Infrared identification of the Criegee intermediates syn- and anti-CH₃CHOO, and their distinct conformation-dependent reactivity

The Criegee intermediates are carbonyl oxides that play critical roles in ozonolysis of alkenes in the atmosphere. So far, the mid-infrared spectrum of only the simplest Criegee intermediate CH₂OO has been reported. Methyl substitution of CH₂OO produces two conformers of CH₃CHOO and consequently complicates the infrared spectrum. Here we report the transient infrared spectrum of syn- and anti-CH₃CHOO, produced from CH₃CHI + O₂ in a flow reactor, using a step-scan Fourier-transform spectrometer. Guided and supported by high-level full-dimensional quantum calculations, rotational contours of the four observed bands are simulated successfully and provide definitive identification of both conformers. Furthermore, anti-CH₃CHOO shows a reactivity greater than syn-CH₃CHOO towards NO/NO₂; at the later period of reaction, the spectrum can be simulated with only syn-CH₃CHOO. Without NO/NO₂, anti-CH₃CHOO also decays much faster than syn-CH₃CHOO. The direct infrared detection of syn- and anti-CH₃CHOO should prove useful for field measurements and laboratory investigations of the Criegee mechanism.