Hydrolysis of SO3 and ClONO2 in water clusters: A combined experimental and theoretical study

The adsorption and reactivity of the atmospherically important molecules SO3 and ClONO2 on large water clusters as prototypes for atmospheric aerosols has been studied using a supersonic molecular beam expansion to generate clusters containing 50-450 water molecules. The experiments show that both SO3 and ClONO2 undergo hydrolysis reactions on water clusters containing a few hundred water molecules. Using both quantum mechanical and molecular mechanical methods, models of these reactions involving both catalytic water molecules directly involved in the reactions and additional solvating waters have been studied computationally. In both cases, the barrier to reaction is predicted to be considerably lowered by the solvating water molecules. For SO3 hydrolysis, such lowering by ca. 10 water molecules effectively removes the barrier to reaction. For the ClONO2 reaction, the initial barrier in the absence of solvation is too high to be sufficiently lowered in the cluster for it to proceed. In this case, our computational results thus suggest that a different mechanism is operable, such as an ionic or acid-catalysed one.