Rate Coefficient and Branching Ratio for the Formation of Criegee Intermediate Syn-/Anti-CH₃CHOO from CH₃CHI + O₂ and the Self-Reaction of Syn-/Anti-CH₃CHOO Determined with Simultaneous IR/UV Probes

Abstract

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 CH 3 CHI 2 , syn-CH 3 CHOO, and anti/syn-CH 3 CHOO, respectively, in the reaction of CH 3 CHI + O 2 . The branching between syn-and anti-CH 3 CHOO was determined to be ≈80:20 from two methods. The concentration temporal profiles of anti-CH 3 CHOO, derived on comparison of infrared and ultraviolet profiles, yielded the rate coefficient for the self-reaction of anti-CH 3 CHOO, k self anti = (6 ± 2) × 10 −10 cm 3 molecule −1 s −1 , ∼4 times the corresponding value, k self syn = (1.4 ± 0.3) × 10 −10 cm 3 molecule −1 s −1 , for syn-CH 3 CHOO; the rate coefficient for the cross-reaction between syn-CH 3 CHOO and anti-CH 3 CHOO was estimated to be (2.1 ± 0.6) × 10 −10 cm 3 molecule −1 s −1 . With determined concentrations of syn-CH 3 CHOO and self-reaction rate coefficients, the rate coefficient for the formation of CH 3 CHOO from CH 3 CHI + O 2 was determined to be k form = (3.8 ± 0.7) × 10 −12 cm 3 molecule −1 s −1 at 298 K, ∼45% of previous reports.