Conformation-Specific Rate Coefficients of CH₃CHOO + HCl Determined with Multiple IR/UV Absorption Probes

Reactions between Criegee intermediates and hydrogen halides significantly impact atmospheric chemistry, particularly in polluted urban environments. We employed a multipass absorption flow cell with four probes to determine the rate coefficient of syn- and anti-CH₃CHOO + HCl. Infrared quantum-cascade lasers near 883 and 1280 cm^-1 provided temporal profiles of syn-CH₃CHOO and a combination of syn-CH₃CHOO and the hydrogen-transferred adduct chloroethyl hydroperoxide (CEHP), respectively. A light-emitting diode at 286 nm probed the precursor CH₃CHI₂, hence [CH₃CHI]₀ upon photolysis at 248 nm. Laser light at 335 nm probed both syn- and anti-CH₃CHOO. By modeling the observed temporal profiles, we derived rate coefficients of syn-CH₃CHOO + HCl and anti-CH₃CHOO + HCl at 298 K as k_HCl^syn = (5.1 ± 1.1) × 10^-11 cm³ molecule^-1 s^-1 and k_HCl^anti = (2.9 ± 0.8) × 10^-10 cm³ molecule^-1 s^-1, respectively. The latter, previously unreported, is ∼ 5.7 times k_HCl^syn, highlighting the conformation-specific reactivity.