Keto–enol tautomerism of dithiomalondialdehyde and dithioacetylacetone. A reinvestigation using version 2.1 of the AM1 method

Abstract

The keto–enol tautomerism of dithiomalondialdehyde (DTMDA) and dithioacetylacetone (DTAA), recently studied by means of the first version of the AM1 program using MNDO parameters for sulphur atoms, has been reinvestigated by version 2.1 of the same AM1 program, in which appropriate parameters for sulphur have been inserted. Bond lengths and bond angles of the minimum energy conformations of DTMDA and DTAA are comparable with the previous ones but the ΔH_f values are remarkably lower. Some tautomers show an S⋯S distance very close to the value of the van der Waals radius of S, whilst the C_2v hydrogen-bonded structures of both compounds (i.e. the H-centred tautomer) appear to be, strangely, the most stable ones. The hydrogen bond strength is estimated roughly to lie in the range 22–25 kJ mol^–1. The new results, on the whole, appear to be worse than predicted by the old AM1 program and the hydrogen bond is overestimated with respect to the well known weakness of the S—H⋯S bridge.