6-巯基嘌呤互变异构体的密度泛函理论计算

在密度泛函B3LYP/6-311G水平下，对8种气相和水相中可能存在的6-巯基嘌呤异构体进行了几何构型的全自由度优化，并计算出它们的总能量、焓、熵、吉布斯自由能．Onsager反应场溶剂模型用于水相的计算．计算结果表明，6-巯基嘌呤在气相和水相中主要以硫酮形式存在．在气相中，硫酮．N(7)(H)要比硫酮-N(9)(H)更稳定，而在水相中，则硫酮-N(9)(H)要比硫酮-N(7)(H)更稳定．计算结果同已有实验结果一致．6-巯基嘌呤的熵效应小，对互变异构平衡几乎没有显著的影响，而焓变对互变异构产生了主要的影响．较详细地讨论了水溶剂化作用对异构体的能量、几何结构、电荷分布和偶极矩的影响，溶剂化吉布斯自由能与异构体的气相偶极矩存在相关性．

DFT Calculations on 6-Thiopurine Tautomers

Molecular structures of eight kinds of 6 thiopurine tautomers were calculated by the B3LYP/6 311G ** method, both in the gas and aqueous phases, with full geometry optimization. The Onsager solvate theory model was employed for aqueous solution calculations. The structures, total energy, standard enthalpy, standard entropy and standard free energy were obtained. The calculations show that 6 thiopurine existing as the thione forms is the predominant isomer in the gas and aqueous phases. The thione N(7)(H) isomer is more stable than the thione N(9)(H) isomer in the gas phase whereas the thione N(9)(H) isomer is more stable than the thione N(7)(H) isomer in the aqueous phase. The results are in good agreement with available experimental results. The entropy effect on the Gibbs free energy of 6 thiopurine base is very small and there is little significance for the tautomeric equilibria of the base. The enthalpic term is dominant in the determination of tautomeric equilibria. The free energy of solvation is well correlated with the dipole moments of 6 thiopurine tautomers.