Effect of Methyl Group Substitution on the Kinetics of Vinyl Radical + O₂ Reaction

The kinetics of (CH₃)₂CCH + O₂ (1) and (CH₃)₂CCCH₃ + O₂ (2) reactions have been measured as a function of temperature (223-600 K) at low pressures (0.4-2 Torr) using a tubular laminar flow reactor coupled to a photoionization mass spectrometer (PIMS). These reactions are important for accurate modeling of unsaturated hydrocarbon combustion. Photolysis of a brominated precursor by a pulsed excimer laser radiation at 248 nm wavelength along the flow reactor axis was used for the production of radicals. The measured bimolecular rate coefficient of reaction 1 shows a negative temperature dependence over the temperature range 223-384 K and becomes temperature independent at higher temperatures. The bimolecular rate coefficient of reaction 2 exhibits a negative temperature dependence throughout the experimental temperature range. The bimolecular rate coefficients of reactions 1 and 2 are expected to be at the high-pressure limit under the current experimental conditions, and the following values are obtained at 298 K: k₁(298 K) = (4.5 ± 0.5) × 10^-12 cm³ s^-1 and k₂(298 K) = (8.9 ± 1.0) × 10^-12 cm³ s^-1. The observed products for reactions 1 and 2 were CH₃COCH₃ and CH₃ + CH₃COCH₃, respectively. Substituting both β-hydrogens in the vinyl radical (CH₂CH) with methyl groups decreases the rate coefficient of the CH₂CH + O₂ reaction by about 50%. However, the rate coefficient of the triply substituted (CH₃)₂CCCH₃ radical reaction with O₂ is almost identical to the CH₂CH + O₂ rate coefficient under the covered temperature range.