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.