Kinetics and Mechanisms of the Allyl plus Allyl and Allyl plus Propargyl Recombination Reactions

The kinetics and mechanisms of the self-reaction of allyl radicals and the cross-reaction between allyl and propargyl radicals were studied both experimentally and theoretically. The experiments were carried out over the temperature range 295-800 K and the pressure range 20-200 Torr (maintained by He or N-2). The allyl and propargyl radicals were generated by the pulsed laser photolysis of respective precursors, 1,5-hexadiene and propargyl chloride, and were probed by using a cavity ring-down spectroscopy technique. The temperature-dependent absorption cross sections of the radicals were measured relative to that of the HCO radical. The rate constants have been determined to be k(C3H5 + C3H5) = 1.40 x 10(-8) T-0.933 exp(-225/T) cm(3) molecule(-1) s(-1) (Delta log(10) k = +/-0.088) and k(C3H5 + C3H3) = 1.71 x 10(-7) T-1.182 exp(-255/T) cm(3) molecule(-1) s(-1) (Delta log(10) k = +/- 0.069) with 2 sigma uncertainty limits. The potential energy surfaces for both reactions were calculated with the CBS-QB3 and CASPT2 quantum chemical methods, and the product channels have been investigated by the steady-state master equation analyses based on the Rice-Ramsperger-Kassel-Marcus theory. The results indicated that the reaction between allyl and propargyl radicals produces five-membered ring compounds in combustion conditions, while the formations of the cyclic species are unlikely in the self-reaction of allyl radicals. The temperature- and pressure-dependent rate constant expressions for the important reaction pathways are presented for kinetic modeling.