Detailed mechanism of the CH₂I + O₂ reaction: Yield and self-reaction of the simplest Criegee intermediate CH₂OO

The application of a new reaction scheme using CH₂I + O₂ to generate the simplest Criegee intermediate, CH₂OO, has stimulated lively research; the Criegee intermediates are extremely important in atmospheric chemistry. The detailed mechanism of CH₂I + O₂ is hence important in understanding kinetics involving CH₂OO. We employed ultraviolet absorption to probe simultaneously CH₂I₂, CH₂OO, CH₂I, and IO in the reaction system of CH₂I + O₂ upon photolysis at 248 nm of a flowing mixture of CH₂I₂, O₂, and N₂ (or SF₆) in the pressure range 7.6-779 Torr to investigate the reaction kinetics. With a detailed mechanism to model the observed temporal profiles of CH₂I, CH₂OO, and IO, we found that various channels of the reaction CH₂I + O₂ and CH₂OO + I play important roles; an additional decomposition channel of CH₂I + O₂ to form products other than CH₂OO or ICH₂OO becomes important at pressure less than 60 Torr. The pressure dependence of the derived rate coefficients of various channels of reactions of CH₂I + O₂ and CH₂OO + I has been determined. We derived a rate coefficient also for the self-reaction of CH₂OO as k = (8 ± 4) × 10^-11 cm³ molecule^-1 s^-1 at 295 K. The yield of CH₂OO from CH₂I + O₂ was found to have a pressure dependence on N₂ and O₂ smaller than in previous reports; for air under 1 atm, the yield of ∼30% is about twice of previous estimates.