Mechanism of the gaseous hydrolysis reaction of SO₂: Effects of NH₃ versus H₂O.

Effects of ammonia and water molecules on the hydrolysis of sulfur dioxide are investigated by theoretical calculations of two series of the molecular clusters SO2-(H2O)n (n = 1-5) and SO2-(H2O)n-NH3 (n = 1-3). The reaction in pure water clusters is thermodynamically unfavorable. The additional water in the clusters reduces the energy barrier for the reaction, and the effect of each water decreases with the increasing number of water molecules in the clusters. There is a considerable energy barrier for reaction in SO2-(H2O)5, 5.69 kcal/mol. With ammonia included in the cluster, SO2-(H2O)n-NH3, the energy barrier is dramatically reduced, to 1.89 kcal/mol with n = 3, and the corresponding product of hydrated ammonium bisulfate NH4HSO3-(H2O)2 is also stabilized thermodynamically. The present study shows that ammonia has larger kinetic and thermodynamic effects than water in promoting the hydrolysis reaction of SO2 in small clusters favorable in the atmosphere.