A computational study of the keto-enol equilibria of sulphur substituted analogues of hydroxycyclopropenone

Keto-enol equilibria in mono- and di-thio substituted analogues of hydroxycyclopropenone is studied using ab initio and density functional theory methods. Five structural isomers of C3H2OS and three isomers of C3H2S2 were fully optimized in the gas phase at HF, MP2 and B3LYP levels of theory in combination with the cc-pVTZ basis set. Two different Self-Consistent Reaction Field approaches, namely, the Onsager-Dipole and the Polarizable Continuum Model were used to investigate the effect of an aqueous solvent on the extent of tautomerisation at the HF/cc-pVTZ and B3LYP/cc-pVTZ model chemistries. In addition, gas phase electronic and Gibbs free energies for the eight molecules were computed using the Complete Basis Set CBS-QB3 method, and the G3 approach, in order to evaluate highly accurate relative energies. As in the case of the hydroxycyclopropenone-cyclopropanedione system, computational results support the fact that the enol forms of the mono- and di-thio substituted analogues are favoured overwhelmingly relative to keto structures. Resonance stabilization arising from aromaticity present in these three-membered cyclic ring systems is posited, and is understood on the basis of Hückel theory.