Infrared Characterization of the Products and Rate Coefficient of the Reaction between Criegee Intermediate CH₂OO and HNO₃

The reactions of Criegee intermediates with HNO₃are important in the polluted urban atmosphere because of their large rate coefficients and the significant concentration of HNO₃. Employing a step-scan Fourier-transform spectrometer, we recorded infrared spectra of transient species and end products in the reaction CH₂OO + HNO₃upon irradiation of a flowing mixture of CH₂I₂/HNO₃/N₂/O₂at 308 nm. Eight bands at 1686, 1426, 1348, 1294, 1052, 965, 891, and 825 cm^-1were assigned to the absorption of the adduct nitrooxymethyl hydroperoxide (NMHP, NO₃CH₂OOH). Additional products from two dissociation channels were observed. Four bands at 1709, 1325, 1276, and 886 cm^-1were assigned to H₂C(O)ONO₂(with coproduct OH), produced from the fission of the O-O bond of internally hot NMHP (NMHP*). Simultaneous detection of H₂CO (1746 cm^-1), NO₂(1617 cm^-1), and HO₂(1392 and 1098 cm^-1) indicated a direct cleavage of the N-OC and C-OO bonds of NMHP*. The relative yields of these three channels in pressure range 10-150 Torr were estimated. At 10 Torr, the absorption of internally excited HNO₃near 885 and 1320 cm^-1was also detected at an early stage of the reaction. We investigated also the rate coefficient of the reaction CH₂OO + HNO₃by probing the temporal profiles of the formation of NMHP and NO₂under total pressures of 40 and 70 Torr at 298 K. The rate coefficient k_HNO3= (2.4 ± 0.4) × 10^-10cm³molecule^-1s^-1is less than half the only literature value, (5.4 ± 1.0) × 10^-10cm³molecule^-1s^-1, reported by Foreman et al. (Angew. Chem. Int. Ed. 2016, 55, 10419-10422).