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Cyclic ethers undergo H-abstraction reactions that yield carbon-centered radicals (Ṙ). The ether functional group introduces a competing set of reaction pathways: ring-opening and reaction with O2 to form peroxy radical adducts, ROȮ, which can result in stereoisomers. ROȮ derived from cyclic ethers can subsequently isomerize into hydroperoxy-substituted carbon-centered radicals, Q̇OOH, which can also undergo ring-opening reactions or pathways prototypical to alkyl oxidation. The balance of reactions that unfold from cyclic ether radicals depends intrinsically on the size of the ring and the structure of any substituents retained in the formation step. The present work examines unimolecular reactions of peroxy radicals from 2-ethyloxetane, a four-membered cyclic ether formed during n-pentane oxidation, and reveals stereoisomer-specific reaction pathways. Automated quantum chemical computations were conducted on constitutional and stereoisomers of ROȮ derived from O2-addition to 2-ethyloxetanyl radicals. Pressure-dependent rate calculations were conducted by solving the master equation from 300 – 1000 K and from 0.01 – 100 atm. Branching fractions were then calculated at 650 K and 825 K, the peak temperatures at which cyclic ethers form in alkane oxidation. Isomer-specific reaction pathways of anti-ROO and syn-ROO and resulting impact on radical production were evident. Q̇OOH ring-opening reactions were significant as were rates of bi-cyclic ether formation common in alkyl radical oxidation. Detailed prescription of rates and reaction mechanisms describing cyclic ether consumption mechanisms are important to enable accurate modeling of reactions of ephemeral Q̇OOH radicals because of the direct, isomer-specific formation pathways. In addition, detailed cyclic ether mechanisms are required to reduce mechanism truncation error. The results herein provide insight on connections between cyclic ethers and chain-reaction pathways yielding ȮH, HOȮ, and other radicals, in addition to pathways leading to performic acid (HOOC(=O)H), the decomposition of which via O–O scission results in an exothermic, chain-branching step.
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Stereoisomer-dependent unimolecular kinetics of 2,4- dimethyloxetanyl peroxy radicals
2,4,dimethyloxetane is an important cyclic ether intermediate that is produced from hydroperoxyalkyl (QOOH) radicals in the low-temperature combustion of n -pentane. However, the reaction mechanisms and rates of consumption pathways remain unclear. In the present work, the pressure- and temperature-dependent kinetics of seven cyclic ether peroxy radicals, which stem from 2,4,dimethyloxetane via H-abstraction and O 2 addition, were determined. The automated kinetic workflow code, KinBot, was used to model the complexity of the chemistry in a stereochemically resolved manner and solve the resulting master equations from 300–1000 K and from 0.01–100 atm. The main conclusions from the calculations include (i) diastereomeric cyclic ether peroxy radicals show significantly different reactivities, (ii) the stereochemistry of the peroxy radical determines which QOOH isomerization steps are possible, (iii) conventional QOOH decomposition pathways, such as cyclic ether formation and HO 2 elimination, compete with ring-opening reactions, which primarily produce OH radicals, the outcome of which is sensitive to stereochemistry. Ring-opening reactions lead to unique products, such as unsaturated, acyclic peroxy radicals, that form direct connections with species present in other chemical kinetics mechanisms through "cross-over" reactions that may complicate the interpretation of experimental results from combustion of n-pentane and, by extension, other alkanes. For example, one cross-over reaction involving 1-hydroperoxy-4-pentanone-2-yl produces 2-(hydroperoxymethyl)-3-butanone-1-yl, which is an iso-pentane-derived ketohydroperoxide (KHP). At atmospheric pressure, the rate of chemical reactions of all seven peroxy radicals compete with that of collisional stabilization, resulting in well-skipping reactions. However, at 100 atm, only one out of seven peroxy radicals undergoes significant well-skipping reactions. The rates produced from the master equation calculations provide the first foundation for the development of detailed sub-mechanisms for cyclic ether intermediates. In addition, analysis of the complex reaction mechanisms of 2,4-dimethyloxetane-derived peroxy radicals provides insights into the effects of stereoisomers on reaction pathways and product yields.
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- Award ID(s):
- 2042646
- PAR ID:
- 10392358
- Date Published:
- Journal Name:
- Faraday Discussions
- Volume:
- 238
- ISSN:
- 1359-6640
- Page Range / eLocation ID:
- 295 to 319
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2FD00029F
