Infrared Characterization of the Products of the Reaction between the Criegee Intermediate CH 3 CHOO 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 CH 3 CHOO + HCl in a flowing mixture of CH 3 CHI 2 /HCl/O 2 /N 2 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 anti -conformer of chloroethyl hydroperoxide ( anti -CEHP or anti- CH 3 CHClOOH). In addition, absorption bands of H 2 O and acetyl chloride [CH 3 C(O)Cl, at 1819.1 cm -1 ] were observed; some of them were produced from the secondary reactions of CH 3 CHClO + O 2 → CH 3 C(O)Cl + HO 2 and OH + HCl → H 2 O + Cl, according to temporal profiles of H 2 O and CH 3 C(O)Cl. These secondary reactions are conceivable because the nascent formation of CH 3 CHClO + OH via decomposition of internally excited CEHP was predicted by theory, and both HCl and O 2 are major species in the system. The nascent formation of CH 3 CHClO + OH appears to be more important than that of CH 3 C(O)Cl + H 2 O, consistent with theoretical predictions. By adding methanol to deplete some anti -CH 3 CHOO, we observed only anti -CEHP with a reduced proportion; this observation indicates that the conversion from syn -CEHP, expected to be produced from syn -CH 3 CHOO + HCl, to anti -CEHP is facile. We also estimated the overall rate coefficient of the reaction syn -/ anti -CH 3 CHOO + HCl to be k HCl = (2.7 ± 1.0) × 10 -10 cm 3 molecule -1 s -1 at ∼70 Torr and 298 K; this rate coefficient is about six times the only literature value k HCl syn = (4.77 ± 0.95) × 10 -11 cm 3 molecule -1 s -1 reported for syn -CH 3 CHOO + HCl by Liu et al., indicating that anti -CH 3 CHOO reacts with HCl much more rapidly than syn -CH 3 CHOO.