Oxiranes are a class of cyclic ethers formed in abundance during lowtemperature combustion of hydrocarbons and biofuels, either via chainpropagating steps that occur from unimolecular decomposition of βhydroperoxyalkyl radicals (β-QOOH) 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-QOOH → 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 theṘ + O 2 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 resonancestabilized ketohydroperoxide-type radicals caused byQOOH ring-opening when the unpaired electron is localized adjacent to the ether group, and cis-trans isomerization ofṘ andQOOH 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 ofṘ + O 2 reactions were detected. Products were also identified from decomposition of ketohydroperoxide-type radicals. The present work

Oxiranes are a class of cyclic ethers formed in abundance during lowtemperature combustion of hydrocarbons and biofuels, either via chainpropagating steps that occur from unimolecular decomposition of βhydroperoxyalkyl 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

Photolytically initiated oxidation experiments were conducted on cyclohexane and tetrahydropyran using multiplexed photoionization mass spectrometry to assess the impact of the ether functional group in the latter species on reaction mechanisms relevant to autoignition. Pseudo-firstorder conditions, with [O 2 ] 0 :[R • ] 0 > 2000, were used to ensure that R • + O 2 → products were the dominant reactions. Quasi-continuous, tunable vacuum ultraviolet light from a synchrotron was employed over the range 8.0−11.0 eV to measure photoionization spectra of the products at two pressures (10 and 1520 Torr) and three temperatures (500, 600, and 700 K). Photoionization spectra of ketohydroperoxides were measured in both species and were qualitatively identical, within the limit of experimental noise, to those of analogous species formed in n-butane oxidation. However, differences were noted in the temperature dependence of ketohydroperoxide formation between the two species. Whereas the yield from cyclohexane is evident up to 700 K, ketohydroperoxides in tetrahydropyran were not detected above 650 K. The difference indicates that reaction mechanisms change due to the ether group, likely affecting the requisite • QOOH + O 2 addition step. Branching fractions of nine species from tetrahydropyran were quantified with the objective of determining the role of ring-opening reactions in diminishing ketohydroperoxide. The results indicate that products formed from unimolecular decomposition of R • and • QOOH radicals via concerted C−C and C−O β-scission are pronounced in tetrahydropyran and are insignificant in cyclohexane oxidation. The main conclusion drawn is that, under the conditions herein, ring-opening pathways reduce the already low steady-state concentration of • QOOH, which in the case of tetrahydropyran prevents • QOOH + O 2 reactions necessary for ketohydroperoxide formation. Carbon balance calculations reveal that products from ring opening of both R • and • QOOH, at 700 K, account for >70% at 10 Torr and >55% at 1520 Torr. Three pathways are confirmed to contribute to the depletion of • QOOH in tetrahydropyran including (i) γ-• QOOH → pentanedial + • OH, (ii) γ-• QOOH → vinyl formate + ethene + • OH, and (iii) γ-• QOOH → 3-butenal + formaldehyde + • OH. Analogous mechanisms in cyclohexane oxidation leading to similar intermediates are compared and, on the basis of mass spectral results, confirm that no such ringopening reactions occur. The implication from the comparison to cyclohexane is that the ether group in tetrahydropyran increases the propensity for ring-opening reactions and inhibits the formation of ketohydroperoxide isomers that precede chainbranching. On the contrary, the absence of such reactions in cyclohexane enables ketohydroperoxide formation up to 700 K and perhaps higher temperature.

No abstract

No abstract