Oxygen-by-sulfur substitutions in glycine: conformational and vibrational effects †

Abstract

Molecular geometries, energies, dipole moments and atomic charges of several conformations of glycine (NH₂CH₂C([double bond, length half m-dash]O)OH) and its oxygen-by-sulfur substituted analogues (NH₂CH₂C([double bond, length half m-dash]O)SH, NH₂CH₂C([double bond, length half m-dash]S)OH and NH₂CH₂C([double bond, length half m-dash]S)SH) were determined by ab initio MO calculations at the MP2/6-31G** level, and vibrational frequencies, infrared and Raman intensities were evaluated within the harmonic approximation, at the HF/6-31G** level. Profiles for the potential energy and several geometric parameters as a function of the NC–C([double bond, length half m-dash]X) (X = O, S) dihedral angle (C–C internal rotation) were obtained using the 3-21G(N*) basis set. Conformational and vibrational effects of oxygen-by-sulfur substitutions are discussed. In particular, the occurrence of intramolecular close contacts between the NH₂CH₂– and –C([double bond, length half m-dash]X)YH (X, Y = O or S) moieties of the molecule is considered, for some of the most stable conformations of each analogue. As a general trend, the results clearly point to the stabilisation of the G/anti forms (: NCC ≊ 60°, NCCX ≊ 180°), highlighting the importance of the N :  · · ·  H(Y) and N :  · · ·  S(H) interactions, e.g. in conformers G/anti/s-trans for the thione form (X = S) and G/anti/s-cis for the thiol one (Y = S), respectively.

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