Investigation of enol-imine/keto-amine tautomerism in (E)-4-[(2-hydroxybenzylidene)amino]phenyl benzenesulphonate by experimental and molecular modelling methods

Abstract

The Schiff base compound (E)-4-[(2-hydroxybenzylidene)amino]phenyl benzenesulphonate has been synthesised from the reaction of 4-aminophenyl benzenesulphonate and salicylaldehyde, and characterised by spectroscopic and single-crystal X-ray diffraction techniques. Quantum chemical calculations employing density functional theory method with the 6-311++G(d,p) basis set were performed to study the molecular, spectroscopic and enol-imine/keto-amine tautomerisation mechanism of the title compound. An acceptable correlation between experimental and theoretical findings is obtained. Enol-imine/keto-amine tautomerisation mechanism was investigated in the gas phase and in solution phase using the polarisable continuum model approximation. The energetic and thermodynamic parameters of the enol-imine -> keto-amine transfer process show that the single proton exchange is unfavoured in all cases. Contrarily, the reverse reaction seems to be feasible with a very low barrier height and is supported by negative values in enthalpy and free energy changes for all cases.