Observation of the simplest Criegee intermediate CH 2 OO in the gas-phase ozonolysis of ethylene

Ozonolysis is one of the dominant oxidation pathways for tropospheric alkenes. Although numerous studies have confirmed a 1,3-cycloaddition mechanism that generates a Criegee intermediate (CI) with form R[subscript 1]R[subscript 2]COO, no small CIs have ever been directly observed in the ozonolysis of alkenes because of their high reactivity. We present the first experimental detection of CH[subscript 2]OO in the gas-phase ozonolysis of ethylene, using Fourier transform microwave spectroscopy and a modified pulsed nozzle, which combines high reactant concentrations with rapid sampling and sensitive detection. Nine other product species of the O[subscript 3] + C[subscript 2]H[subscript 4] reaction were also detected, including formaldehyde, formic acid, dioxirane, and ethylene ozonide. The presence of all these species can be attributed to the unimolecular and bimolecular reactions of CH[subscript 2]OO, and their abundances are in qualitative agreement with published mechanisms and rate constants.National Science Foundation (U.S.) (CHE-1058063)Camille & Henry Dreyfus Foundation (Postdoctoral Program in Environmental Chemistry