KINETICS OF REACTIONS OF CH3O2 AND HOCH2CH2O2 RADICALS PRODUCED BY THE PHOTOLYSIS OF IODOMETHANE AND 2-IODOETHANOL

The molecular modulation technique coupled with UV absorption spectroscopy has been used to investigate the UV spectra and kinetics of reactions of the methylperoxy radical (CH3O2) and the 2-hydroxyethylperoxy radical (HOCH2CH2O2), generated by the 254-nm photolysis of the organic iodides CH3I and HOCH2CH2I: RI + h-nu(lambda = 254 nm) --> R + I (7) and R + O2 + M --> RO2 + M (8). Measurements of the UV spectra of both RO2 radicals were complicated by the production of additional transient species absorbing strongly at wavelengths above 240 nm. These are believed to be CH3OOI and HOCH2CH2OOI formed as intermediates in the RO2-chaperoned recombination of iodine atoms. Both CH3O2 and HOCH2CH2O2 were found to obey second-order kinetic behavior owing to removal by a series of reactions initiated by the self-reactions: CH3O2 + CH3O2 --> products (9) and HOCH2CH2O2 + HOCH2CH2O2 --> products (10). The parameter k9obs/sigma (where k9obs is the observed second-order rate coefficient) had a value of (1.01 +/- 0.09) x 10(5) cm s-1 at 230 nm, independent of pressure in the range 10.8-760 Torr, at 298 K. Additional measurements made over the temperature range 268-350 K indicated that this parameter displays a weak negative temperature dependence. E/R was found to have a value of -220 +/- 72 K at 760 Torr, and a value of -92 +/- 53 K at 10.8 Torr. The parameter k10obs/sigma had a value of (6.8 +/- 0.4) x 10(5) cm s-1 at 230 nm (p = 760 Torr, T = 298 K). Assuming the photolysis of HOCH2CH2I leads exclusively to the production of HOCH2CH2O2, the following values of sigma(230 nm) and k10obs were concluded: sigma(230nm) = (2.35 +/- 0.25) x 10(-18) cm2 molecule-1 and k10obs = (1.60 +/- 0.17) x 10(-12) cm3 molecule-1 s-1. At lower pressures, HO2 was also generated in the HOCH2CH2I system in significant quantities, enabling investigation of reaction 11 at 10 Torr and 298 K: HOCH2CH2O2 + HO2 --> products (11). A value of k11 = (4.8 +/- 1.5) x 10(-12) cm3 molecule-1 s-1 was concluded from these measurements, based on the above value of sigma(230 nm).