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

The structure and energetics of the four lowest-energy conformers of glycine were determined at the MP2/aug-cc-pVDZ level of theory. The optimized structural parameters for these conformers agree with previous theoretical results obtained by highly correlated ab initio methods and with available experimental data. The only structure with planar heavy atom arrangement is conformer I (global minimum), the other conformers have nonplanar heavy atom arrangements. In accordance with temperature dependence studies of the vibrational spectra in various rare gas environments, conformers III and IV have small interconversion barriers to conformer I (940 and 740 cm-(1)). Our calculations have shown that full-dimensional anharmonic treatment is required for an accurate description of the vibrational modes in various glycine conformers. The most pronounced effect has been observed for conformer II with the intramolecular O-H ... N bond. The theoretical results obtained at the MP2/aug-cc-pVDZ level reproduce quantitatively the argon matrix experiments. The calculation uses the quartic force field approximation in the framework of second-order perturbation theory. An estimate of the higher-order correction is also given. (C) 2000 American Institute of Physics. [S0021-9606(00)30335-X].