Density functional theory B3LYP method at the 6-311＋G* basis set was used to investigate the mechanism of tautomerism between enol form and keto form of 3-hydropyridazine as well as its methyl, nitro or chloro derivatives, with the geometric structures of reactants, transition states and products optimized completely. In addition, the transition states have been explored and proved by vibration analysis and intrinsic reaction coordination analysis, and meanwhile, the total energies and reaction activation energies have been calculated. The results showed that the most stable isomers are compounds with the structure of 2-hydropyridazin-3-one or its derivatives compared with the other corresponding isomers, which was in good agreement with the previous reports that predicted the ratio of isomers by experimental data. The mechanism of the isomerization was discussed on the basis of the theoretical values.

Key words: 3-hydropyridazine, density functional theory, tautomerism, transition state, prototropy