Hydrogen bonding and tautomerism in 3-substituted β-thioxoketones: an ab initio molecular orbital study

Abstract

The molecular geometry of some 3-substituted β-thioxoketones (R = CN, OH, F, CH₃) and of 3-methyl-4-mercaptopent-3-en-2-one (6) has been fully optimised by ab initio molecular orbital calculations using the 3-21G basis set. The energetics of the intramolecular hydrogen bonding and tautomerism in these compounds have been studied using 6-31G** and MP2/6-31G^** basis sets. The 3-substitution does not change the geometry of the hydrogen bridge except in the sterically hindered compound 6. Accordingly, the hydrogen-bonding strength (E_{H B}) is little influenced by the substitutent, except in 6, where it increases by ca. 58% with respect to the parent compound, reaching 90 kJ mol^–1. E_{H B} does not follow Hammett-type behaviour in parallel with the observed variation in the chelate proton chemical shift of the related β-diketones. This is supported by the molecular orbital and population analysis which, inter alia, shows that the β-thioxoketone skeleton behaves as a strong σ-electron donor towards the 3-substituent, including the OH group. Very strong hydrogen bonds in this class of compounds can be achieved only by steric effects, electronic effects of the substituent being damped by conjugative effects and charge redistributions outside the H–bridge. The tautomeric equilibrium is markedly shifted towards the enolic form in the CN— and CH₃— derivatives, whilst in the OH— derivative the enethiolic structure is favoured. The interconversion (Z)-enol ⇌(Z)-enethiol process cannot be regarded as an internal acid–base reaction. The tautomeric energy, relative to the parent compound, rather reflects differential conjugative effects. On the basis of the calculated dipole moment values, the stable gas-phase tautomer is expected to be relatively even more stable in solution.