A novel mechanism for the isomerization of N2O4 and its implication for the reaction with H2O and acid rain formation

Abstract

We have discovered, by high-level quantum-chemical calculations, a new and predominant isomerization mechanism for N2O4 -> ONONO2 via a roaming-like transition state occurring unimolecularly or bimolecularly during collision with H2O. The new mechanism allows N2O4 to react with H2O with a significantly lower barrier (< 13.1 kcal/mol) than the commonly known tight transition state (similar to 30-45 kcal/mol) by concurrent stretching of the N-N bond and rotation of one of the NO2 groups to form trans-ONONO2, which then undergoes a rapid metathetical reaction with H2O in the gas phase and in aqueous solution. The results have a significant implication for the hydrolysis of N2O4 in water to produce HONO and HNO3. Rate constants for the isomerization and hydrolysis reactions have been predicted for atmospheric modeling applications.