A shock tube kinetic study of allyl + allyl and allyl + OH recombination reactions at high temperatures

In this work, we investigated the self-reaction of allyl radicals and its cross-reaction with hydroxyl radical by employing shock tube and laser absorption techniques. We carried out the experiments behind reflected shock waves over the temperature range of 800-1200 K and pressures of 1.1-2.5 bar. We generated allyl (C₃H₅) and OH radicals by fast thermal decomposition of allyl iodide (C₃H₅I) and tert-butyl hydroperoxide (TBHP), respectively, and monitored reaction progress by detecting OH near 306.69 nm and C₃H₅ near 220 nm using UV laser absorption. At the detection wavelength, we measured the temperature dependence of the absorption cross-sections of C₃H₅ and C₃H₅I. Rate coefficient for the self-recombination reaction of allyl radicals showed a small negative temperature dependence and no noticeable fall-off behavior over 1.15-1.96 bar giving a mean value of ^{_{kC3 H5+C3 H5 =(1.0±0.2)×10-11cm3molecule-1 s-1}} . Likewise, the cross-reaction of allyl and OH radicals did not exhibit discernible pressure and temperature dependence under our experimental conditions indicating a barrierless reaction, and an average value of ^{_{kC3 H5+OH=(9.3±0.7)×10-11cm3molecule-1 s-1}} best illustrates our measured rate coefficients. These measurements represent the first direct experimental determinations of the rate coefficients for these important radical-radical reactions at high temperatures and pressures.