标题： 钠离子在液态水中的水合数 显示英文标题

标题： The hydration number of Na+ in liquid water

摘要： 一种在水溶液中钠离子的从头算分子动力学模拟被提出并进行了讨论。 该计算在周期性边界条件下处理一个钠离子和32个水分子，立方体积由在正常密度和344±24 K温度下该溶质在水中的零部分摩尔体积估算确定。对12 ps轨迹后半部分的分析显示，平均有4.6个水分子占据钠离子的内层水合壳层，其中5个是最可能的占据数。 观察到的钠离子自扩散系数为1.0 x 10$^{-5}$ cm$^2$/s。 溶液的准化学理论为另外两个计算提供了框架。 首先，一个补充计算基于离子-水团簇的电子结构结果以及外层水合贡献的介电连续体模型，预测了平均水合壳层占据数为4.0。 这种低估归因于团簇的谐波近似以及对外层贡献的近似介电连续体模型处理。 最后，通过内层占据数的最大熵拟合得到的不敏感复合自由能近似表明，在这些条件下，钠离子（aq）的水合自由能约为-68 kcal/mol，且没有提供包装或范德华相互作用的贡献。 显示英文摘要

摘要： An `ab initio' molecular dynamics simulation of a Na+ ion in aqueous solution is presented and discussed. The calculation treats a Na+ ion and 32 water molecules with periodic boundary conditions on a cubic volume determined by an estimate of zero partial molar volume for this solute in water at normal density and at a temperature of 344+/-24 K. Analysis of the last half of the 12 ps trajectory shows 4.6 water molecules occupying the inner hydration shell of the Na+ ion on average, with 5 being the most probable occupancy. The self-diffusion coefficient observed for the Na+ is 1.0 x 10$^{-5}$ cm$^2$/s. The quasi-chemical theory of solutions provides the framework for two more calculations. First a complementary calculation, based on electronic structure results for ion-water clusters and a dielectric continuum model of outer sphere hydration contributions, predicts an average hydration shell occupancy of 4.0. This underestimate is attributed to the harmonic approximation for the clusters in conjunction with the approximate dielectric continuum model treatment of outer sphere contributions. Finally, a maximum entropy fitting of inner sphere occupancies that leads to insensitive composite free energy approximations suggests a value in the neighborhood of -68 kcal/mol for the hydration free energy of Na+ (aq) under these conditions with no contribution supplied for packing or van der Waals interactions.