Rate coefficients for hydrogen abstraction from cis– and trans-2,3-dimethyloxirane, two cyclic ethers produced from low-temperature oxidation of n-butane, were calculated in the high-pressure limit with ab initio transition state theory from 300 – 1000 K. Pressure-dependent rate coefficients were also calculated from 0.01–100 atm using AutoMech and the MESS codes. Six abstractors relevant at combustion conditions were selected for the calculations: ȮH, Ḣ, HOȮ, ĊH₃, CH₃Ȯ, and CH₃OȮ. H-abstraction creates four unique 2,3-dimethyloxiranyl radicals with the radical site located either on the primary or tertiary carbon and with stereochemical orientation retained, at least initially. The dependence of H-abstraction reactions on stereochemistry was quantified for the first time.

For most abstractors, including ȮH, significant stereochemical differences were observed for abstraction at the primary carbon site, but not at the tertiary site. Ring strain in the oxirane group enables facile ring-opening of 2,3-dimethyloxiranyl radicals prior to collisional stabilization. As a result, pressure-dependent non-Boltzmann (prompt) effects influence the product branching between H-abstraction leading to intact carbon-centered ring radicals and ring-opened products. The results show that prompt reactions comprise a significant fraction of the total abstraction rate, with the precise branching depending strongly on the temperature and pressure conditions, as well as the abstraction site and the abstractor. For example, prompt ring-opening accounted for >50% of the total abstraction rate below 30 atm for ȮH-initiated abstraction at the primary carbon site and >90% from 0.01–100 atm at the tertiary site. The results herein affirm that the inclusion of rate coefficients in chemical kinetics mechanisms for site-specific H-abstraction reactions leading to thermalized cyclic ether radicals and prompt reactions leading directly to ring-opened products is critical due to the effects on the competition between O₂-addition and ring-opening of 2,3-dimethyloxiranyl radicals, which impacts radical populations and species profiles.