The catalytic effect of water on the keto-enol tautomerisation reaction of thioformic acid

A detailed theoretical study of the proton transfer in the HC(O)SH reversible arrow HC(S) OH reaction in the gas phase and catalysed by water is presented. The mechanism is analysed within the framework provided by the reaction force; structural parameters and electronic properties are analysed along the reaction path. A complete description of the electronic activity that takes place along the reaction coordinate emerges when analysing the reaction electronic flux, thus allowing one to make a clear distinction between the two reaction mechanisms. Results show that the barrier height of the water-catalysed proton transfer reaction is about a third of that for the intramolecular reaction, thus emphasising the importance of the catalytic effect of H2O.