Unimolecular Reaction Rate Measurement of syn-CH₃CHOO

Abstract

The unimolecular reactions of Criegee intermediates (CIs) are thought to be one of the significant sources of atmospheric OH radicals. However, stark discrepancies exist in the unimolecular reaction rate of the methyl-substituted CI CH3CHOO, typically from ozonolysis of alkenes such as trans-2-butene, between the results of ozonolysis of alkene experiments and the up-to-date theoretical calculations. That no further progress has been made since the method that directly produces CIs in the laboratory was developed is mostly attributed to the existence of two conformers, syn- and anti-CH3CHOO, and the methodological limitations of sensitive conformer-specific detection. We report a conformer-specific measurement of the unimolecular reaction rate of syn-CH3CHOO by using a high-repetition-rate laser-induced fluorescence method. At 298 K, the observed value of 182 ± 66 s–1 is in good agreement with recent theoretical calculations.

Kinetics of dimethyl sulfide (DMS) reactions with isoprene-derived Criegee intermediates studied with direct UV absorption

“…At higher laser fluences, more CH 2 OO and radical byproducts are generated, resulting in shorter CH 2 OO lifetimes 125 (see Fig. S7: plot of k 0 against [CH 2 I 2 ]I 308nm ), similar to previous works (Smith et al, 2016;Li et al, 2020;Zhou et al, https://doi.org/10.5194/acp-2020-326 Preprint. Discussion started: 23 April 2020 c Author(s) 2020.…”

Unimolecular decomposition rates of a methyl-substituted Criegee intermediate syn-CH3CHOO

“…The thermal decomposition rate at 298 K (25 to 100 torr) is reported to be 182 AE 66 s À1 . 19 Their results also support that the thermal decomposition is the main atmospheric process for syn-CH 3 CHOO.…”

“…7. As will be discussed below, Zhou et al 19 may have underestimated their diffusion loss and thus overestimated their k uni values, especially for data at lower pressures. Overall, the observed pressure dependences are roughly consistent with the theoretical trend.…”

“…In previous works on the thermal decomposition reactions of (CH 3 ) 2 COO by Smith et al 27 and of syn-CH 3 CHOO by Zhou et al, 19 the authors have considered the contributions of second-order reactions (including the reaction with iodine atoms and self-reaction of Criegee intermediates 26 ). Following the notation of Smith et al, the corresponding kinetic equation can be expressed as below.…”

“…Recently Zhou et al measured the unimolecular rates of syn-CH 3 CHOO by probing the OH products with laser-induced uorescence. 19 Because the barrier of OH production of anti-CH 3 CHOO is much higher than that for syn-CH 3 CHOO, 13,15 Zhou et al do not need to scavenge anti-CH 3 CHOO. Their measurements were under 10 to 100 torr, 19 limited by the lower sensitivity of the OH laser-induced uorescence at higher pressure.…”