Isomer‐dependent reaction mechanisms of cyclic ether intermediates: cis‐2,3‐dimethyloxirane and trans‐2,3‐dimethyloxirane

Abstract

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

Quantitative Detection of Products and Radical Intermediates in Low-Temperature Oxidation of Cyclopentane

“…Dissociative ionization has been observed previously for ethyl peroxy and propyl peroxy, and stable parent cations have been reported for methyl peroxy and alkenyl peroxy . However, the absolute PI cross section of ROO we report here is only the third such determination to date, building on the recent PI spectra of methyl peroxy and ethoxyethyl peroxy from our group.…”

“…The high-pressure experiments, conducted with ∼50–500 higher oxygen partial pressure than at 10 Torr, lead to much higher rate coefficients for O 2 addition and much more efficient collisional relaxation of chemically activated radicals. Our previous work on alkane, ether, and ketone oxidation revealed that high- P , high-oxygen conditions in the HPR enhance second-O 2 addition relative to the LPR. ,,− We therefore assigned the m / z = 67, 84, 100, and 116 peaks at 3000 and 7500 Torr to species formed by γ-QOOH + O 2 , as shown in Figure . In addition, the time dependence of m / z = 57 and 82 peaks (marked with asterisks in Figure ) suggests that they are also likely due to DI fragments of second-O 2 addition products; however, we could not definitively assign them.…”

Kinetic study of the atmospheric oxidation of a series of epoxy compounds by OH radicals

“…Our recent study on the reactivity of a series of epoxides towards chlorine atoms in gas phase using quantum mechanical calculations agrees with these findings. As we discussed in Tovar et al (2021), if we consider the case of cisand trans-epoxybutane, when the compound has lost one H of the CH 3 group, it becomes trigonally (sp 2 ) hybridized, facilitating the possible formation of a double bond with the singly occupied p-orbital after abstraction of H. This effect could explain the formation of acrolein from Cl-initiated oxidation of both cis-2,3dimethyloxirane and trans-2,3-dimethyloxirane with yields of 36.49 ± 0.55 and 49.08 ± 1.29 at 650 and 800 K, respectively, in the work presented by Doner et al (2021). The formation of the vinyl oxirane from β hydrogen abstraction and the subsequent formation of the keto hydro peroxy radical reported by Christianson et al (2021) for the oxidation of 1,2-epoxybutane also confirm this theory.…”

Stereoisomer-dependent unimolecular kinetics of 2,4-dimethyloxetanyl peroxy radicals

“…This is largely due to the lack of insight on the consumption reactions, which include site-specific H-abstraction reactions, the ring-opening of cyclic ether radicals, and the reactions of cyclic ether radicals with O 2 . The latter can traverse QOOH-mediated pathways 17–19 in a manner similar to hydrocarbons but can have more complicated chemical behavior as well, due to the C–O bonds in the molecular structure. It has been shown before that the unimolecular reactions of cyclic ether peroxy radicals can lead to ketohydroperoxide-type radicals 17,18 and dicarbonyls.…”