Mallik Bhabani S, Semparithi A, Chandra Amalendu
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India.
J Chem Phys. 2008 Nov 21;129(19):194512. doi: 10.1063/1.3006032.
A theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions is presented from first principles without employing any empirical potential models. The present calculations are based on ab initio molecular dynamics for trajectory generation and wavelet analysis of the simulated trajectories for time dependent frequency calculations. Results are obtained for two different deuterated aqueous solutions: the first one is a relatively dilute solution of a single Cl(-) ion and the second one is a concentrated solution of NaCl ( approximately 3M) dissolved in liquid D(2)O. It is found that the frequencies of OD bonds in the anion hydration shell, i.e., those which are hydrogen bonded to the chloride ion, have a higher stretch frequency than those in the bulk water. Also, on average, the frequencies of hydration shell OD modes are found to increase with increase in the anion-water hydrogen bond distance. On the dynamical side, when the vibrational spectral diffusion is calculated exclusively for the hydration shell water molecules in the first solution, the dynamics reveals three time scales: a short-time relaxation ( approximately 200 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation ( approximately 3 ps) corresponding to the lifetimes of chloride ion-water hydrogen bonds, and another longer-time constant ( approximately 20 ps) corresponding to the escape dynamics of water from the anion hydration shell. Existence of such three time scales for hydration shell water molecules was also reported earlier for water containing a single iodide ion using classical molecular dynamics [B. Nigro et al., J. Phys. Chem. A 110, 11237 (2006)]. Hence, the present study confirms the basic results of this earlier work using a different methodology. However, when the vibrational spectral diffusion is calculated over all the OD modes, only two time scales of approximately 150 fs and approximately 2.7 ps are found without the slowest component of approximately 20 ps. This is likely because of the very small weight that the hydration shell water molecules carry to the overall spectral diffusion in the solution containing a single ion. For the concentrated solution also, the slowest component of approximately 20 ps is not found in the spectral diffusion of all water molecules because a distinct separation between the hydration shell and bulk water in terms of their stretch frequencies does not hold at this high concentration regime. The present first principles results are compared with those of the available experiments and classical simulations.
本文从第一性原理出发，对水合离子溶液中的振动光谱扩散和氢键动力学进行了理论研究，未采用任何经验势模型。当前的计算基于从头算分子动力学来生成轨迹，并对模拟轨迹进行小波分析以进行随时间变化的频率计算。得到了两种不同的氘代水溶液的结果：第一种是单个Cl(-)离子的相对稀溶液，第二种是溶解在液态D₂O中的NaCl浓溶液（约3M）。研究发现，阴离子水合壳层中OD键的频率，即那些与氯离子形成氢键的OD键，其伸缩频率高于本体水中的OD键。此外，平均而言，水合壳层OD模式的频率随阴离子 - 水氢键距离的增加而增加。在动力学方面，当仅对第一种溶液中水合壳层水分子的振动光谱扩散进行计算时，动力学显示出三个时间尺度：对应于完整离子 - 水氢键动力学的短时间弛豫（约200 fs）、对应于氯离子 - 水氢键寿命的较慢弛豫（约3 ps）以及对应于水从阴离子水合壳层逃逸动力学的另一个较长时间常数（约20 ps）。使用经典分子动力学方法，对于含有单个碘离子的水，此前也曾报道过水合壳层水分子存在这样三个时间尺度[B. Nigro等人，《物理化学杂志A》110, 11237 (2006)]。因此，本研究使用不同方法证实了该早期工作的基本结果。然而，当对所有OD模式计算振动光谱扩散时，仅发现了约150 fs和约2.7 ps的两个时间尺度，没有约20 ps这个最慢的成分。这可能是因为在含有单个离子的溶液中，水合壳层水分子对整体光谱扩散的贡献非常小。对于浓溶液，在所有水分子的光谱扩散中也未发现约20 ps这个最慢的成分，因为在这种高浓度体系中，水合壳层和本体水在伸缩频率方面没有明显的区分。将当前的第一性原理结果与现有实验和经典模拟结果进行了比较。
