Exploration of the Mechanism of the Activation of ClONO2by HCl in Small Water Clusters Using Electronic Structure Methods

High-level electronic structure calculations were used to study the mechanism of the reaction of ClONO 2 with HCl in neutral water clusters containing one to five solvating water molecules. For the reaction between molecular HCl and ClONO 2, the barrier decreases from 42 kcal mol -1 (uncatalyzed) to essentially zero when catalyzed by only two water molecules, where the reaction products involve Cl 2 and HONO 2 . The calculations thus predict that the gas-phase reaction may be important in the stratospheric reactivation of ClONO 2 . The reaction between ClONO 2 and solvated H 3 O + Cl -, as on the polar stratospheric cloud (PSC) surface, was investigated with clusters involving up to seven water molecules. The ice-catalyzed reaction involves an ionic mechanism whereby charge transfer to ClONO 2 from the attacking nucleophile leads to significant ionization along the Cl−ONO 2 bond. The effect of the size of the first solvation shell of Cl - is addressed by our calculations. In a cluster containing three waters and a five-water cluster structurally related to hexagonal ice, ClONO 2 reacts spontaneously with HCl to yield Cl 2 /HONO 2 in the three-water reaction and Cl 2 /H 3 O + NO 3 - in the five-water-catalyzed reaction. The calculations thus predict that the reaction of ClONO 2 with HCl on PSC ice aerosols can proceed spontaneously via an ionic pathway.