How water affects mercury-halogen interaction in the atmosphere.

The solvation of mercury and halogens ions in water is essential for studying the reaction kinetics of various mercury depletion reactions in the atmosphere. Here, we use two approaches. The first one is the implementation of transition state theory to study the recombination reactions of Hg2+and Hal- with the introduction of a water molecule as a third body part. The inclusion of solvation corrections to the total energy enables one to localize the barrier for such diatomic systems with explicit water molecule participation. The second approach is the molecular modeling of three mercuric halide ion pairs in water complexes [HgHal(H2O)n]+ (Hal = Cl, Br, I) by using the semiempirical tight-binding molecular dynamics combined with density functional theory calculations. Various [Hg-Hal]+ ion pairs behave similarly when hydrated and tend to adopt clathrate-like configurations with a [Hg2+(H2O)6] central motif and halogen ions residing on the external surface of the water complex. Contact ion pairs are energetically favorable for all complexes up to 50 water molecules. Further increase in the level of hydration stabilized the solvent-separated forms of [Hg-Hal]+ ion pairs, which matches the water affinity rule. The balance between the contact and the solvated ion pairs was shown to be ion-pair specific and temperature dependent. Graphical abstractThe structure of stable water complexes of mercury halides reflects the competition between water-water, Hg2+ -water, and Hal -water interactions.