Exploration of the mechanism of the activation of ClONO2 by HCl in small water clusters using electronic structure methods

High-level electronic structure calculations were used to study the mechani sm of the reaction of ClONO2 with HCl in neutral water clusters containing one to five solvating water molecules. For the reaction between molecular H Cl and ClONO2, the barrier decreases from 42 kcal mol(-1) (uncatalyzed) to essentially zero when catalyzed by only two water molecules, where the reac tion products involve Cl-2 and HONO2. The calculations thus predict that th e gas-phase reaction may be important in the stratospheric reactivation of ClONO2. The reaction between ClONO2 and solvated H3O+Cl-, as on the polar s tratospheric cloud (PSC) surface, was investigated with clusters involving up to seven water molecules. The ice-catalyzed reaction involves an ionic m echanism whereby charge transfer to ClONO2 from the attacking nucleophile l eads to significant ionization along the Cl-ONO2 bond. The effect of the si ze of the first solvation shell of Cl- is addressed by our calculations. In a cluster containing three waters and a five-water cluster structurally re lated to hexagonal ice, ClONO2 reacts spontaneously with HCl to yield Cl-2/ HONO2 in the three-water reaction and Cl-2/H3O+-NO3- in the five-water-cata lyzed reaction. The calculations thus predict that the reaction of ClONO2 w ith HCl on PSC ice aerosols can proceed spontaneously via an ionic pathway.