2，6-二硫代黄嘌呤互变异构体的密度泛函理论研究

Density functional theory calculations on 2,6-dithioxanthine tautomers

在密度泛函B3LYP/6-311G^(**)水平下,对14种气相和水相中可能存在的2,6-二硫代黄嘌呤异构体进行了几何构型全自由度优化,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,2,6-二硫代黄嘌呤在气相和水相中主要以双硫酮形式存在。在气相和水相中,硫酮-N(7)(H)均比硫酮-N(9)(H)稳定,计算结果同已有实验结果一致。2,6-二硫代黄嘌呤互变异构的熵效应小,对互变异构平衡没有显著影响,而焓变对互变异构产生了主要影响。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。溶剂化自由能与异构体的偶极矩存在相关性。并且以在水相中最稳定的两种异构体为对象,采用超分子模型探讨了它们与水分子之间氢键作用的情况。

Molecular structures of fourteen 2,6-dithioxanthine 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 2,6-dithioxanthine existing as the dithione form is the predominant isomers in the gas and aqueous phases. The dithione-N（7）（H） isomer is more stable than the dithione-N（9）（H） isomer in the gas and aqueous phases. The results are in good agreement with available experimental results. The entropy effect on the Gibbs free energy of 2,6-dithioxanthine base is very small and there is little significan cefor 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 2,6-dithioxanthine tautomers. Additionally, in order to taste the possible importance of some specific solvent effects like H-bonds, calculations with the inclusion of one water molecule to the two most energetically favored tautomers have been carried out.