Laboratory of Physical Chemistry, ETH Zürich, CH-8093 Zürich, Switzerland.
J Chem Phys. 2011 Apr 14;134(14):144104. doi: 10.1063/1.3567022.
The raw single-ion solvation free energies computed from atomistic (explicit-solvent) simulations are extremely sensitive to the boundary conditions and treatment of electrostatic interactions used during these simulations. However, as shown recently [M. A. Kastenholz and P. H. Hünenberger, J. Chem. Phys. 124, 224501 (2006); M. M. Reif and P. H. Hünenberger, J. Chem. Phys. 134, 144103 (2010)], the application of appropriate correction terms permits to obtain methodology-independent results. The corrected values are then exclusively characteristic of the underlying molecular model including in particular the ion-solvent van der Waals interaction parameters, determining the effective ion size and the magnitude of its dispersion interactions. In the present study, the comparison of calculated (corrected) hydration free energies with experimental data (along with the consideration of ionic polarizabilities) is used to calibrate new sets of ion-solvent van der Waals (Lennard-Jones) interaction parameters for the alkali (Li(+), Na(+), K(+), Rb(+), Cs(+)) and halide (F(-), Cl(-), Br(-), I(-)) ions along with either the SPC or the SPC/E water models. The experimental dataset is defined by conventional single-ion hydration free energies [Tissandier et al., J. Phys. Chem. A 102, 7787 (1998); Fawcett, J. Phys. Chem. B 103, 11181] along with three plausible choices for the (experimentally elusive) value of the absolute (intrinsic) hydration free energy of the proton, namely, ΔG(hyd)(⊖)[H(+)] = -1100, -1075 or -1050 kJ mol(-1), resulting in three sets L, M, and H for the SPC water model and three sets L(E), M(E), and H(E) for the SPC/E water model (alternative sets can easily be interpolated to intermediate ΔG(hyd)(⊖)[H(+)] values). The residual sensitivity of the calculated (corrected) hydration free energies on the volume-pressure boundary conditions and on the effective ionic radius entering into the calculation of the correction terms is also evaluated and found to be very limited. Ultimately, it is expected that comparison with other experimental ionic properties (e.g., derivative single-ion solvation properties, as well as data concerning ionic crystals, melts, solutions at finite concentrations, or nonaqueous solutions) will permit to validate one specific set and thus, the associated ΔG(hyd)(⊖)[H(+)] value (atomistic consistency assumption). Preliminary results (first-peak positions in the ion-water radial distribution functions, partial molar volumes of ionic salts in water, and structural properties of ionic crystals) support a value of ΔG(hyd)(⊖)[H(+)] close to -1100 kJ·mol(-1).
从原子（显式溶剂）模拟中计算出的原始单离子溶剂化自由能对模拟过程中使用的边界条件和静电相互作用的处理方法非常敏感。然而，正如最近[M. A. Kastenholz 和 P. H. Hünenberger，J. Chem. Phys. 124, 224501 (2006)；M. M. Reif 和 P. H. Hünenberger，J. Chem. Phys. 134, 144103 (2010)]所示，应用适当的修正项可以得到与方法无关的结果。修正后的数值仅与基础分子模型有关，包括离子-溶剂范德华相互作用参数，这些参数决定了离子的有效尺寸和色散相互作用的大小。在本研究中，通过与实验数据（同时考虑离子极化率）进行比较，计算出（修正后的）水合自由能被用于校准新的碱（Li(+)、Na(+)、K(+)、Rb(+)、Cs(+)）和卤化物（F(-)、Cl(-)、Br(-)、I(-)）离子与 SPC 或 SPC/E 水模型的溶剂化范德华（Lennard-Jones）相互作用参数。实验数据集由传统的单离子水合自由能[Tissandier 等人，J. Phys. Chem. A 102, 7787 (1998)；Fawcett，J. Phys. Chem. B 103, 11181]以及质子绝对（内在）水合自由能的三个合理选择（实验上难以捉摸），即ΔG(hyd)(⊖)[H(+)] = -1100、-1075 或 -1050 kJ·mol(-1)，这导致 SPC 水模型的三个数据集 L、M 和 H 以及 SPC/E 水模型的三个数据集 L(E)、M(E)和 H(E)（可以轻松插值到中间的ΔG(hyd)(⊖)[H(+)]值）。还评估了计算出的（修正后的）水合自由能对体积-压力边界条件和修正项计算中进入的有效离子半径的剩余敏感性，发现其非常有限。最终，预计与其他实验离子性质（例如，单离子溶剂化性质的导数，以及有关离子晶体、熔体、有限浓度溶液或非水溶液的数据）的比较将验证特定的数据集，从而验证相关的ΔG(hyd)(⊖)[H(+)]值（原子一致性假设）。初步结果（离子-水径向分布函数的第一峰位置、离子盐在水中的偏摩尔体积以及离子晶体的结构性质）支持ΔG(hyd)(⊖)[H(+)]接近-1100 kJ·mol(-1)的值。
