Reaction mechanism and kinetics of Criegee intermediate CH2OO with CH2 = C(CH3)CHO

Abstract

The comprehensive mechanism and kinetics for CH2OO with methacrolein (CH2 = C(CH3)CHO, MACR) reaction were investigated at the CCSD(T)/6-311+G(2df,2p)//M06-2X/6-311+G(2df,2p) level of theory. The results show that three kinds of mechanisms including cycloaddition, oxidation and insertion have been identified. Among them, the cycloaddition of CH2OO adding to CO bond of MACR is more favorable, which occurs via a deeply submerged barrier leading to the formation of energized secondary ozonide (SOZ). The nascent SOZ mainly further decomposes into methacrylic acid and formaldehyde or formic acid and methacrolein, in which formic acid generation follows stepwise and concerted mechanism involving a catalysis for rearrangement of CH2OO by methacrolein. The calculated rate constant for the title reaction at 300 K using canonical variational transition state theory with small curvature tunneling correction is 1.00 × 10−12 cm3 molecule−1 s−1, which is close to the experimental value of (4.4 ± 1.0) × 10−13 cm3 molecule−1 s−1. The results reveal that the reaction provides a new pathway to produce organic acids. However, the contribution of acids from this reaction is small as its relative lower rate, compared to atmospheric CH2OO loss occurring through the hydrolysis of CH2OO + (H2O)n (n = 1–2).