Specific Ion Effects on Hydrogen-Bond Rearrangements in the Halide–Dihydrate Complexes

Small aqueous ionic clusters represent ideal systems to investigate the microscopic hydrogen-bonding structure and dynamics in ion hydration shells. In this context, halide-dihydrate complexes are the smallest systems where the interplay between halide–water and water–water interactions can be studied simultaneously. Here, quantum molecular dynamics simulations unravel specific ion effects on the temperature-dependent structural transition in X^-(H₂O)₂ complexes (X = Cl, Br and I) which is induced by the breaking of the water–water hydrogen bond. A systematic analysis of the hydrogen-bonding rearrangements at low temperature provides fundamental insights into the competition between halide–water and water–water interactions depending on the properties of the halide ion. While the halide–water hydrogen-bond strength decreases going from Cl^-(H₂O)₂ to I^-(H₂O)₂, the opposite trend in observed in the strength of the water–water hydrogen-bond, suggesting that non-trivial many-body effects may also be at play in the hydration shells of halide ions in solution, especially in frustrated systems (e.g., interfaces) where the water molecules can have dangling OH bonds.