Rate Coefficient and Branching Ratio for the Formation of Criegee Intermediate S<i>yn</i>-/A<i>nti</i>-CH₃CHOO from CH₃CHI + O₂ and the Self-Reaction of S<i>yn</i>-/A<i>nti</i>-CH₃CHOO Determined with Simultaneous IR/UV Probes

A flow reactor coupled with a light-emitting diode at 286 nm, an infrared quantum-cascade laser near 11 μm, and an ultraviolet laser at 335 nm was implemented to probe the precursor CH₃CHI₂, <i>syn</i>-CH₃CHOO, and <i>anti</i>/<i>syn</i>-CH₃CHOO, respectively, in the reaction of CH₃CHI + O₂. The branching between <i>syn</i>- and <i>anti</i>-CH₃CHOO was determined to be ≈80:20 from two methods. The concentration temporal profiles of <i>anti</i>-CH₃CHOO, derived on comparison of infrared and ultraviolet profiles, yielded the rate coefficient for the self-reaction of <i>anti</i>-CH₃CHOO, <i>k</i>_self ^anti = (6 ± 2) × 10^–10 cm³ molecule^–1 s^–1, ∼4 times the corresponding value, <i>k</i>_self ^syn = (1.4 ± 0.3) × 10^–10 cm³ molecule^–1 s^–1, for <i>syn</i>-CH₃CHOO; the rate coefficient for the cross-reaction between <i>syn</i>-CH₃CHOO and <i>anti</i>-CH₃CHOO was estimated to be (2.1 ± 0.6) × 10^–10 cm³ molecule^–1 s^–1. With determined concentrations of <i>syn</i>-CH₃CHOO and self-reaction rate coefficients, the rate coefficient for the formation of CH₃CHOO from CH₃CHI + O₂ was determined to be <i>k</i>_form = (3.8 ± 0.7) × 10^–12 cm³ molecule^–1 s^–1 at 298 K, ∼45% of previous reports.