Development of Sub-Mechanisms for Cyclic Ethers: Alkyl-Substituted Oxiranes

Oxiranes are a class of cyclic ethers formed in abundance during the low-temperature combustion of hydrocarbons and biofuels from the unimolecular decomposition of hydroperoxyalkyl radicals (Q̇OOH). For example, ethyloxirane, cis-2,3-dimethyloxirane, and trans-2,3-dimethyloxirane are produced as intermediates during the oxidation of n-butane. While rate coefficients for the unimolecular decomposition of Q̇OOH to the aforementioned species are well-characterized, questions remain as to the propensity of cyclic ethers to undergo peroxy radical-mediated reactions. The lack of insight results in the oversimplification of oxirane consumption reactions in chemical kinetics mechanisms by convoluting several elementary reactions into one step, which can lead to mechanism truncation error – uncertainty derived from missing or incomplete chemistry.

The present work provides fundamental insight into the reaction mechanisms of ethyloxirane and 2,3-dimethyloxirane to support ongoing efforts to minimize mechanism truncation error. Specifically, sub-mechanisms for each species were constructed automatically using Reaction Mechanism Generator (RMG). Species thermochemistry and reaction rates were produced via estimation methods within RMG due to the lack of experimental values. The branching algorithm within RMG was employed to ensure that important low-throughput reactions were included in the model. Each model was compared against measurements from recent experiments on ethyloxiranyl and 2,3-dimethyloxiranyl radicals using multiplexed photoionization mass spectrometry conducted at 10 Torr, 650 K, and 800 K. The development of sub-mechanisms for species directly related to Q̇OOH provides a useful benchmark for theoretical rate calculations that are necessary for high-fidelity combustion modeling.