Takahashi Hideaki, Satou Wataru, Hori Takumi, Nitta Tomoshige
Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan.
J Chem Phys. 2005 Jan 22;122(4):44504. doi: 10.1063/1.1839858.
A novel quantum chemical approach recently developed has been applied to an ionic dissociation of a water molecule (2H(2)O-->H(3)O(+)+OH(-)) in ambient and supercritical water. The method is based on the quantum mechanical/molecular mechanical (QM/MM) simulations combined with the theory of energy representation (QM/MM-ER), where the energy distribution function of MM solvent molecules around a QM solute serves as a fundamental variable to determine the hydration free energy of the solute according to the rigorous framework of the theory of energy representation. The density dependence of the dissociation free energy in the supercritical water has been investigated for the density range from 0.1 to 0.6 g/cm(3) with the temperature fixed at a constant. It has been found that the product ionic species significantly stabilizes in the high density region as compared with the low density. Consequently, the dissociation free energy decreases monotonically as the density increases. The decomposition of the hydration free energy has revealed that the entropic term (-TDeltaS) strongly depends on the density of the solution and dominates the behavior of the dissociation free energy with respect to the variation of the density. The increase in the entropic term in the low density region can be attributed to the decrease in the translational degrees of freedom brought about by the aggregation of solvent water molecules around the ionic solute.
最近开发的一种新型量子化学方法已应用于环境水和超临界水中水分子的离子解离（2H₂O→H₃O⁺+OH⁻）。该方法基于量子力学/分子力学（QM/MM）模拟与能量表示理论（QM/MM-ER）相结合，其中围绕量子力学溶质的分子力学溶剂分子的能量分布函数作为基本变量，根据能量表示理论的严格框架来确定溶质的水合自由能。在温度恒定的情况下，研究了超临界水中解离自由能在0.1至0.6 g/cm³密度范围内的密度依赖性。结果发现，与低密度相比，产物离子物种在高密度区域显著稳定。因此，解离自由能随密度增加而单调降低。水合自由能的分解表明，熵项（-TΔS）强烈依赖于溶液的密度，并在解离自由能随密度变化的行为中起主导作用。低密度区域熵项的增加可归因于离子溶质周围溶剂水分子聚集导致的平动自由度降低。
