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

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 SO₂, 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 cm³ s^-1 (95% confidence limits, 298 K, 10 Torr) and demonstrate the formation of SO₃ 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 SO₂ and obtain modified Arrhenius fits to our predictions for the reaction of both syn and anti conformers of MECI with SO₂ (k_syn = 4.42 × 10¹¹ T^-7.80exp(-1401/T) cm³ s^-1 and k_anti = 1.26 × 10¹¹ T^-7.55exp(-1397/T) cm³ s^-1). Our experimental and theoretical rate coefficients (which are in reasonable agreement at 298 K) show that the reaction of MECI with SO₂ is significantly faster than MVK-oxide + SO₂, demonstrating the substantial effect of resonance stabilization on Criegee intermediate reactivity.