Riemenschneider Julian, Holzmann Jörg, Ludwig Ralf
Institut für Chemie, Abteilung Physikalische Chemie, Universität Rostock, Dr.-Lorenz-Weg 1, D-18051 Rostock, Germany.
Chemphyschem. 2008 Dec 22;9(18):2731-6. doi: 10.1002/cphc.200800571.
We study aqueous solutions of alkaline chlorides (NaCl, KCl, RbCl and CsCl) with a combination of attenuated total reflection infrared (ATR-IR) spectroscopy and molecular dynamics (MD) simulations using the TIP4P-Ew water model (Horn et al., J. Chem. Phys. 2004, 120, 9665), covering concentration ranges between 0.1 and 6 M. Spectral modifications in the OH stretch region are evaluated and correlated to the various salts and salt concentrations. By taking the difference spectra between the spectral line shapes of aqueous salt solutions and those of pure water, we specifically focus on the small quasi-"free OH" band appearing at the highest wavenumbers in the spectra. This free-OH feature is found constantly at 3650 cm(-1) for all salts and salt concentrations, but it shows a characteristic intensity depending on the chosen cation. In the order from Na(+) to Cs(+), the free OH intensity decreases compared to that of pure water. To interpret the experimental results, we performed MD simulations for similar salt solutions. The experimentally observed effects can be correlated with structural alterations indicated by differences between the site-site pair correlation functions of water in aqueous salt solutions and those of pure water.
我们结合衰减全反射红外（ATR - IR）光谱和分子动力学（MD）模拟，使用TIP4P - Ew水模型（Horn等人，《化学物理杂志》，2004年，120卷，9665页）研究了碱性氯化物（NaCl、KCl、RbCl和CsCl）的水溶液，涵盖了0.1至6 M的浓度范围。对OH伸缩振动区域的光谱变化进行了评估，并与各种盐类和盐浓度相关联。通过获取盐水溶液和纯水光谱线形之间的差谱，我们特别关注光谱中最高波数处出现的小的准“自由OH”带。对于所有盐类和盐浓度，该自由OH特征在3650 cm⁻¹处恒定出现，但它显示出取决于所选阳离子的特征强度。从Na⁺到Cs⁺，与纯水相比，自由OH强度降低。为了解释实验结果，我们对类似的盐溶液进行了MD模拟。实验观察到的效应可以与盐水溶液中水分子的位点 - 位点对关联函数与纯水之间的差异所表明的结构变化相关联。
