Direct kinetics study of CH2OO + methyl vinyl ketone and CH2OO + methacrolein reactions and an upper limit determination for CH2OO + CO reaction

Abstract

Methyl vinyl ketone (MVK) and methacrolein (MACR) are important intermediate products in atmospheric degradation of volatile organic compounds, especially of isoprene. This work investigates the reactions of the smallest Criegee intermediate, CH₂OO, with its co-products from isoprene ozonolysis, MVK and MACR, using multiplexed photoionization mass spectrometry (MPIMS), with either tunable synchrotron radiation from the Advanced Light Source or Lyman-α (10.2 eV) radiation for photoionization. CH₂OO was produced via pulsed laser photolysis of CH₂I₂ in the presence of excess O₂. Time-resolved measurements of reactant disappearance and of product formation were performed to monitor reaction progress; first order rate coefficients were obtained from exponential fits to the CH₂OO decays. The bimolecular reaction rate coefficients at 300 K and 4 Torr are k(CH₂OO + MVK) = (5.0 ± 0.4) × 10⁻¹³ cm³ s⁻¹ and k(CH₂OO + MACR) = (4.4 ± 1.0) × 10⁻¹³ cm³ s⁻¹, where the stated ±2σ uncertainties are statistical uncertainties. Adduct formation is observed for both reactions and is attributed to the formation of a secondary ozonides (1,2,4-trioxolanes), supported by master equation calculations of the kinetics and the agreement between measured and calculated adiabatic ionization energies. Kinetics measurements were also performed for a possible bimolecular CH₂OO + CO reaction and for the reaction of CH₂OO with CF₃CHCH₂ at 300 K and 4 Torr. For CH₂OO + CO, no reaction is observed and an upper limit is determined: k(CH₂OO + CO) < 2 × 10⁻¹⁶ cm³ s⁻¹. For CH₂OO + CF₃CHCH₂, an upper limit of k(CH₂OO + CF₃CHCH₂) < 2 × 10⁻¹⁴ cm³ s⁻¹ is obtained.