Hori Takumi, Takahashi Hideaki, Nitta Tomoshige
Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka Osaka 560-8531, Japan.
J Comput Chem. 2003 Jan 30;24(2):209-21. doi: 10.1002/jcc.10134.
A hybrid real space quantum mechanical/molecular mechanical (RS-QM/MM) method has been applied to an ionic S(N)2 reaction (OH- + CH3Cl --> CH3OH + Cl-) in water solution to investigate dynamic solvation effects of the supercritical water (SCW) on the reaction. It has been demonstrated that the approaching process of OH- to methyl group is prevented by water molecules in the ambient water (AW), while the reaction takes place easily in the gas phase. Almost the same solvation effect on the dynamics of OH- is observed in the SCW, though the bulk density of water is substantially reduced compared with that of the AW. It has been shown that the solvation of the SCW around the OH anion is locally identical to that of the AW due to the strong ion-dipole interactions between OH- and water molecules. At the transition state, the QM/MM simulations have revealed that the excess electron is quite flexible, and the charge volume, as well as the fractional charges on atoms, vary seriously depending on the instantaneous solvent configurations. However, it has been found that the solvation energy in the SCW can be qualitatively related to the HOMO volume of the system by Born's equation.
一种混合实空间量子力学/分子力学（RS-QM/MM）方法已应用于水溶液中的离子型双分子亲核取代反应（S(N)2）（OH- + CH3Cl --> CH3OH + Cl-），以研究超临界水（SCW）对该反应的动态溶剂化效应。结果表明，在环境水（AW）中，水分子会阻止OH-接近甲基，而该反应在气相中则容易发生。在SCW中观察到对OH-动力学几乎相同的溶剂化效应，尽管与AW相比，水的体密度大幅降低。研究表明，由于OH-与水分子之间强烈的离子-偶极相互作用，SCW围绕OH阴离子的溶剂化在局部上与AW相同。在过渡态，QM/MM模拟表明，多余的电子非常灵活，电荷体积以及原子上的分数电荷会根据瞬时溶剂构型而严重变化。然而，已发现SCW中的溶剂化能可通过玻恩方程与系统的最高占据分子轨道（HOMO）体积定性相关。
