Kinetics and Mechanisms of the Allyl + Allyl and Allyl + 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 N₂). 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 × 10^–8 T^–0.933 exp(−225/T) cm³ molecule^–1 s^–1 (Δ log₁₀ k = ± 0.088) and k(C₃H₅ + C₃H₃) = 1.71 × 10^–7 T^–1.182 exp(−255/T) cm³ molecule^–1 s^–1 (Δ log₁₀ k = ± 0.069) with 2σ 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.