Infrared Characterization of the Products of the Reaction between the Criegee Intermediate CH₃CHOO and HCl

The rapid reactions between Criegee intermediates and hydrogen halides play important roles in atmospheric chemistry, particularly in the polluted urban atmosphere. Employing a step-scan Fourier transform spectrometer, we recorded infrared absorption spectra of transient species and end products of the reaction CH₃CHOO + HCl in a flowing mixture of CH₃CHI₂/HCl/O₂/N₂ irradiated at 308 nm. Bands at 1453.6, 1383.7, 1357.9, 1323.8, 1271.8, 1146.2, 1098.2, 1017.5, 931.5, and 847.0 cm^–1 were observed and assigned to the <i>anti</i>-conformer of chloroethyl hydroperoxide (<i>anti</i>-CEHP or <i>anti-</i>CH₃CHClOOH). In addition, absorption bands of H₂O and acetyl chloride [CH₃C(O)Cl, at 1819.1 cm^–1] were observed; some of them were produced from the secondary reactions of CH₃CHClO + O₂ → CH₃C(O)Cl + HO₂ and OH + HCl → H₂O + Cl, according to temporal profiles of H₂O and CH₃C(O)Cl. These secondary reactions are conceivable because the nascent formation of CH₃CHClO + OH via decomposition of internally excited CEHP was predicted by theory, and both HCl and O₂ are major species in the system. The nascent formation of CH₃CHClO + OH appears to be more important than that of CH₃C(O)Cl + H₂O, consistent with theoretical predictions. By adding methanol to deplete some <i>anti</i>-CH₃CHOO, we observed only <i>anti</i>-CEHP with a reduced proportion; this observation indicates that the conversion from <i>syn</i>-CEHP, expected to be produced from <i>syn</i>-CH₃CHOO + HCl, to <i>anti</i>-CEHP is facile. We also estimated the overall rate coefficient of the reaction <i>syn</i>-/<i>anti</i>-CH₃CHOO + HCl to be <i>k</i>_HCl = (2.7 ± 1.0) × 10^–10 cm³ molecule^–1 s^–1 at ∼70 Torr and 298 K; this rate coefficient is about six times the only literature value <i>k</i>_HCl^<i>syn</i> = (4.77 ± 0.95) × 10^–11 cm³ molecule^–1 s^–1 reported for <i>syn</i>-CH₃CHOO + HCl by Liu et al., indicating that <i>anti</i>-CH₃CHOO reacts with HCl much more rapidly than <i>syn</i>-CH₃CHOO.