Infrared absorption spectrum of the simplest criegee intermediate CH ₂OO

The Criegee intermediates are carbonyl oxides postulated to play key roles in the reactions of ozone with unsaturated hydrocarbons; these reactions constitute an important mechanism for the removal of unsaturated hydrocarbons and for the production of OH in the atmosphere. Here, we report the transient infrared (IR) absorption spectrum of the simplest Criegee intermediate CH ₂OO, produced from CH₂I + O₂ in a flow reactor, using a step-scan Fourier-transform spectrometer. The five observed bands provide definitive identification of this intermediate. The observed vibrational frequencies are more consistent with a zwitterion rather than a diradical structure of CH₂OO. The direct IR detection of CH₂OO should prove useful for kinetic and mechanistic investigations of the Criegee mechanism.