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Abstract
The conformational potential energy surfaces (PES) of the neutral forms of glycine and its sulphur analogue dithioglycine (NH2CH2C(─S)SH) were studied by using ab initio (SCF-HF/6-31G) and semiempirical (AM1, PM3) molecular orbital calculations. Fully optimized molecular geometries, electric dipole moments and atomic charges were calculated for the different conformers with the various methods used. The conformational dependence of some relevant structural parameters was used to characterise the most important intramolecular interactions present in the molecules studied. In both cases, all methods predicted the planar conformer with both X─C-X-H and N-C-C─X dihedral angles equal to 0° and a Lp-N-C-C dihedral angle of 180° (X=O or S and Lp=lone pair) as the most stable form. However, important differences were found between the conformational behavior of the two molecules studied, which could be explained in terms of both the different sizes and electronic properties of the oxygen and sulphur substituents and the relative strength of the various possible intramolecular interactions.