Kinetic study of the reactions CH3O2+ CH3O2 and CH3O2+ HO2 using molecular modulation spectroscopy

Abstract

Infrared diode laser and ultraviolet spectroscopic techniques have been used to observe the behaviour of HO₂ and CH₃O₂ radicals generated in the gas phase by the near-ultraviolet modulated photolysis of flowing Cl₂–CH₄–H₂–O₂ and Cl₂–CH₄–H₂O₂–O₂ mixtures. This has enabled measurement of the rate coefficient for the reaction HO₂+ CH₃O₂→ products (3) which was found to have a value of (5.4 ± 1.1)× 10^–12 cm³ molecule^–1 s^–1 at 10 Torr and 300 K. CH₃O₂ produced during the photolysis of Cl₂–CH₄–O₂ mixtures displayed second-order kinetic behaviour owing to a series of reactions initiated by the reaction CH₃O₂+ CH₃O₂→ products. (1) The parameter k_obs/σ(where k_obs is the observed second-order rate constant) was measured for 210 ⩽λ/nm ⩽ 270 at 760 Torr and 298 K, and had a value of 1.11 × 10⁵ cm s^–1 at 250nm. σ(250 nm) was determined to be (4.25 ± 0.5)× 10^–18 cm² molecule^–1, leading toi a value of (4.7 ± 0.5)× 10^–13 cm³ molecule^–1 s^–1 for k_obs. The time dependence of HDO production during the continuous near-ultraviolet photolysis of a static Cl₂, CD₄, CH₃OH, O₂ mixture has also been measured in the infrared. It seems likely that reaction (18a) accounts for the majority of the observed HDO with the remainder being formed following the production of OH radicals in the system. The observed results are consistent with the existence of two channels for reaction (18), with k_18a/(k_18a+k_18b)≈ 0.4: CD₃O₂+ HO₂→ HDO + CD₂O + O₂(18a), →CD₃OOH + O₂. (18b)