Kinetic insights into the reaction of hydroxyl radicals with 1,4-pentadiene: A combined experimental and theoretical study

Abstract

Olefinic hydrocarbons are one of the main intermediates of the oxidation of large hydrocarbons, and they are also found in distillate transportation fuels. Combustion characteristics of olefinic species, particularly non-conjugated ones, are not well understood. This work reports chemical insights into the reaction of OH radicals with 1,4-pentadiene (14PTDN), an important non-conjugated diolefin. High-temperature rate coefficients of OH + 14PTDN were measured in a shock tube over T = 881 – 1314 K and p ∼ 1150 Torr. The progress of the reaction was monitored by detecting OH radicals near 307 nm using a UV laser-absorption technique. OH radicals were generated by the fast thermal decomposition of tert‑butyl hydroperoxide (TBHP). Stochastic RRKM-based Master Equation (RRKM-ME) simulations were carried out on the potential energy surface computed at M06–2X/aug-cc-pVTZ level of theory to gain mechanistic insights. The theoretical model captured high-temperature experimental data from this work and previously reported data at low temperatures (294 - 468 K). This combined experimental and theoretical study reveals: (i) no discernible pressure dependence of the total rate constants, (ii) mechanism shift occurs with temperature, (iii) similar to conjugated dienes + OH reactions, the addition of OH radicals to 14PTDN dominates compared to abstraction pathways for T < 500 K, (iv) among the bimolecular product channels originating from the OH-adducts, only vinyl alcohol + allyl radical is important, contributing ∼28% of the reactant consumption at 0.1 Torr and 400 K, (iv) beyond 1000 K, the addition channel contributes negligibly small for all pressures. To our knowledge, this is the first detailed experimental and theoretical kinetic study of the reaction of 1,4-pentadiene with OH radicals. The expression for the theoretical total rate coefficients (in units of cm3 molecule−1s−1) is provided as k(T)=0.172×T−3.82exp(−125.6KT)+1.49×10−20T3.09exp(−53.7KT) over T = 200 – 2000 K and P = 760 Torr.