Infrared characterization of the products and the rate coefficient of the reaction between Criegee intermediate CH2OO and HCl

Abstract

Reactions between Criegee intermediates and hydrogen halides might be significant, particularly in the polluted urban atmosphere, because of their large rate coefficients. Employing a Fourier-transform spectrometer in a step-scan mode or a continuous-scan mode, we recorded infrared spectra of transient species and end products in a flowing mixture of CH₂I₂/HCl/N₂/O₂ irradiated at 308 nm. Five bands near 823.2, 1061.1, 1248.4, 1309.2, and 1359.6 cm⁻¹ were observed and assigned to the gauche-conformer of chloromethyl hydroperoxide (CMHP, CH₂ClOOH). At a later time of the reaction, absorption bands of H₂O and formyl chloride (CHClO) at 1782.9 cm⁻¹ were observed; these species were likely produced from the secondary reactions of CH₂ClO + O₂ → CHClO + HO₂ and OH + HCl → H₂O + Cl according to temporal profiles of CMHP, H₂O, and CHClO; formation of CH₂ClO + OH via decomposition of internally excited CMHP was predicted by theory and both HCl and O₂ are major species in the system. We investigated also the rate coefficient of the reaction CH₂OO + HCl on probing CH₂OO with a continuous-wave infrared quantum-cascade laser absorption system under total pressure 5.2–8.2 torr at 298 K. The rate coefficient k_HCl = (4.8 ± 0.4) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, is comparable to the only literature value k_HCl = (4.6 ± 1.0) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ reported by Foreman et al.