KINETICS AND PRODUCT STUDY OF THE SELF-REACTIONS OF ALLYL AND ALLYL PEROXY-RADICALS AT 296-K

The laser flash photolysis technique, coupled with UV absorption spectroscopy, has been used to investigate the UV spectra and kinetics of reactions of the allyl radical (CH2-CHCH2) and the allyl peroxy radical (CH2-CHCH2O2) at 296 K and total pressures near atmospheric. CH2-CHCH2 radicals were generated by the 193 nm photolysis of hexa-1,5-diene-N2 mixtures, or the 248 nm photolysis of allyl iodide-N2 mixtures. The 193 nm photolysis of hexa-1,5-diene-02-N2 mixtures was used to generate CH2-CHCH2O2. The low resolution spectrum of CH2-CHCH2 Was characterised in the range 210-232.5 nm. The absolute absorption cross-section near the maximum, sigma (220 nm) = (5.8 +/- 0.8) x 10(-17) cm2 molecule-1, calibrated relative to the loss of allyl iodide, is in good agreement with the single published determination. The observed time dependence of CH2-CHCH2 in the various chemical systems allowed measurement of rate coefficients for the following reactions 2CH2=CHCH2 (+M) --> CH2=CHCH2CH2CH=CH2 (+M) (10) CH2=CHCH2 + I (+M) --> CH2=CHCH2I (+M) (12) CH2=CHCH2 + 02 (+M) --> CH2=CHCH2O2 (+M) (7) The parameters obtained were k10 = (3.0 +/- 0.5) x 10(-11), k12 = (1.6 +/- 0.6) x 10(-10), and k7 = (6 +/- 2) x 10(-13) cm3 molecule-1 s-1. The UV absorption spectrum of CH2=CHCH2O2, characterised in the wavelength range 210-300 nm, is typical of an organic peroxy radical, with a peak cross-section of (6.2 +/- 0.9) x 10(-18) CM2 molecule-1 at 235 nm. CH2=CHCH2O2 displayed second-order kinetic behaviour indicative of its removal via the self-reaction. 2CH2=CHCH2O2 --> 2CH2=CHCH2O + O2 (13a) --> CH2=CHCHO + CH2=CHCH2OH + O2 (13b) The observed rate coefficient, k13obs = (1-1 +/- 0.2) x 10(-12) cm3 molecule-1 s-1, is greater than the elementary coefficient, k13, owing to secondary removal of CH2=CHCH2O2. The observed and elementary coefficients are related by the expression k13obs = (1 + alpha13)k13, where alpha13 = k13a/k13. Associated long pathlength Fourier transform infrared (FTIR) measurements of the products of the 253.7 nm initiated photo-oxidation of allyl iodide, allowed the product channels of reaction (13) to be identified and quantified, and a value of alpha13 = 0.61 +/- 0.07 was determined, leading to k13 = (6.8 +/- 1.3) x 10(3) CM-13 Molecule-1 s-1. The kinetic and mechanistic data obtained for CH2=CHCH2O2, and those measured previously in this laboratory for HOCH2CH2O2, are used to infer average rate coefficients for peroxy radicals (RO2) formed in the OH-initiated oxidation of isoprene. A simple box model of the planetary boundary layer is used to demonstrate the potential impact of elevated concentrations of isoprene on ambient levels of OH, HO2, RO2, O3 and NO(x).