School of Chemistry, University of East Anglia, Norwich NR4 7TJ, UK.
J Phys Chem B. 2011 Mar 3;115(8):1863-73. doi: 10.1021/jp111239v. Epub 2011 Feb 3.
A detailed picture of aqueous solvation of ions is central to the understanding of diverse phenomena in chemistry and biology. In this work, we report polarization resolved THz time domain measurements of the Raman spectral density of a wide range of aqueous salt solutions. In particular, the isotropic Raman spectral density reveals the frequency of the hydrogen bond formed between the halide ion and water. The frequency of this mode is measured for the series Cl(-), Br(-), and I(-) as a function of concentration, cation size, and charge. The frequencies extrapolated to zero concentration permit an estimation of the force constant of the mode, which is found to decrease with increasing halide mass and to be similar to the force constant associated with the water-water hydrogen bond. This result is consistent with recent calculations. The extrapolation of the frequency of the chloride hydrogen bond to zero concentration reveals a dependence of the frequency on the nature of the cation. This is ascribed to an interaction between the solvated anion and cation even at the lowest concentration studied here (<0.15 M). It is suggested that this behavior reflects the influence of the electric field of the cation on the hydrogen bond of an adjacent anion. Such interactions should be taken into account when modeling experimental data recorded at concentrations of ions in excess of 0.1 M. These measurements of the isotropic Raman spectral density are compared with those for the anisotropic response, which reflects the frequencies of the full range of hydrogen bonds in aqueous salt solutions. The anisotropic spectral density recovered can be modeled in terms of a concentration-dependent population of water-water H-bonds with a frequency unaffected by the ions, the halide-water hydrogen bonds, and a low-frequency collision-induced contribution.
离子在水溶液中的详细情况是理解化学和生物学中各种现象的核心。在这项工作中，我们报告了宽范围的水溶液盐溶液的各向异性喇曼光谱密度的极化分辨太赫兹时域测量。特别是，各向同性喇曼光谱密度揭示了卤化物离子和水之间形成的氢键的频率。该模式的频率被测量为 Cl(-)、Br(-)和 I(-)系列，作为浓度、阳离子大小和电荷的函数。零浓度下的频率外推允许估计该模式的力常数，发现该力常数随着卤化物质量的增加而减小，并且与水-水氢键的力常数相似。这一结果与最近的计算结果一致。氯化氢氢键的频率外推到零浓度揭示了频率对阳离子性质的依赖关系。这归因于溶剂化阴离子和阳离子之间的相互作用，即使在研究的最低浓度（<0.15 M）下也是如此。有人认为，这种行为反映了阳离子电场对相邻阴离子氢键的影响。在模型化浓度超过 0.1 M 的离子的实验数据时，应考虑到这种相互作用。各向异性喇曼光谱密度的这些测量与各向异性响应的测量进行了比较，各向异性响应反映了水溶液盐溶液中氢键的全范围的频率。恢复的各向异性光谱密度可以用与浓度相关的水-水 H 键的种群来建模，该种群的频率不受离子、卤化物-水氢键和低频碰撞诱导贡献的影响。
