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

Molecular geometries, energies, dipole moments and atomic charges of several conformations of glycine (NH2CH2C(O)OH) and its oxygen-by-sulfur substituted analogues (NH2CH2C(O)SH, NH2CH2C(S)OH and NH2CH2C(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(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(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.