2,6-二巯基嘌呤质子转移异构化的密度泛函理论研究

采用密度泛函B3LYP方法, 在6-311＋G(d,p)基组水平上对2,6-二巯基嘌呤质子转移引起的硫醇式与硫酮式互变异构反应进行了计算研究, 获得了互变异构过程的反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等性质. 计算结果表明, 2,6-二巯基嘌呤无论是孤立分子还是一水合物, 其二硫酮式R是最稳定异构体. 由二硫酮式通过分子内质子转移向二硫醇式异构化共有6条反应通道, 其主通道(1)速控步骤的活化能为139.1 kJ?mol^－1, 速率常数为2.16×10^－12 s^－1; 当水分子参与反应以双质子转移机理异构化时, 活化能显著降低, 有利于硫酮式向硫醇式转变, 其主通道(7)速控步骤的活化能为61.3 kJ?mol^－1, 速率常数为1.33×10 s^－1. 计算结果还表明, 氢键作用在增大2,6-二巯基嘌呤氢键一水合物稳定性、降低质子转移异构化反应活化能等方面起着重要的作用.

DFT Theoretical Study on the Proton Transfer Isomerization of 2,6-Dithiopurine

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.