M. G. Christianson, A. C. Doner, M. M. Davis, A. L. Koritzke, J. M. Turney, H. F. Schaefer III, L. Sheps, D. L. Osborn, C. A. Taatjes, B. Rotavera
International Journal of Chemical Kinetics (Special Issue in Honor of Joe V. Michael), Vol. 53
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. Ethyloxirane is one of four alkyl-substituted isomers produced as an intermediate from n-butane oxidation. While rate coefficients for β-Q̇OOH → ethyloxirane + ȮH are reported extensively, subsequent reaction mechanisms of the cyclic ether are not. As a result, chemical kinetics mechanisms commonly adopt simplified chemistry to describe ethyloxirane consumption by convoluting several elementary reactions into a single step, which may introduce mechanism truncation error – uncertainty derived from missing or incomplete chemistry.

The present work provides fundamental insight on reaction mechanisms of ethyloxirane in support of ongoing efforts to minimize mechanism truncation error. Reaction mechanisms are inferred from the detection of products during chlorine atom-initiated oxidation experiments using multiplexed photoionization mass spectrometry (MPIMS) conducted at 10 Torr and temperatures of 650 K and 800 K. To complement the experiments, calculations of stationary point energies were conducted using the ccCA-PS3 composite method on Ṙ + O2 potential energy surfaces for the four ethyloxiranyl radical isomers, which produced barrier heights for 24 reaction pathways.

In addition to products from Q̇OOH → cyclic ether + ȮH and Ṙ + O2 → conjugate alkene + HOȮ, both of which were significant pathways and are prototypical to alkane oxidation, other species were identified from ring-opening of both ethyloxiranyl and Q̇OOH radicals. The latter occurs when the unpaired electron is localized on the ether group, causing the initial Q̇OOH structure to ring-open and form a resonance-stabilized ketohydroperoxide-type radical. The present work provides the first analysis of ethyloxirane oxidation chemistry and reveals that consumption pathways are complex and may require an expansion of sub-mechanisms in order to increase the fidelity of chemical kinetics mechanisms.