AM1 study of tautomerism and intramolecular hydrogen bonding in thiomalondialdehyde and thioacetylacetone

Abstract

All the keto, enol and enethiol tautomers of thiomalondialdehyde (TMDA) and thioacetylacetone (TAA) have been fully optimized by means of the AM1 method in order to determine the most stable isomer and evaluate the hydrogen-bond energies of the O—H⋯S and S—H⋯O bridges. The geometries agree fairly well with those coming from ab initio calculations. Both in TMDA and in TAA, the chelated enethiol tautomer is predicted to be more stable than the corresponding enol tautomer, in agreement with recent ab initio results.

The hydrogen-bond energy of the O—H⋯S bridge is estimated to be 42.1 and 40.0 kJ mol^–1 in TMDA and TAA, respectively, whilst values of 25.4 and 20.3 kJ mol^–1 are found for the S—H⋯O bridge.