Theoretical investigation on the reaction mechanism and kinetics of a Criegee intermediate with ethylene and acetylene

The detailed reaction mechanism of the Criegee intermediate CH₂OO with ethylene and acetylene has been investigated by using the HL//M06-2X/AUG-cc-pVTZ method. The 1,3-cycloaddition of CH₂OO to the unsaturated bond of ethylene or acetylene forms a five-membered ring adduct. For the reaction of CH₂OO with ethylene, the subsequent ring-opening, H-shift isomerization and decomposition result in the formation of ethenol + HCHO and acetaldehyde + HCHO, and for the reaction of CH₂OO with acetylene, the adduct proceeds via ring-opening and H-shift isomerization forming malonaldehyde. The calculated overall rate constant increases in the temperature range of 200-500 K, and at 298 K, it is 3.91 × 10^-15 cm³ molecule^-1 s^-1 for the CH₂OO + C₂H₄ reaction and 1.27 × 10^-16 cm³ molecule^-1 s^-1 for the CH₂OO + C₂H₂ reaction. The product branching ratio of the CH₂OO + C₂H₄ reaction is pressure dependent, and the adduct tends to decompose to ethenol + HCHO and acetaldehyde + HCHO at lower pressures and higher temperatures. For the CH₂OO + C₂H₂ reaction, the adduct isomerizes completely to malonaldehyde in the temperature range of 200-500 K and the pressure range of 100-1000 Torr.