Flash photolysis study of the CH3O2+ CH3O2 and CH3O2+ HO2 reactions between 600 and 719 K: unimolecular decomposition of methylhydroperoxide
Abstract
The reactions: CH3O2+ CH3O2→ 2CH3O + O2(1a), → CH3OH + HCHO + O2(1b) and CH3O2+ HO2→ CH3OOH + O2(2) have been studied between 600 and 719 K and at atmospheric pressure, using the flash photolysis/UV absorption method. The peroxy radicals were generated via the photolysis of molecular oxygen around 200 nm in the presence of CH4(for CH3O2) and/or CH3OH (for HO2). Results for k1 and k2 are in good agreement with earlier lower-temperature work in this laboratory. Reanalysis of all results from this laboratory to date, using recently reported temperature-dependent absorption cross-sections for CH3O2 and HO2 gives, for the temperature range 248–700 K: k1/cm3 molecule–1 s–1=(1.0 ± 0.1)× 10–13 exp[(416 ± 32)K/T], k2/cm3 molecule–1 s–1=(2.9 ± 0.3)× 10–13 exp[(862 ± 44)K/T] Above 600 K, the unimolecular decomposition of methylhydroperoxide becomes important: CH3OOH + M → CH3O + OH + M (3) Under most conditions, CH3O and OH are converted rapidly to HO2 and CH3O2 respectively, effectively reversing reaction (2). The occurrence of reaction (3) alters the form and increases the timescale of the radical decay, as the only remaining termination reaction for hydroperoxy radicals is their self-reaction. An Arrhenius fit gives: k3/s–1= 10(14.8±0.7) exp[–(177 ± 9)kJ mol–1/RT] Errors are 1σ.