QUANTUM-CHEMICAL STUDIES OF HYDROGEN-BONDING INVOLVING THIOXOKETONES, THIENOLS, THIOFORMALDEHYDE AND HYDROGEN-SULFIDE WITH SPECIFIC REFERENCE TO THE STRENGTH OF INTRAMOLECULAR HYDROGEN-BONDS

The results of ab initio quantum-chemical calculations are reported for rotamers of S(CH)3OH (thioxoketone) and HS(CH)3O (thienol), several complexes of these with H2O and H2S as well as a range of other hydrogen-bonded complexes involving H2S, H2O, H2CS and H2CO. The calculations were performed largely at the SCF level of theory using bases of triple zeta plus polarization functions quality yielding equilibrium geometries and energies. Correlation corrections to the energies were obtained using second-order Moller-Plesset (MP2) perturbation theory. The results demonstrate that both thioxoketone and thienol are capable of forming fairly strong hydrogen bonds with H2O and H2S and can also form intramolecular hydrogen bonds. In order to estimate the strengths of such intramolecular H bonds so that comparison with H-bonded dimers is meaningful we propose the use of appropriate, well chosen reference structures. Detailed analysis of the results obtained indicates that enhanced delocalization in thioxoketone gives rise to a strong H bond. Such a mechanism seems to be absent in thienol where the H bond is very weak.