Halide Ion Microhydration: Structure, Energetics, and Spectroscopy of Small Halide–Water Clusters

Replica exchange molecular dynamics simulations and vibrational spectroscopy calculations are performed using halide–water many-body potential energy functions to provide a bottom-up analysis of the structures, energetics, and hydrogen-bonding arrangements in X–(H2O)n (n = 3–6) clusters, with X = F, Cl, Br, and I. Independently of the cluster size, it is found that all four halides prefer surface-type structures in which they occupy one of the vertices in the underlying three-dimensional hydrogen-bond networks. For fluoride–water clusters, this is in contrast to previous reports suggesting that fluoride prefers interior-type arrangements, where the ion is fully hydrated. These differences can be ascribed to the variability in how various molecular models are capable of reproducing the subtle interplay between halide–water and water–water interactions. Our results thus emphasize the importance of a correct representation of individual many-body contributions to the molecular interactions for a quantitative description of halide ion hydration