Roaming radicals in the thermal decomposition of dimethyl ether: Experiment and theory

Abstract

The thermal dissociation of dimethyl ether has been studied with a combination of reflected shock tube experiments and ab initio dynamics simulations coupled with transition state theory based master equation calculations. The experiments use the extraordinary sensitivity provided by H-atom ARAS detection with an unreversed light source to measure both the total decomposition rate and the branching to radical products versus molecular products, with the molecular products arising predominantly through roaming according to the theoretical analysis. The experimental observations also provide a measure of the rate coefficient for H + CH 3OCH 3. An evaluation of the available experimental results for H + CH 3OCH 3 can be expressed by a three parameter Arrhenius expression as, k = 6.54 × 10 - 24 T 4.13 exp ( - 896 / T ) cm 3 molecule - 1 s - 1 ( 273 - 1465 K ) The potential energy surface is explored with high level ab initio electronic structure theory. The dynamics of roaming versus radical formation is studied with a reduced dimensional trajectory approach. The requisite potential energy surface is obtained from an interpolative moving least squares fit to wide-ranging ab initio data for the long-range interactions between methyl and methoxy. The predicted roaming and radical micro-canonical fluxes are incorporated in a master equation treatment of the temperature and pressure dependence of the dissociation process. The tight (i.e., non-roaming) transition states leading to a variety of additional molecular fragments are also included in the master equation analysis, but are predicted to have a negligible contribution to product formation. The final theoretical results reliably reproduce the measured dissociation rate to radical products reported here and are well reproduced over the 500–2000 K temperature range and the 0.01–300 bar pressure range by the following modified Arrhenius parameters for the Troe falloff format: k 1 , ∞ ( T ) = 2.33 × 10 19 T - 0.661 exp ( - 42345 / T ) s - 1 k 1 , 0 ( T ) = 2.86 × 10 35 T - 11.4 exp ( - 46953 / T ) cm 3 molecule - 1 s - 1 F cent ( T ) = exp ( - T / 880 ) The experimentally observed branching ratio of 0.19 ± 0.07 provides a direct measure of the contribution from the roaming radical mechanism. The theoretical analysis predicts a much smaller roaming contribution of 0.02.