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

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 (∼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.