Kinetic studies of the allylperoxyl radical self-reaction and reaction with HO2

Abstract

The laser flash photolysis technique has been used to study the kinetics of the following reactions of the allylperoxy radical at atmospheric pressure and as a function of temperature: CH₂CHCH₂O₂+ CH₂CHCH₂O₂→ products (1), CH₂CHCH₂O₂+ HO₂→ CH₂CHCH₂O₂H + O₂(2) The radicals were generated by the photolysis of suitable hexa-1,5-diene–O₂–N₂ and 3-chloropropene–HCHO–O₂–N₂ mixtures at λ= 193 nm, the resulting total absorbance being measured as a function of time by UV absorption spectrometry. Knowledge of the secondary chemistry and of radical and product absorption spectra, combined with reasonable assumptions of their variations with temperature, allowed the rate coefficients of interest to be estimated by an iterative procedure involving numerical integration of decay profiles recorded at appropriate analysis wavelengths. The resulting Arrhenius expression for reaction (1) is k₁=(5.4 ± 1.1)× 10^–14 exp[(760 ± 70)/T] cm³ molecule^–1 s^–1(T= 286–394 K), yielding k₁(296 K)=(7.0 ± 0.2)× 10^–13 cm³ molecule^–1 s^–1 and in very good agreement with the only other room-temperature measurement of this rate coefficient (M. E. Jenkin et al., J. Chem. Soc., Faraday Trans., 1993, 89, 433). The determination of k₂ was limited by experimental conditions to T= 393–426 K, within which no variation with temperature could be distinguished, and giving k₂=(5.6 ± 0.4)× 10^–12 cm³ molecule^–1 s^–1. Extrapolation to 298 K then suggests k₂≈ 1 × 10^–11 cm³ molecule^–1 s^–1. The implications of these results for our understanding of isoprene degradation under conditions of low NO_x concentrations and for general trends in peroxyl radical reactivity are discussed.