Department of Chemistry and Green-Nano Materials Research Center, College of Natural Sciences, Kyungpook National University, Taegu 702-701, Korea.
J Phys Chem B. 2013 Aug 8;117(31):9273-9. doi: 10.1021/jp4049346. Epub 2013 Jul 25.
The formation of like-ion pairs, Na(+)-Na(+) and Cl(-)-Cl(-), in aqueous solution was studied by high-level ab initio methods, classical molecular dynamics (MD), QM/TIP5P, and QM/EFP MD (quantum mechanics/effective fragment potential molecular dynamics). Ab initio calculations on model clusters revealed that the Na(+)(H2O)nNa(+) (n = 2-4) clusters were significantly more stabilized by bridged waters than the corresponding Cl(-)(H2O)nCl(-) clusters. QM/EFP MD simulations in solution also predicted a clear local minimum near 3.6 Å only for the Na(+)-Na(+) pair, suggesting that Na(+)-Na(+) pairs may be more likely to form than Cl(-)-Cl(-) pairs in solution. Analysis of the hydration structures further showed that two-water bridged Na(+)-Na(+) pairs were dominant at the local minimum. The preferred formation of Na(+) like-ion pairs in solution appeared to come from significant short-range effects, in particular, charge delocalization (polarization) between the bridged oxygen p and the vacant valence Na(+) orbitals.
在水溶液中，通过高级从头算方法、经典分子动力学（MD）、QM/TIP5P 和 QM/EFP MD（量子力学/有效片段势能分子动力学）研究了同离子对 Na(+)-Na(+)和 Cl(-)-Cl(-)的形成。对模型簇的从头算计算表明，桥接水分子显著稳定了 Na(+)(H2O)nNa(+)（n = 2-4）簇，而相应的 Cl(-)(H2O)nCl(-)簇则不稳定。在溶液中的 QM/EFP MD 模拟也预测了仅在 Na(+)-Na(+)对附近存在明显的局部极小值，这表明在溶液中 Na(+)-Na(+)对可能比 Cl(-)-Cl(-)对更容易形成。对水合结构的进一步分析表明，在局部最小值处，双水分子桥接的 Na(+)-Na(+)对占主导地位。溶液中 Na(+)同离子对的优先形成似乎来自于显著的短程效应，特别是桥接氧 p 和空的价 Na(+)轨道之间的电荷离域（极化）。
