Kinetics of the O ₂ Reactions with CH ₃ OCO and CH ₂ OCHO Formed from Methyl Formate

Methyl formate (CH₃OCHO, MF) is the simplest ester, a class of species that is of interest for combustion and atmospheric chemistry studies. H-abstraction reactions from the two sites of MF leads to radicals, R, that react with oxygen to form RO₂ species that are important in the oxidation of MF under atmospheric and low temperature combustion conditions. This work reports the kinetics of the O₂ reactions with R formed through the reaction of Cl with MF from 213 K to ∼420 K and in the range ∼5-100 Torr of N₂ or Ar. The rate coefficients kR+O₂ were determined by the analysis of the kinetic profiles of the OH radicals formed by the prompt dissociation of RO₂, which were measured using the pulsed laser flash photolysis-laser induced fluorescence technique. At 294 K in N₂ (5-100 Torr), kR+O₂ was pressure independent, within experimental error, with a value of (5.0 ± 0.4) × 10⁻¹² cm³ molecule⁻¹ s⁻¹. Isotopic studies using CH₃OCDO were used to differentiate between the kinetics of the CH₃OCO and CH₂OCHO associations with O₂. CH₂OCDO reacted faster than CH₃OCO by approximately 40% at 213 K, rising to 200% at 472 K. The enhanced reactivity of the CH₂OCHO with O₂ can be explained by a more attractive potential energy surface and a looser transition state. It was also possible to determine the rate coefficients of the unimolecular decomposition of CH₃OCO, kdec (348-470 K). The experimental values of kdec were fitted with the master equation application, MESMER, with the literature (ANL0F″) barrier height of 57.3 kJ mol⁻¹, and floating the average energy transfer parameter at 298 K, <ΔE>d, 298 K, and the temperature exponent of <ΔE>d. A good fit to the data was obtained with: <ΔE>d(Ar) = (110 ± 30) × (T/298 K)(0.0 ± 0.7) cm⁻¹ and <ΔE>d(He) = (34.4 ± 6.2) × (T/298 K)(1.0 ± 0.5) cm⁻¹.