Mechanism of the hydrolysis of halogen nitrates in small water clusters studied by electronic structure methods

High-level electronic structure calculations have been used to study the me chanism of the hydrolysis of bromine nitrate in neutral water clusters cont aining one to eight solvating water molecules. The calculations clarify som e of the uncertainties in the mechanism of halogen nitrate hydrolysis on PS C ice aerosols. The free energy barrier decreases from 42.4 kcal mol(-1) (s ingle water) to essentially zero when catalyzed by six water molecules. As the size of the water cluster is increased, BrONO2 shows increasing ionizat ion along the Br-ONO2 bond, consistent with the proposed predissociation in which the electrophilicity of the bromine atom is enhanced, thus making it more susceptible to nucleophilic attack from a surface water molecule. A s pecies akin to the experimentally proposed intermediate, H2OBr+NO3-, is fou nd to be stable in clusters containing both three and six water molecules, where the ion pair is separated by single and double layers of water molecu les, respectively. For a cluster containing six water molecules, which has a structure related to that of ordinary hexagonal ice, BrONO2 is hydrolyzed to yield HOBr and ionized nitric acid (H3O+NO3-. The calculations thus pre dict an ionic mechanism for the hydrolysis of halogen nitrates on PSC ice a erosols.