Reactions of Ions in Water Involving Halogen-Bonded Complexes

Reactions of ions in water are important in many areas of chemistry and biology, including the chemistry of seawater, atmospheric chemistry, and processes in proteins. Despite their importance, the microscopic mechanisms of these inherently complex and multistep reactions are mostly unknown so far. In this Outlook, we outline recent progress on atmospheric reactions of halogenated species with anions in water. These are found to proceed via halogen-bonded complexes, which are stabilized by the water environment. Such complexes were recently characterized experimentally in the gas phase with structures similar to those computed in water. We highlight two such reactions: one that leads to the formation of a dihalogen and is enhanced with acidity. The other reaction is of halogen exchange, which involves participation of water molecules that greatly catalyze the process. These findings suggest searches for many other reactions that may involve halogen-bonded complexes as precursors. On the basis of the present studies, searches for such reaction mechanisms can be effectively pursued by ab initio molecular dynamics (AIMD) simulations. The halogen-bonded complexes can be complementarily characterized by vibrational spectroscopy in mass spectrometry experiments.