摘要
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6-硫代黄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-硫代黄嘌呤在气相中和水相中主要以硫酮的形式存在.在气相和水相中,硫酮-N7(H)均比硫酮-N9(H)更稳定.计算结果同已有实验结果一致.6-硫代黄嘌呤异构化的熵效应小,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.
Molecular structures of fourteen 6-thioxanthine tautomers were calculated by the B3LYP/6-311G** method, both in gas and aqueous phases, with full geometry optimization. The Onsager solvate theory model was employed for aqueous solution calculations. The structure, total energy, standard enthalpy, standard entropy and standard Gibbs free energy were obtained. The calculations show that 6-thioxanthine exists predominantly as the thione forms in the gas and aqueous phases. The thione-N7(H) isomer is more stable than the thione-N9(H) isomer in the gas and aqueous phase. The results are in good agreement with available experimental results. The entropy effect on the Gibbs free energies of 6-thioxanthine 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 Gibbs free energies of solvation are well correlated with the dipole moments of 6-thioxanthine tautomers.
出处
《物理化学学报》
SCIE
CAS
CSCD
北大核心
2004年第5期503-506,共4页
Acta Physico-Chimica Sinica
基金
国家自然科学基金(20171034)资助项目~~
