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

A flow reactor coupled with a light-emitting diode at 286 nm, an infrared quantum-cascade laser near 11 mu m, and an ultraviolet laser at 335 nm was implemented to probe the precursor CH3CHI2, syn-CH3CHOO, and anti/syn-CH3CHOO, respectively, in the reaction of CH3CHI + O-2. The branching between syn- and anti-CH3CHOO was determined to be approximate to 80:20 from two methods. The concentration temporal profiles of anti-CH3CHOO, derived on comparison of infrared and ultraviolet profiles, yielded the rate coefficient for the self-reaction of anti-CH3CHOO, k(self)(anti) = (6 +/- 2) x 10(-10) cm(3) molecule(-1) s(-1), similar to 4 times the corresponding value, k(self)(syn) = (1.4 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1), for syn-CH3CHOO; the rate coefficient for the cross-reaction between syn-CH3CHOO and anti-CH3CHOO was estimated to be (2.1 +/- 0.6) x 10(-10) cm(3) molecule(-1) s(-1). With determined concentrations of syn-CH3CHOO and self-reaction rate coefficients, the rate coefficient for the formation of CH3CHOO from CH3CHI + O-2 was determined to be k(form) = (3.8 +/- 0.7) x 10(-12) cm(3) molecule(-1) s(-1) at 298 K, similar to 45% of previous reports.