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Degenerate chain-branching mechanisms of n -alkanes are centered on the formation of hydroperoxy-alkyl radicals (̇QOOH), formed via ̇R + O 2 reactions, and the ensuing competition between unimolecular decomposition and second-O 2 -addition. Quantitative measurements of partially oxidized intermediates formed via reactions of ̇QOOH provide critical constraints that are required for accurate modeling of combustion chemistry. To examine the influence of temperature and oxygen concentration on intermediates from unimolecular decomposition of ̇QOOH, isomer-resolved speciation measurements were conducted on n -butane oxidation at 835 Torr in a jet-stirred reactor (JSR) from 500 – 900 K. Resulting from negative-temperature coefficient behavior, cyclic ether formation peaked at two temperatures, 650 K and 800 K, which were selected for separate experiments to quantify the O 2 -dependence of species profiles using O 2 concentrations of 4.2 · 1017 – 1.1 · 1019 molecules cm–3. Utilizing vacuum-ultraviolet absorption spectroscopy and electron-impact mass spectrometry, cyclic ether isomers were quantified separately, including explicit resolution of cis - and trans - isomers of 2,3-dimethyloxirane. Stereoisomers of 2-butene were also quantified explicitly. For all cyclic ethers, a common trend in O 2 -dependence emerged: species concentrations reach a maximum near 3.0 · 1018 molecules cm–3 (equivalence ratio of 0.5). Although quantitative disparities are evident, chemical kinetics modeling qualitatively reproduces the O 2 dependence of species at 650 K. However, at 800 K, weak dependence on O 2 is predicted, which is in contrast with the measurements. Two carbonyls, diacetyl and methyl vinyl ketone, were also quantified and follow similar dependence on [O 2 ] and temperature as the cyclic ethers, which indicates some fraction forms via ̇QOOH-mediated reactions. The discrepancies between the measured and model-predicted species profiles indicate that sub-mechanisms for important intermediates may require additional elementary reactions, including stereochemical-specific reactions, to improve the fidelity of n -alkane combustion modeling. [ABSTRACT FROM AUTHOR]

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‐̇QOOH) or from reactions of HȮO with alkenes. The cis‐ and trans‐isomers of 2,3‐dimethyloxirane are intermediates of n‐butane oxidation, and while rate coefficients for β‐̇QOOH → 2,3‐dimethyloxirane + ̇OH 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 ̇R + 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 ̇QOOH ring‐opening when the unpaired electron is localized adjacent to the ether group, and cis‐trans isomerization of ̇R and ̇QOOH 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. Neither of the two conjugate alkene isomers prototypical of ̇R + 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 submechanisms in chemical kinetics mechanisms. [ABSTRACT FROM AUTHOR]

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‐̇QOOH) or from reactions of HOȮ with alkenes. Ethyloxirane is one of four alkyl‐substituted cyclic ether isomers produced as an intermediate from n‐butane oxidation. While rate coefficients for β‐̇QOOH → 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 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 ̇R + O2 potential energy surfaces for the four ethyloxiranyl radical isomers, which produced barrier heights for 24 reaction pathways. In addition to products from ̇QOOH → cyclic ether + ȮH and ̇R + 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 ̇QOOH radicals. The latter occurs when the unpaired electron is localized on the ether group, causing the initial ̇QOOH structure to ring‐open and form a resonance‐stabilized ketohydroperoxide‐type radical. The present work provides the first analysis of ethyloxirane oxidation chemistry, which reveals that consumption pathways are complex and may require an expansion of submechanisms to increase the fidelity of chemical kinetics mechanisms. [ABSTRACT FROM AUTHOR]