A quantum chemical study on the relative thermodynamic stabilities of the two isomeric species of C2H3S

Non-empirical SCF-MO wave functions were computed for the two limiting structures of C2H3S+ with full geometry optimization using three different atomic orbital bases. It was found that the bridged structure (thiirenium ion) was somewhat more stable (1–14 kcal mol–1 depending on the basis set) than the open structure (β-thiovinyl cation). The barrier of interconversion between these two valence tautomers was calculated to be 12·8 kcal mol–1 without geometry optimization. The pyramidal inversion at sulphur in the thiirenium ion was computed to be 72·9 kcal mol–1.