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

Molecular geometries, energies, dipole moments and atomic charges of severa l conformations of glycine (NH2CH2C(=O)OH) and its oxygen-by-sulfur substit uted analogues (NH2CH2C(=O)SH, NH2CH2C(=S)OH and NH2CH2C(=S)SH) were determ ined by ab initio MO calculations at the MP2/6-31G** level, and vibrational frequencies, infrared and Raman intensities were evaluated within the harm onic approximation, dt the HF/6-31G** level. Profiles for the potential ene rgy 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 subs titutions are discussed. In particular, the occurrence of intramolecular cl ose contacts between the NH2CH2- and -C(=X)YH (X, Y = O or S) moieties of t he molecule is considered, for some of the most stable conformations of eac h analogue. As a general trend, the results dearly point to the stabilisati on of the G/anti forms (:NCC congruent to 60 degrees, NCCX congruent to 180 degrees), highlighting the importance of the N:... H(Y) and N:... S(H) int eractions, e.g. in conformers G/antils-trans for the thione form (X = S) an d G/anti/s-cis for the thiol one (Y = S), respectively.