Analysis of the Kinetics and Yields of OH Radical Production from the CH₃OCH₂ + O₂ Reaction in the Temperature Range 195–650 K: An Experimental and Computational study

The methoxymethyl radical, CH₃OCH_2, is an important intermediate in the low temperature combustion of dimethyl ether. The kinetics and yields of OH from the reaction of the methoxymethyl radical with O₂ have been measured over the temperature and pressure ranges of 195–650 K and 5–500 Torr by detecting the hydroxyl radical using laser-induced fluorescence following the excimer laser photolysis (248 nm) of CH₃OCH₂Br. The reaction proceeds via the formation of an energized CH₃OCH₂O₂ adduct, which either dissociates to OH + 2 H₂CO or is collisionally stabilized by the buffer gas. At temperatures above 550 K, a secondary source of OH was observed consistent with thermal decomposition of stabilized CH₃OCH₂O₂ radicals. In order to quantify OH production from the CH₃OCH₂ + O₂ reaction, extensive relative and absolute OH yield measurements were performed over the same (T, P) conditions as the kinetic experiments. The reaction was studied at sufficiently low radical concentrations (∼10¹¹ cm^–3) that secondary (radical + radical) reactions were unimportant and the rate coefficients could be extracted from simple bi- or triexponential analysis. Ab initio (CBS-GB3)/master equation calculations (using the program MESMER) of the CH₃OCH₂ + O₂ system were also performed to better understand this combustion-related reaction as well as be able to extrapolate experimental results to higher temperatures and pressures. To obtain agreement with experimental results (both kinetics and yield data), energies of the key transition states were substantially reduced (by 20–40 kJ mol^–1) from their ab initio values and the effect of hindered rotations in the CH₃OCH₂ and CH₃OCH₂OO intermediates were taken into account. The optimized master equation model was used to generate a set of pressure and temperature dependent rate coefficients for the component nine phenomenological reactions that describe the CH₃OCH₂ + O₂ system, including four well-skipping reactions. The rate coefficients were fitted to Chebyshev polynomials over the temperature and density ranges 200 to 1000 K and 1 × 10¹⁷ to 1 × 10²³ molecules cm^–3 respectively for both N₂ and He bath gases. Comparisons with an existing autoignition mechanism show that the well-skipping reactions are important at a pressure of 1 bar but are not significant at 10 bar. The main differences derive from the calculated rate coefficient for the CH₃OCH₂OO → CH₂OCH₂OOH reaction, which leads to a faster rate of formation of O₂CH₂OCH₂OOH.