AM1 study of intramolecular hydrogen bonding in the dithio analogues of malondialdehyde and acetylacetone

Abstract

All the molecular structures of the dithioanalogues of malondialdehyde and acetylacetone were fully optimized by the AM1 method in order to study the thioketo–enethiol tautomerism and to evaluate the hydrogen-bond strength. Whilst the hydrogen-bonded tautomer of the dithioacetylacetone is still predicted to be the most stable conformer, an enethiol trans structure of dithiomalondialdehyde is calculated to be slightly more stable than the hydrogen-bonded one. In any case and equilibrium between more than two enethiol conformers is to be expected, with an insignificant percentage of the diketo tautomer. The hydrogen-bond strengths are 18.06 and 15.66 kJ mol^–1 in dithiomalondialdehyde and dithioacetylacetone, respectively. The H-centred structure is the most probable transition state in the proton-transfer process and a barrier lower than that in the corresponding oxygenated compounds is predicted.