Mechanism and kinetics of the reaction of the Criegee intermediate CH 2 OO with acetic acid studied using a step-scan Fourier-transform IR spectrometer.

Acetic acid, CH 3 C(O)OH, plays an important role in the acidity of the troposphere. The reactions of Criegee intermediates with CH 3 C(O)OH have been proposed to be a potential source of secondary organic aerosol in the atmosphere. We investigated the detailed mechanism and kinetics of the reaction of the Criegee intermediate CH 2 OO with CH 3 C(O)OH. The time-resolved infrared absorption spectra of transient species produced upon irradiation at 308 nm of a flowing mixture of CH 2 I 2 /O 2 /CH 3 C(O)OH at 298 K were recorded using a step-scan Fourier-transform infrared spectrometer. The decrease in the intensity of the bands of CH 2 OO was accompanied by the appearance of bands near 886, 971, 1021, 1078, 1160, 1225, 1377, 1402, 1434, and 1777 cm -1 , assigned to the absorption of hydroperoxymethyl acetate [CH 3 C(O)OCH 2 OOH, HPMA], the hydrogen-transferred adduct of CH 2 OO and CH 3 C(O)OH. Two types of conformers of HPMA, an open form and an intramolecularly hydrogen-bonded form, were identified. At a later reaction period, bands of the open-form HPMA became diminished, and new bands appeared at 930, 1045, 1200, 1378, 1792, and 1810 cm -1 , assigned to formic acetic anhydride [CH 3 C(O)OC(O)H, FAA], a dehydrated product of HPMA. The intramolecularly hydrogen-bonded HPMA is more stable. From the temporal profiles of HPMA and FAA, we derived a rate coefficient k = (1.3 ± 0.3) × 10 -10 cm 3 molecule -1 s -1 for the reaction CH 2 OO + CH 3 C(O)OH to form HPMA and a rate coefficient k = 980 ± 40 s -1 for the dehydration of the open-form HPMA to form FAA. Theoretical calculations were performed to elucidate the CH 2 OO + CH 3 C(O)OH reaction pathway and to understand the distinct reactivity of these two forms of HPMA.