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, E>d, 298 K, and the temperature exponent of E>d. A good fit to the data was obtained with: E>d(Ar) = (110 ± 30) × (T/298 K)(0.0 ± 0.7) cm-1 and E>d(He) = (34.4 ± 6.2) × (T/298 K)(1.0 ± 0.5) cm-1.
Onel et al. (Thu,) studied this question.