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

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: (1) CH2=CHCH2O2 + CH2=CHCH2O2 --> products (2) CH2=CHCH2O2 + HO2 --> CH2=CHCH2O2H + O-2 The radicals were generated by the photolysis of suitable hexa-1,5-diene-O-2-N-2 and 3-chloropropene-HCHO-O-2-N-2 mixtures at lambda = 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 k(1) = (5.4 +/- 1.1) x 10(-14) exp[(760 +/- 70)/T] cm(3) molecule(-1) s(-1) (T = 286-394 K), yielding k(1)(296 K) = (7.0 +/- 0.2) x 10(-13) cm(3) 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 k, was limited by experimental conditions to T = 393-426 K, within which no variation with temperature could be distinguished, and giving k(2) = (5.6 +/- 0.4) x 10(-12) cm(3) molecule(-1) s(-1). Extrapolation to 298 K then suggests k(2) approximate to 1 x 10(-11) cm(3) molecule(-1) s(-1). The implications of these results for our understanding of isoprene degradation under conditions of low NOx concentrations and for general trends in peroxyl radical reactivity are discussed.