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

Abstract

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 anti-conformer of chloroethyl hydroperoxide (anti-CEHP or anti-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 anti-CH3CHOO, we observed only anti-CEHP with a reduced proportion; this observation indicates that the conversion from syn-CEHP, expected to be produced from syn-CH3CHOO + HCl, to anti-CEHP is facile. We also estimated the overall rate coefficient of the reaction syn-/anti-CH3CHOO + HCl to be kHCl = (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 kHClsyn = (4.77 ± 0.95) × 10-11 cm3 molecule-1 s-1 reported for syn-CH3CHOO + HCl by Liu et al., indicating that anti-CH3CHOO reacts with HCl much more rapidly than syn-CH3CHOO.