A. C. Doner, N. S. Dewey, B. Rotavera
Journal of Physical Chemistry A (Marsha Lester Festschrift)
Publication year: 2023

Alkyl-substituted cyclic ethers are intermediates formed in abundance during low-temperature oxidation of hydrocarbons and biofuels via a chain-propagating step with ȮH. Subsequent reactions of cyclic ether radicals involve a competition between ring-opening and reaction with O2, the latter of which enables mechanisms mediated by hydroperoxy-substituted carbon-centered radicals (Q̇OOH). Due to the resultant implications of competing unimolecular and bimolecular reactions on overall populations of ȮH, detailed insight into the chemical kinetics of cyclic ethers remains critical to high-fidelity numerical modeling of combustion.

Cl-initiated oxidation experiments were conducted on 2-methyloxetane – an intermediate of n-butane oxidation – using multiplexed photoionization mass spectrometry (MPIMS) in tandem with calculations of stationary point energies on potential energy surfaces for unimolecular reactions of 2-methyloxetanyl and 2-methyloxetanylperoxy isomers. The potential energy surfaces were mapped using the KinBot algorithm with stationary points calculated at the CCSD(T)-F12/cc-pVDZ-F12 level of theory. The experiments were conducted at 6 Torr and two temperatures (650 K and 800 K) under pseudo-first-order conditions to facilitate Ṙ + O2 reactions. Photoionization spectra were measured from 8.5 – 11.0 eV in 50-meV steps and relative yields were quantified for species consistent with Ṙ → products and Q̇OOH → products.

Product species detected in the MPIMS experiments are linked to specific radicals of 2-methyloxetane. Species from Ṙ → products include methyl, ethene, formaldehyde, propene, ketene, 1,3-butadiene, and acrolein. Ion signals consistent with products from alkyl radical oxidation were detected, including for Q̇OOH-mediated species, which are also low-lying channels on respective potential energy surfaces. In addition to species common to alkyl oxidation pathways, ring-opening reactions of Q̇OOH radicals derived from 2-methyloxetane produced ketohydroperoxide species (performic acid and 2-hydroperoxyacetaldehyde), which may impart additional chain-branching potential, and dicarbonyl species (3-oxobutanal and 2-methylpropanedial) that often serve as proxies for modeling decomposition rates of ketohydroperoxides. The experimental and computational results underscore that reactions of cyclic ethers are inherently more complex than prescribed in chemical kinetics models utilized for combustion.