In aqueous solution, many chemical and biological substances, from simple salts to proteins and DNA, are charged. The solvation of ions in water is accompanied by the formation of a hydration shell – a layer of water molecules, which interacts strongly with the ions. Intermolecular thermal fluctuations of a hydration shell is essential in the substitution and elimination reactions, where they provide the energy needed to surmount the reaction barrier. Due to the altered intermolecular forces, the spectrum of thermal fluctuations of a hydration shell is significantly different from that of the bulk water, as revealed by the far-infrared absorption spectroscopy. However, little is known about the inhomogeneity of these intermolecular vibrations. Here, we report the use the two-dimensional Terahertz-InfraRed-Visible (2D TIRV) spectroscopy to measure the inhomogeneity of the intermolecular terahertz vibrations in the hydration shells of simple inorganic ions. The influence of an ion on the surrounding water molecules depends on its charge density: the higher is the charge density, the stronger is the interaction between the ion and water. We found that even ions with modest charge density, such as Na+ and Cl-, can induce significant inhomogeneity of the terahertz vibrations of water (Fig. 1). In the hydration shells of these ions, the water molecules with stronger hydrogen bonds have lower frequencies of the intermolecular vibrations. Our results thus provide an insight into the correlation between the strength of the hydrogen bonds and the terahertz fluctuations of the solvating water molecules.

Inhomogeneity of the intermolecular terahertz modes in aqueous salt solutions

2019 Fall National Meeting

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