Kinetics and thermochemistry of cyclohexadienes reactions with hydroxyl radicals

Abstract

Cyclohexadienes (CHDs) are important intermediates in the pyrolysis and oxidation of many C6 cyclic hydrocarbons. Under combustion relevant conditions, these dienes may either eliminate H2 directly or lose H atoms sequentially to produce the first aromatic ring, i.e., benzene. This is one of the major reaction pathways for the formation of benzene from C6 cyclic hydrocarbons. Despite its importance in PAH and soot formation process, the chemistry of CHD has attracted less attention from the scientific community. In this work, we have investigated hydrogen abstraction reactions of 1,3- and 1,4-CHD by OH radicals using a shock tube and UV laser absorption over the temperature range of 900 – 1100 K and pressures of ∼ 1 – 3 atm. Reaction kinetics was followed by monitoring OH radicals near 307 nm. At high temperatures, both isomers of CHDs are found to exhibit comparable reactivity with OH radicals. These reactions show a weak positive temperature dependence. Measured rate coefficients can be represented by the following Arrhenius expressions in units of cm3 molecule−1s − 1:k1(1,3−CHD+OH)=1.76×10−10exp(−1761KT)k2(1,4−CHD+OH)=2.23×10−10exp(−1813KT) To gain further insights into these reactions, various ab initio methods were employed to compute relevant thermochemical data. This study reports the first high-temperature kinetic data for cyclohexadienes which will be beneficial to improve the performance of the oxidation kinetic models of C6 cyclic hydrocarbons.