Intermolecular proton transfer process in sulfur tautomeric heterocycles. I. A quantum chemical approach

Molecular structures of two tautomeric aromatic heterocycles, 2-mercaptobenzoxazole and 2-mercaptobenzothiazole, were analyzed from a quantum chemical approach by means of AM1 semiempirical molecular orbital calculations. Thiones are the preferential tautomeric structures of these compounds in solid phase and solutions, and the AM1 profiles of potential wells agree the experimental evidences. In spite of the good description of the thione and thiol structures by the AM1 approach, the experimental tautomerism observed at room temperature can not be explained by means of an intramolecular protonic transfer mechanism. The potential barrier AM1 calculations of both tautomers range from 40 to 50 kcal/mol, approximately. Therefore, we have postulated a tautomeric process through a geometrical model of co-planar dimers in these heterocyclic compounds. These dimers are more stable than the respective monomers and this model permit to assume a simultaneous intermolecular proton transfer process between sulfur and nitrogen centers in the thiocyanate (NCS) syntonic units. Our results give a coherent mechanistic explanation to several experimental observations in chemical reactions and spectroscopic data.