LASER FLASH-PHOTOLYSIS STUDY OF THE UV SPECTRUM AND KINETICS OF REACTIONS OF HOCH2CH2O2 RADICALS

The UV absorption spectrum of the HOCH2CH2O2 radical has been determined by the laser flash photolysis of H2O2 at 248 nm in the presence of C2H4 and O2. The absorption cross-sections were determined relative to the known cross-section of C2H5O2 at 250 nm and are in good agreement with results of a molecular modulation study in which mixtures of H2O2, C2H4 and O2 were photolysed at 254 nm. The spectrum is similar in shape and magnitude to that of C2H5O2 with a peak cross-section of 5.1 x 10(-18) cm2 molecule-1 occurring at ca. 245 nm. The cross-sections determined in these studies are a factor of two higher than determined previously in this laboratory by the modulated photolysis of 2-iodoethanol, although the shape of the spectra are similar. A plausible explanation has been proposed for the low cross-sections determined in the previous study. It is concluded that the 254 nm photolysis of 2-iodoethanol produces HOCH2CH2O2 with ca. 50% yield. The removal of HOCH2CH2O2 followed second-order kinetics with an observed rate coefficient, k(obs) = (3.0 +/- 0.5) x 10(-12) cm3 molecule-1 s-1 at 296 K and 740 Torrt (N2), the error referring to precision only at the 95% confidence level. The ratio k(obs)/sigma-250 was calculated to be (6.6 +/- 1.1) x 10(5) cm s-1 in very good agreement with values determined by the molecular modulation method in this study using H2O2 [(6.8 +/- 0.6) x 10(5) cm s-1] and in the previous study using 2-iodoethanol [(6.5 +/- 0.4) x 10(5) cm s-1] as the radical sources. The decay of HOCH2CH2O2 is attributed to a sequence of reactions initiated by the self-reaction [GRAPHICS] A reinterpretation of the previous results, accounting for the 50% yield of HOCH2CH2O2, leads to a revised value of alpha = 0.36 +/- 0.07 for the branching ratio of the alkoxy radical product channel, 2HOCH2CH2O + O2 (6a). A value of k6 = (2.2 +/- 0.5) x 10(-12) cm3 molecule-1 s-1 (95% confidence level) is concluded based on this value of alpha and the above value of k(obs). The 193 nm photolysis of 2-chloroethanol in the presence of O2 yields HOCH2CH2O2 and HO2 simultaneously and allowed the determination of the rate coefficient for reaction (7), k7 = (1.2 +/- 0.3) x 10(-11) cm3 molecule-1 s-1. [GRAPHICS]