Mechanism of a Halogen Exchange Reaction in Water: Catalysis by Aqueous Media

Reactions of Cl^-, Br^-, and I^- ions in seawater with incoming molecules from the gas phase are of major atmospheric importance, but their mechanisms are mostly unknown. In this study, using ab initio molecular dynamics (AIMD) simulations, the microscopic mechanism of the halogen exchange reaction in water, HOCl + I^- → HOI + Cl^-, is unraveled. The main findings are as follows: (1) The reaction proceeds through a halogen-bonded isomer of the complex of HOCl with I^-, which is present in water and has a significant lifetime. The hydrogen-bonded isomer of the complex seems to play no role in the reaction. (2) Several water molecules act to catalyze the reaction through a Grotthuss-like mechanism that is totally different from that of halogen exchange in the gas phase. These results may have important implications for the chemistry of seawater, in particular for other reactions involving halogenated species.