Detailed mechanism of the CH2I + O-2 reaction: Yield and self-reaction of the simplest Criegee intermediate CH2OO

Abstract

The application of a new reaction scheme using CH2I + O-2 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 + O-2 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 + O-2 upon photolysis at 248 nm of a flowing mixture of CH2I2, O-2, and N-2 (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 + O-2 and CH2OO + I play important roles; an additional decomposition channel of CH2I + O-2 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 + O-2 and CH2OO + I has been determined. We derived a rate coefficient also for the self-reaction of CH2OO as k = (8 +/- 4) x 10(-11) cm(3) molecule(-1) s(-1) at 295 K. The yield of CH2OO from CH2I + O-2 was found to have a pressure dependence on N-2 and O-2 smaller than in previous reports; for air under 1 atm, the yield of similar to 30% is about twice of previous estimates. (C) 2014 AIP Publishing LLC.