Ab initio study of the thioketo-enethiol tautomerism of dithiomalondialdehyde and dithioacetylacetone

Abstract

The molecular geometries of all the possible conformations of dithiomalondialdehyde (DTMDA) and dithioacetylacetone (DTAA) were fully optimized by ab initio 4-31G calculations, in order to verify the reliability of results obtained by semiempirical methods. All of the enethiol conformers of DTMDA were found to be more stable than the keto conformers, even though the chelate structures are expected to be less stable than most of the unchelated isomers. The hydrogenbonded structure of DTAA is predicted to be energetically favoured with respect to the other enethiol isomers, but a non-planar thioketo structure is found to be the most stable conformation. However, two or more isomers of DTMDA and DTAA may be in equilibrium in the gas phase as well as in solution. In both compounds, the H-centred ( C 2v) structure is predicted to be less stable than the C 8 one and the former is proposed as the transition state in the proton transfer pathway between the two sulphur atoms. The H-bonding energy is 11.14 kJ mol −1 in DTMDA and 12.44 kJ mol −1 in DTAA. Adoption of the 6-31G ∗∗ basis set lowers the hydrogen bond strength of DTMDA to 4.55 kJ mol −1. The 3-21G ∗ basis set predicts no hydrogen-bonding in DTMDA and very weak hydrogen-bonding (1.02 kJ mol- −1) in DTAA.