For researching the mechanism of tautomerism between enol form and keto form of 3,6-dihydroxypyridazine, the potential energy surface of the isomerization was studied employing DFT at B3LYP/6-311+G(d). It was found that there were at least 34 isomers and 43 transition states in possible reaction pathways. All the possible processes of reaction were studied. The results showed that the energy of 2,3-dihydro-6-hydroxy-pyridazin-3-one is lower than those of other isomers in the form of monomer, hydrate or dimer. So it was the most stable one. It was consistent with the former report that predicted the ratio of isomers by experimental data. In three possible reaction pathways, the activation energy needed by intromolecular prototropy was 142.2 kJ/mol, that by water catalysis proton transfer was 41.3 kJ/mol which became 59.2 kJ/mol after correction with the effect of solvent activation energy and that by double-proton transfer in dimer was 16.8 kJ/mol which was the lowest activation energy. The last pathway was possible at room temperature. It implied that the hydrogen bond played an important role in depressing the activation energy of reaction.

Key words: 3,6-dihydroxypyridazine, density functional theory, tautomerism, transition state