A. C. Doner, M. M. Davis, A. L. Koritzke, M. G. Christianson, J. M. Turney, H. F. Schaefer III, L. Sheps, D. L. Osborn, C. A. Taatjes, B. Rotavera
International Journal of Chemical Kinetics (Joe V. Michael Special Edition), Vol. 53, pp. 127 – 145
Publication year: 2021

Oxiranes are a class of cyclic ethers formed in abundance during low-temperature combustion of hydrocarbons and biofuels, either via chain-propagating steps that occur from unimolecular decomposition of β-hydroperoxyalkyl radicals (β-Q̇OOH) or from reactions of HOȮ with alkenes. The cis– and trans– isomers of 2,3-dimethyloxirane are intermediates of n-butane oxidation, and while rate coefficients for β-Q̇OOH → 2,3-dimethyloxirane + ȮH are reported extensively, subsequent reaction mechanisms of the cyclic ethers are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe the consumption of 2,3-dimethyloxirane by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error –uncertainty derived from missing or incomplete chemistry.

The present research 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). The experiments were conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, the enthalpies of stationary points on the Ṙ + O2 surfaces were computed at the ccCA-PS3 level of theory. In total, 28 barrier heights were computed on the 2,3-dimethyloxiranylperoxy surfaces. Two notable aspects are low-lying pathways that form resonance-stabilized ketohydroperoxide-type radicals caused by Q̇OOH ring-opening when the unpaired electron is localized adjacent to the ether group, and cistrans isomerization of the Ṙ and Q̇OOH radicals, via inversion, which enable reaction pathways otherwise restricted by stereochemistry.

Several species were identified in the MPIMS experiments from ring-opening of 2,3-dimethyloxiranyl radicals, and neither of the two conjugate alkene isomers prototypical of Ṙ + O2 reactions were detected. Products were also identified from decomposition of ketohydroperoxide-type radicals. The present work provides the first analysis of 2,3-dimethyloxirane oxidation chemistry and reveals that consumption pathways are complex and require the expansion of sub-mechanisms in chemical kinetics mechanisms.