Water-catalyzed decomposition of the simplest Criegee intermediate CH2OO

A comprehensive theoretical study on the gas-phase decomposition of the CH2OO intermediate is performed at the CCSD(T)//B3LYP/6-311+G(2df,2p) level in the presence of water molecules [n(H2O), n = 1-3]. The calculated results show that the most favorable route is the formation of dioxirane pathway (R3) in the unimolecular decomposition of CH2OO. For the reactions of CH2OO with uni-, bi-, and trimolecular water, the predominant pathway is the CH2OO with bimolecular water reaction as the major product is cis-HMHP. Compared with the barrier of CH2OO reaction with unimolecular water, an addition of water molecule contributes to a reduction of 4.91 kcal mol(-1) for the barrier. The elimination processes of cis-HMHP have two parallel competitive pathways: trans-HCOOH + H2O (R9) and HCOH + H2O2 (R10). The apparent activation barriers of these two reactions are significantly reduced with the increase in the number of water molecules involved. The barrier of R9 is higher than that of R10 about 8-15 kcal mol(-1) in the presence of water molecules, meaning that the favorable route is the formation of HCOH + H2O2 in the decomposition of cis-HMHP. The rate coefficients of CH2OO reaction with unimolecular water satisfy a positive temperature coefficient behavior at 298-500 K, whereas the rate coefficients of CH2OO reaction with bimolecular water exhibit a negative temperature dependence.