Kinetics of the Methyl-Vinyl Radical + O₂ Reactions Associated with Propene Oxidation

The bimolecular rate coefficients of reactions CH₃CCH₂ + O₂ (1) and cis/ trans-CH₃CHCH + O₂ (2a/3a) have been measured using a tubular laminar flow reactor coupled with a photoionization mass spectrometer (PIMS). These reactions are relevant in the combustion of propene. Pulsed excimer laser photolysis of a ketone or a bromide precursor molecule at 193 or 248 nm wavelength was used to produce radicals of interest homogeneously along the reactor. Time-resolved experiments were performed under pseudo-first-order conditions at low pressure (0.3-1.5 Torr) over the temperature range 220-660 K. The measured bimolecular rate coefficients were found to be independent of bath gas concentration. The bimolecular rate coefficients possess negative temperature dependence at low temperatures ( T < 420 K) and appear to be independent of temperature at high temperatures ( T > 420 K). Observed products of the reaction CH₃CCH₂ + O₂ were CH₃ and H₂CO, while for the reaction cis/trans-CH₃CHCH + O₂, observed products were CH₃CHO and HCO. Current results indicate that the reaction mechanism of both reactions is analogous to that of C₂H₃ + O₂. Methyl substitution of the vinyl radical changes its reactivity toward O₂ upward by ca. 50% if it involves the α-position and downward by ca. 30% if the methyl group takes either of the β-positions, respectively.