More Criegee Sightings: The reaction of ozone with unsaturated hydrocarbons produces short-lived molecules termed Criegee intermediates. The simplest such molecule, H ₂ CO ₂ , was recently detected and monitored in the laboratory. Su et al. (p. 174 ; see the Perspective by Vereecken ) have obtained its vibrational spectrum, which could ultimately enable direct measurements of its reactivity in the atmosphere. Taatjes et al. (p. 177 ; see the Perspective by Vereecken ) report on the laboratory preparation and reactivity of the next heavier Criegee intermediate, which bears a methyl group in place of one of the hydrogen atoms.

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.