Computational Chemical Kinetics for the Reaction of Criegee Intermediate CH 2 OO with HNO 3 and Its Catalytic Conversion to OH and HCO.

The kinetics and mechanisms for the reaction of the Criegee intermediate CH2 OO 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 NO3 CH2 OOH* 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 CH2 OO 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 CH2 OO + 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 NO3 CH2 OOH and 0.79 for CH2 (O)NO3 + OH at a pressure of 40 Torr (N2 ), and 0.79 for the formation of NO3 CH2 OOH and 0.21 for CH2 (O)NO3 + OH at 760 Torr (N2 ). This new catalytic conversion of CH2 OO to HCO + OH by HNO3 might have significant impact on atmospheric chemistry.