Unimolecular Reactions of 2,4-Dimethyloxetanyl Radicals

Abstract

Alkyl-substituted oxetanes are cyclic ethers formed via unimolecular reactions of QOOH radicals produced via a six-membered transition state in the preceding isomerization step of organic peroxy radicals, ROO. Owing to radical isomer-specific formation pathways, cyclic ethers are unambiguous proxies for inferring QOOH reaction rates. Therefore, accounting for subsequent oxidation of cyclic ethers is important in order to accurately determine rates for QOOH → products. Cyclic ethers can react via unimolecular reaction (ring-opening) or via bimolecular reaction with O 2 to form cyclic ether-peroxy adducts. The computations herein provide reaction mechanisms and theoretical rate coefficients for the former type in order to determine competing pathways for the cyclic ether radicals. Rate coefficients of unimolecular reactions of 2,4-dimethyloxetanyl radicals were computed using master equation modeling from 0.01 to 100 atm and from 300 to 1000 K. Coupled-cluster methods were utilized for stationary-point energy calculations, and uncertainties in the computed rate coefficients were accounted for using variation in barrier heights and in well depths. The potential energy surfaces reveal accessible channels to several species via crossover reactions, such as 2-methyltetrahydrofuran-5-yl and pentanonyl isomers. For the range of temperature over which 2,4-dimethyloxetane forms during n-pentane oxidation, the following are the major channels: 2,4dimethyloxetan-1-yl → acetaldehyde + allyl, 2,4-dimethyloxetan-2-yl → propene + acetyl, and 2,4-dimethyloxetan-3-yl → 3-butenal + methyl, or, 1-penten-3-yl-4-ol. Well-skipping reactions were significant in a number of channels and also exhibited a markedly different pressure dependence. The calculations show that rate coefficients for ring-opening are approximately an order of magnitude lower for the tertiary 2,4-dimethyloxetanyl radicals than for the primary and secondary 2,4-dimethyloxetanyl radicals. Unlike for reactions of the corresponding ROO radicals, however, unimolecular rate coefficients are independent of the stereochemistry. Moreover, rate coefficients of cyclic ether radical ring-opening are of the same order of magnitude as O 2 addition, underscoring the point that a competing network of reactions is necessary to include for accurate chemical kinetics modeling of species profiles for cyclic ethers.

Combustion in a Sustainable World: From Molecules to Processes

“…Cyclic ethers provide an important marker of the flux through QOOH radicals, which are the central transient intermediate in low temperature chain branching. Doner et al report a stereochemically specific ab initio master equation theoretical study of the sequence of decomposition reactions for the five radicals derived from the prototypical cyclic ether, 2,4-dimethyloxetane . These reactions compete with oxygen addition reactions in their influence on the low-temperature reactivity …”

“…Doner et al report a stereochemically specific ab initio master equation theoretical study of the sequence of decomposition reactions for the five radicals derived from the prototypical cyclic ether, 2,4-dimethyloxetane. 10 These reactions compete with oxygen addition reactions in their influence on the low-temperature reactivity. 10 Specific reactions have also been investigated in the context of aromatics formation and pyrolysis.…”

“…10 These reactions compete with oxygen addition reactions in their influence on the low-temperature reactivity. 10 Specific reactions have also been investigated in the context of aromatics formation and pyrolysis. 11,12 For example, Hanamirian et al employed their automated kinetics code (EStokTP) and master equation lumping to obtain ab initio kinetic models for the H-abstraction reaction from methylcyclopentadiene and the unimolecular decomposition of the resulting methylcyclopentadienyl radical.…”

Unimolecular Reactions of 2-Methyloxetanyl and 2-Methyloxetanylperoxy Radicals

“…Subsequent reactions of cyclic ethers, both in the context of biofuels , and as oxidation intermediates, are complex − because of competing reactions of ring-opening and reaction with O 2 . Common Q̇OOH-derived intermediates from alkane oxidation are substituted three-membered cyclic ethers (alkyloxiranes) and four-membered cyclic ethers (alkyloxetanes).…”

“…Reaction pathways of alkyloxiranes involve Q̇OOH ring-opening reactions into resonance-stabilized ketohydroperoxide-type radicals − and also exhibit stereochemical dependence . Similar competing pathways also exist for radicals derived from 2,4-dimethyloxetane. , Doner et al computed stationary points on potential energy surfaces for 2,4-dimethyloxetanyl radicals at the CCSD­(T)-F12/cc-pVTZ-F12//ωB97XD/6-311++G­( d , p ) level of theory. Solutions to the master equation were then conducted in order to determine the branching fractions of products from ring opening.…”