A Molecular Perspective on Ion Hydration

Hydration of anions, particularly halide ions, presents a particularly challenging problem where due to strong intermolecular interactions the ion can significantly alter the hydrogen bonding network of water. The extent to which varies greatly depending on the nature of ion-water interactions. An accurate description of the interplay between ion-water and water-water interactions is necessary to achieve a molecular level understanding of ion hydration. In this work, we present a bottom-up analysis of the structure, energetics, vibrational spectroscopy and hydrogen bond arrangement of small halide-water clusters (X-(H2O)n, X- = F-, Cl-, Br-, I-) using state-of-the-art computational chemistry tools. We begin by developing ab initio based many-body potential energy functions PEFs, called MB-nrg, for describing halide-water intermolecular interactions that include many-body effects for all system sizes by taking into account explicitly the two-body and three-body interactions, and all higher order interactions implicitly through a mean field approximation. To directly probe the strength of halide-water intermolecular interactions, full dimensional vibrational spectra are calculated for both X-(H2O) and X-(D2O) dimers at the quantum-mechanical level. Followed by an analysis of the structure, hydrogen bond arrangement and temperature dependent dynamics of the I-(H2O)2 and I-(D2O)2 through quantum path integral molecular dynamics simulations. Tunneling pathways leading hydrogen bond rearrangement were identified and the corresponding tunneling splitting patterns were calculated using the ring polymer instanton method. Finally, we studied the structural, thermodynamic and spectroscopic properties of small X-(H2O)n clusters where X- = F-, Cl-, Br-, I-), n=3-6, using replica exchange molecular dynamics simulations. Across all sizes, fluoride-water clusters exhibit qualitatively different structures and properties compared to the chloride-, bromide- and iodide-water clusters which, on the other hand, are found to be similar to each other. This is a direct consequence of the exceptionally strong fluoride-water intermolecular interactions, which significantly affect the water-water hydrogen bonding strength and arrangement in the vicinity. Through extensive comparisons between the MB-nrg PEFs and classical polarizable force fields and approximate ab initio methods like density functional theory and MP2, our results emphasize the importance of an accurate description of the quantum mechanical many-body intermolecular interactions for a robust molecular level understanding of halide ion hydration. Follow-up studies of larger cluster sizes will focus on the evolution of the hydration shells in a systematic way