Theoretical Investigation of the Reaction Mechanism of ClONO₂ + HCl → HNO₃ + Cl₂ on (H₂O)_<i>n</i> (<i>n</i> = 0–3) Cluster

Hydrated chlorine nitrate and hydrogen chloride ClONO₂·HCl·(H₂O)_<i>n</i> (<i>n</i> = 0–3) clusters were investigated by using the MP2/aug-cc-pVTZ level of theory to clarify the reaction mechanism of Cl₂ production. Isomeric stable structures found in <i>n</i> = 2 and 3 clusters have equivalent binding energies and the reaction barrier drastically decreases to be 2.1 kcal/mol at <i>n</i> = 3. The plausible reaction pathways were proposed according to calculated structures and energies, where the zero-point-energy corrections are important to determine the energy profiles of reactions especially for the <i>n</i> = 3 cluster. The kinetic analysis using the transition state theory suggested that the reaction rate constant from the original reactants to the product of <i>n</i> = 3 is 1.8 × 10⁵ times larger than that of <i>n</i> = 2 cluster. Even though the small amount of the molar concentration of HCl­(H₂O)₃ is obtained, the overall reaction rate of the trihydrate reaction is still 35 times faster than that of the dihydrate