Temperature- and Pressure-Dependent Kinetics of CH2OO + CH3COCH3 and CH2OO + CH3CHO: Direct Measurements and Theoretical Analysis

Abstract

The rate coefficients of the gas-phase reactions CH₂OO + CH₃COCH₃ and CH₂OO + CH₃CHO have been experimentally determined from 298–500 K and 4–50 Torr using pulsed laser photolysis with multiple-pass UV absorption at 375 nm, and products were detected using photoionization mass spectrometry at 10.5 eV. The CH₂OO + CH₃CHO reaction's rate coefficient is ∼4 times faster over the temperature 298–500 K range studied here. Both reactions have negative temperature dependence. The T dependence of both reactions was captured in simple Arrhenius expressions:

kCH₂OO+CH₃CoCH₃(T) = (7 ± 2.5) x 10⁻¹⁵ cm3 molecule⁻¹s⁻¹ exp[+(9.3 ± 2.9)kJ mol⁻¹ /RT]

kCH₂OO+CH₃CHO (T) = (3 ± 0.8 x 10⁻¹⁴ cm3 molecule⁻¹s⁻¹ exp[+(9.1 ± 2.7)kJ mol⁻¹ /RT]

The rate of the reactions of CH₂OO with carbonyl compounds at room temperature is two orders of magnitude higher than that reported previously for the reaction with alkenes, but the A factors are of the same order of magnitude. Theoretical analysis of the entrance channel reveals that the inner 1,3-cycloaddition transition state is rate limiting at normal temperatures. Predicted rate-coefficients (RCCSD(T)-F12a/cc-pVTZ-F12//B3LYP/MG3S level of theory) in the low-pressure limit accurately reproduce the experimentally observed temperature dependence. The calculations only qualitatively reproduce the A factors and the relative reactivity between CH₃CHO and CH₃COCH₃. The rate coefficients are weakly pressure dependent, within the uncertainties of the current measurements. The predicted major products are not detectable with our photoionization source, but heavier species yielding ions with masses m/z = 104 and 89 are observed as products from the reaction of CH₂OO with CH₃COCH₃. The yield of m/z = 89 exhibits positive pressure dependence that appears to have already reached a high-pressure limit by 25 Torr.