Infrared identification of the Criegee intermediates syn- and anti-CH(3)CHOO, 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 CH(2)OO has been reported. Methyl substitution of CH(2)OO produces two conformers of CH(3)CHOO and consequently complicates the infrared spectrum. Here we report the transient infrared spectrum of syn- and anti-CH(3)CHOO, produced from CH(3)CHI + O(2) 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-CH(3)CHOO shows a reactivity greater than syn-CH(3)CHOO towards NO/NO(2); at the later period of reaction, the spectrum can be simulated with only syn-CH(3)CHOO. Without NO/NO(2), anti-CH(3)CHOO also decays much faster than syn-CH(3)CHOO. The direct infrared detection of syn- and anti-CH(3)CHOO should prove useful for field measurements and laboratory investigations of the Criegee mechanism.