Abstract: To determine the tautomerism mechanism between the enol form and the keto form of 2- (2-hydroxybenzylidenamino)pyrimidine-4,6-diol (M1) the potential energy surface of the isomerization was studied using density functional theory (DFT) calculations at the B3LYP/6-311+G(d, p) level. We found that there were at least 8 isomers and 10 transition states in the possible reaction pathways. All the possible processes of the reaction were studied. The results showed that the energy of 6-hydroxy-2-(2- hydroxybenzylideneamino) pyrimidine-4(3H)-one (M6) was lower than those of the other isomers in the form of a monomer, a hydrate, and a dimer. Therefore, it was the most stable isomer. In these possible reaction pathways the activation free energy required for intramolecular prototropy was 143.8 kJ·mol^-1 and for the proton transfer process that was catalyzed by water was 38.9 kJ·mol^-1. The activation free energy in the double-proton transfer of the dimer was 0.6 kJ·mol^-1, which was the lowest value. The latter pathway was feasible at room temperature. This implies that hydrogen bonding plays an important role in depressing the activation energy of the reaction.

Key words: 2-(2-Hydroxybenzylidenamino)pyrimidine)-4,6-diol, Density functional theory, Tautomerism, Proton transfer