Examining Cyclic Ether Consumption in Low-Temperature Oxidation

Oxiranes are a class of cyclic ethers formed in abundance during low-temperature combustion of hydrocarbons and biofuels. While rate coefficients for the formation of 2,3-dimethyloxirane are reported extensively, subsequent reaction mechanisms of the cyclic ether are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe the consumption of 2,3-dimethyloxirane, which may introduce mechanism truncation error – uncertainty derived from missing or incomplete chemistry.

The present work examines the isomer-dependence of 2,3-dimethyloxirane reaction mechanisms in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred via the detection of products from Cl-initiated oxidation of both cis-2,3-dimethyloxirane and trans-2,3-dimethyloxirane using multiplexed photoionization mass spectrometry (MPIMS). To complement the experiments, the enthalpies of stationary points on the Ṙ + O2 surfaces were computed at the ccCA-PS3 level of theory. Theoretical computations revealed low-lying pathways that form resonance-stabilized ketohydroperoxide-type Q̇OOH radicals, which were confirmed in experiment by detection of several decomposition products of such radicals. Isomerization of Ṙ and Q̇OOH radicals by inversion enables reaction pathways otherwise restricted by stereochemistry. The present work provides the first analysis of 2,3-dimethyloxirane oxidation chemistry and reveals that consumption pathways are complex and require the expansions of sub-mechanisms in chemical kinetics mechanisms.