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

Abstract

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 cm 3 s-1 and k2(298 K) = (8.9 ± 1.0) × 10-12 cm 3 s-1. The observed products for reactions (1) and (2) were CH3COCH3 and CH3 + CH3COCH3, respectively. Substituting both β-hydrogens in vinyl radical (CH2CH) with methyl groups decreases the rate coefficient of 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.

Direct Kinetic Measurements and Master Equation Modelling of the Unimolecular Decomposition of Resonantly-Stabilized CH2CHCHC(O)OCH3 Radical and an Upper Limit Determination for CH2CHCHC(O)OCH3 + O2 Reaction

“…To rule out a presence of vinyl-or alkyl-type isomers (see Scheme 1), a parallel set of experiments were conducted by adding a high concentration of molecular oxygen, [O 2 ] = (1.3 ± 0.1) × 10 15 molecules cm −3 , to the reaction mixture (labeled as superscript " d " in Table 1). An [O 2 ] this high should rapidly, well within 1 ms, consume all vinyl-and alkyl-type radicals [25]. No difference in unimolecular decay rate coefficients was detected when molecular oxygen was added into the reaction mixture, so we conclude that vinyl-and alkyl-type radicals were not present in the reactor, see Figure S4.…”