Thiocarbonyl reactivity and molecular orbital correlations

A set of 41 thiocarbonyl compounds for which the structural environment of the C = S group has been modified to cover the widest range of nucleophilic reactivity is studied within the AM1 framework to rationalise the experimental rate constants of these compounds with Mel. The rate constants in acetone, methanol and acetonitrile are correlated to each other and correlated with rate constants of these compounds with MeOTs. These correlations suggest that a molecular orbital treatment of the series of derivatives could account for the variations in reactivity. The total charge on the sulfur is roughly correlated with the measured nucleophilic reactivity of the thiocarbonyl group; the most polarisable compounds however react far faster than their charge would suggest. A rough correlation between the calculated first and second HOMOs and nucleophilic reactivity is also displayed by this set of compounds whereas the third HOMO displays no correlation at all. The preceding correlation is however improved if the nucleophilicity is correlated with the energy of the σ orbital of highest energy (grossly sulfur lone pair) which can be, depending upon the structure of the compounds, either the HOMO or the next HOMO.The AM1 method accounts reasonably for the energies of the first two experimental HOMOs (P.E.S.), except for some non cyclic compounds where steric factors seem to interfere.A correlation between the P.E.S. measured σ higher energies and nucleophilic reactivity confirms the trend hinted at by AM1 calculations. The significance of these results in terms of Frontier Molecular Orbital Theory is discussed