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.