Computational Chemical Kinetics for the Reaction of Criegee Intermediate CH₂OO with HNO₃ and Its Catalytic Conversion...

The kinetics and mechanisms for the reaction of the Criegee intermediate CH₂OO with HNO₃ and the unimolecular decomposition of its reaction product CH₂(O)NO₃ are important in atmospheric chemistry. The potential-energy profile of the reactions predicted with the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ method shows that the initial association yields a prereaction complex that isomerizes by H migration to yield excited intermediate nitrooxymethyl hydroperoxide NO₃CH₂OOH* with internal energy ∼44 kcal mol^-1. A fragmentation of this excited intermediate produces CH₂(O)NO₃ + OH with its transition state located 5.0 kcal mol^-1 below that of the reactants. Further decomposition of CH₂(O)NO₃ produces HCO + HNO₃, forming a catalytic cycle for destruction of CH₂OO by HNO₃. The rate coefficients and product-branching ratios were calculated in the temperature range 250-700 K at pressure 20-760 Torr (N₂) using the variational-transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories. The predicted total rate coefficient for reaction CH₂OO + HNO₃ at 295 K, 5.1 × 10^-10 cm³ molecule^-1 s^-1, agrees satisfactorily with the experimental value, (5.4 ± 1.0) × 10^-10 cm³ molecule^-1 s^-1. The predicted branching ratios at 295 K are 0.21 for the formation of NO₃CH₂OOH and 0.79 for CH₂(O)NO₃ + OH at a pressure of 40 Torr (N₂), and 0.79 for the formation of NO₃CH₂OOH and 0.21 for CH₂(O)NO₃ + OH at 760 Torr (N₂). This new catalytic conversion of CH₂OO to HCO + OH by HNO₃ might have significant impact on atmospheric chemistry.