Effect of hydrogen bonding on the methyl conformation of thioacetamide: an ab initio study

Abstract

The methyl rotational barrier height of thioacetamide was studied by ab initio molecular orbital calculations including electron correlation corrections up to the MP4/6-311+G**//MP2/6-31 G* level. The calculations indicate that the methyl group of isolated thioacetamide shows a slight tendency to eclipse the CS bond. This result was also found for the thiocarbonyl compounds thioacetone and thioacetaldehyde which are included in this work. Centrosymmetrical dinners and hexamers of thioacetamide are proposed as a simple simulation of the crystal field and ab initio calculations were carried out to investigate the effects of hydrogen bonding on the methyl conformation and on the molecular structure of thioacetamide. The oligomer models are capable of reproducing the changes in the π-character of the CS and C–N bonds, the variations in the valence angles and the rotation of the methyl group observed by comparing the geometry of the isolated molecule with that of the crystal molecules. The structural perturbations due to self-association have been rationalized supposing that the contribution of the polar canonical form where the sulfur atom is negatively charged increases progressively in going from the monomer to the dimer and hexamer of thioacetamide molecule.