Determination of rate constants for a thermoneutral H-abstraction reaction: Allylic hydrogen abstraction from 1,5-hexadiene by allyl radical

Thermoneutral H-abstraction reactions, which involve the abstraction by resonantly stabilized radicals from olefins to form other resonantly stabilized radicals, have recently been found to be important in high-temperature combustion. They will shift the distribution of resonantly stabilized radicals and affect the molecular weight growth kinetics, thus changing the prediction for aromatic species. The present work provides the first experimental study on a typical thermoneutral H-abstraction reaction: C₃H₅-A + 1,5-C₆H₁₀ = C₃H₆ + C₆H₉-A (R1). A rapid compression machine, together with a fast sampling technique, was used to study the high temperature kinetics of 1,5-hexadiene. Experimental conditions that can be used to derive the rate constant of R1 were first identified by sensitivity analysis of a kinetic model for 1,5-hexadiene combustion. The concentration of propene was found to be highly sensitive to the rate constant of R1 over the temperature range of 893-1007 K at 25 bar. By fitting the measured propene profiles, the rate constant of R1 was deduced under selected conditions. Furthermore, the concentration profiles of propene in 1,5-hexadiene pyrolysis from a flow reactor measurement by Wang et al. (Fuel 208, 779-790, 2017) and oxidation from a jet stirred reactor measurement by Vermeire et al. (Proc. Combust. Inst. 37, 1091-1098, 2019) were also used to derive the rate constant of R1 over wider temperature ranges. The rate coefficient of R1 was further calculated by ab initio transition state theory, with energies obtained at the level of DLPNO-CCSD(T)/aug-cc-pVTZ//B2PLYP/cc-pVTZ. The theoretical predictions were found to be in good agreement with the experimentally derived rate constants. The final rate coefficient determined is: k_R1 = 15.058 × T^3.425 exp(-14492.073/RT) cm³ mol^-1 s^-1 (500 - 2000 K), with an estimated uncertainty of a factor of 1.5.