Lasser flash photolysis study of the UV spectrum and kinetics of reactions of HOCH2CH2O2 radicals

Abstract

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 × 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)× 10^–12 cm³ molecule^–1 s^–1 at 296 K and 740 Torr (N₂), the error referring to precision only at the 95% confidence level. The ratio k_obs/σ₂₅₀ was calculated to be (6.6 ± 1.1)× 10⁵ cm s^–1 in very good agreement with values determined by the molecular modulation method in this study using H₂O₂[(6.8 ± 0.6)× 10⁵ cm s^–1] and in the previous study using 2-iodoethanol [(6.5 ± 0.4)× 10⁵ cm s^–1] as the radical sources. The decay of HOCH₂CH₂O₂ is attributed to a sequence of reactions initiated by the self-reaction. HOCH₂CH₂O₂+ HOCH₂CH₂O₂→ products (6) A reinterpretation of the previous results, accounting for the 50% yield of HOCH₂CH₂O₂, leads to a revised value of α= 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)× 10^–12 cm³ molecule^–1 s^–1(95% confidence level) is concluded based on this value of α 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)× 10^–11 cm³ molecule^–1 s^–1. HOCH₂CH₂O₂+ HO₂→ products (7)