Exploration of the mechanism of the hydrolysis of chlorine nitrate in small water clusters using electronic structure methods

High-level electronic structure calculations have been used to study the me chanism of hydrolysis of chlorine nitrate in neutral water clusters contain ing three to eight solvating water molecules. The calculations clarify some of the current uncertainties in the hydrolysis mechanism. As the size of t he water cluster is increased, ClONO2 shows increasing ionization along the O2NO-Cl bond consistent with the proposed predissociation in which the ele ctrophilicity of the chlorine atom is enhanced, thus making it more suscept ible to nucleophilic attack from a surface water molecule. A species akin t o the experimentally observed intermediate, H2OCl+ is found to be stable in a cluster containing eight water molecules. The hydrolysis products, ioniz ed nitric (H3O+/NO3-) and molecular hypochlorous (HOCl) acids, are found to be stable in two different types of structures, containing six and eight w ater molecules. For the water cluster containing six water molecules, which has a structure related to ordinary hexagonal ice, ClONO2 is hydrolyzed to yield H3O+/NO3-/HOCl, with essentially no barrier. The calculations thus p redict that hydrolysis of ClONO2 on PSC ice aerosols can proceed spontaneou sly in small neutral water clusters.