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.