperature dependence studies of the vibr ational spectra in various rare gas environments, conformers III and IV hav e small interconversion barriers to conformer I (940 and 740 cm-(1)). Our c alculations have shown that full-dimensional anharmonic treatment is requir ed for an accurate description of the vibrational modes in various glycine conformers. The most pronounced effect has been observed for conformer II w ith the intramolecular O-H ... N bond. The theoretical results obtained at the MP2/aug-cc-pVDZ level reproduce quantitatively the argon matrix experim ents. The calculation uses the quartic force field approximation in the fra mework of second-order perturbation theory. An estimate of the higher-order correction is also given. (C) 2000 American Institute of Physics. [S0021-9 606(00)30335-X].