KINETIC-STUDIES OF THE ALLYLPEROXYL RADICAL SELF-REACTION AND REACTION WITH HO2

The laser flash photolysis technique has been used to study the kineti cs of the following reactions of the allylperoxy radical at atmospheri c pressure and as a function of temperature: (1) CH2=CHCH2O2 + CH2=CHC H2O2 --> products (2) CH2=CHCH2O2 + HO2 --> CH2=CHCH2O2H + O-2 The rad icals 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 r adical and product absorption spectra, combined with reasonable assump tions of their variations with temperature, allowed the rate coefficie nts of interest to be estimated by an iterative procedure involving nu merical 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) c m(3) molecule(-1) s(-1) and in very good agreement with the only other room-temperature measurement of this rate coefficient (M. E. Jenkin e t al., J. Chem. Soc., Faraday Trans., 1993, 89, 433). The determinatio n of k, was limited by experimental conditions to T = 393-426 K, withi n which no variation with temperature could be distinguished, and givi ng k(2) = (5.6 +/- 0.4) x 10(-12) cm(3) molecule(-1) s(-1). Extrapolat ion to 298 K then suggests k(2) approximate to 1 x 10(-11) cm(3) molec ule(-1) s(-1). The implications of these results for our understanding of isoprene degradation under conditions of low NOx concentrations an d for general trends in peroxyl radical reactivity are discussed.