The reaction mechanism of thione-thiol tautomerism for 2,6-dithiopurine obtained from proton transfer has been investigated by DFT B3LYP method with 6-311＋G(d,p) basis set. The reaction enthalpies, activation energies, activation free energies and the rate constants of tautomerism were obtained. The results showed that the dithione R is the most stable tautomer of all the tautomers in the form of isolated or hydrated isomers. The 6 reaction pathways from dithione to dithiol through intramolecular proton transfer were found. The activation energy needed in the main pathway (1) is 139.1 kJ•mol^－1 and the rate constant is 2.16×10^－12 s^－1. In water catalysis, double proton transfer was found and the activation energies were dramatically decreased, which was preferable to transform from dithione to dithiol tautomers. And the main reaction pathway needed 61.3 kJ•mol^－1 activation energy, the rate constant of which was 1.33×10 s^－1. It was found that the effect of hydrogen bonds played an important role in increasing the stabilities of 2,6-dithiopurine hydrated tautomers and lowering the activation energies of proton transfer.

Key words: 2,6-dithiopurine, proton transfer, tautomerism, activation energy