Stereoisomer-dependent rate coefficients and reaction mechanisms of 2-ethyloxetanylperoxy radicals

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Cyclic ethers undergo H-abstraction reactions that yield carbon-centered radicals (R˙). The ether functional group introduces a competing set of reaction pathways: ring-opening and reaction with O2 to form peroxy radical adducts, ROO˙, which can result in stereoisomers. ROO˙ derived from cyclic ethers can subsequently isomerize into hydroperoxy-substituted carbon-centered radicals, Q˙OOH, 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 ROO˙ 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. Q˙OOH 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 Q˙OOH 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 O˙H, HOO˙, 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.