Bimolecular Reaction of Methyl-Ethyl-Substituted Criegee Intermediate with SO₂

Abstract

Methyl-ethyl-substituted Criegee intermediate (MECI) is a four-carbon carbonyl oxide that is formed in the ozonolysis of some asymmetric alkenes. MECI is structurally similar to the isoprene-derived methyl vinyl ketone oxide (MVK-oxide) but lacks resonance stabilization, making it a promising candidate to help us unravel the effects of size, structure, and resonance stabilization that influence the reactivity of atmospherically important, highly functionalized Criegee intermediates. We present experimental and theoretical results from the first bimolecular study of MECI in its reaction with SO 2 , a reaction that shows significant sensitivity to the Criegee intermediate structure. Using multiplexed photoionization mass spectrometry, we obtain a rate coefficient of (1.3 ± 0.3) × 10 −10 cm 3 s −1 (95% confidence limits, 298 K, 10 Torr) and demonstrate the formation of SO 3 under our experimental conditions. Through high-level theory, we explore the effect of Criegee intermediate structure on the minimum energy pathways for their reactions with SO 2 and obtain modified Arrhenius fits to our predictions for the reaction of both syn and anti conformers of MECI with SO 2 (k syn = 4.42 × 10 11 T −7.80 exp(−1401/T) cm 3 s −1 and k anti = 1.26 × 10 11 T −7.55 exp(−1397/T) cm 3 s −1 ). Our experimental and theoretical rate coefficients (which are in reasonable agreement at 298 K) show that the reaction of MECI with SO 2 is significantly faster than MVKoxide + SO 2 , demonstrating the substantial effect of resonance stabilization on Criegee intermediate reactivity.

Reaction Kinetics of CH2OO and syn-CH3CHOO Criegee Intermediates with Acetaldehyde

“…Its primary atmospheric sink is through reaction with water vapor, particularly water dimers. − The second smallest CI, CH 3 CHOO, exists in two conformers that differ based on the orientation of the methyl group relative to the terminal oxygen atom. The unimolecular reaction of anti -CH 3 CHOO is relatively slow , compared to its bimolecular reaction with water vapor and SO 2 under atmospheric conditions. ,,− In contrast, the primary sink of syn- CH 3 CHOO is through its unimolecular reaction, which predominantly generates OH radicals. , …”

Collection on the Spectroscopy, Structure, and Reactivity of Stabilized Criegee Intermediates

“…The unusual electronic character of the sCIs influences their chemical reactions, which typically proceed via cyclic transition states associated with cycloaddition or 1,2- or 1,4-insertion pathways. − The latter insertion reactions tend to be faster, in some cases approaching the gas-kinetic limit. Not only is there fundamental interest in the structural and conformer dependence of the bimolecular reaction rates, but many of the studies of the rates of these bimolecular reactions are also motivated by their potential importance in atmospheric oxidation chemistry.…”