Probing O2 dependence of hydroperoxy-butyl reactions via isomer-resolved speciation

Hartness, Samuel W.; Dewey, Nicholas S.; Christianson, Matthew G.; Koritzke, Alanna L.; Doner, Anna C.; Webb, Annabelle R.; Rotavera, Brandon

doi:10.1016/j.proci.2022.09.066

Cited by 6 publications

(13 citation statements)

References 27 publications

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“…The selected temperatures correspond to peak yields of 2methyloxetane in n-butane oxidation. 29 The experimental procedure is similar to that in Rotavera et al 24 Photolysis of Cl 2 , using unfocused 351 nm light from an excimer laser operating at 2.5 W, generated C ̇l atoms instantaneously and homogeneously along both the radial and longitudinal axes of the reactor. The photoionization cross-section from Sander et al 40 (0.18 Mb) enabled calculation of the initial chlorine atom concentration, [C ̇l] 0 .…”

Section: Experimental Methodmentioning

confidence: 99%

“…Two temperatures, 650 K and 800 K, were utilized at a pressure of 6 Torr. The selected temperatures correspond to peak yields of 2-methyloxetane in n -butane oxidation . The experimental procedure is similar to that in Rotavera et al…”

Section: Experimental and Computational Approachmentioning

confidence: 99%

“…In contrast, Q̇OOH decomposition pathways leading to alkyloxetanes are among the highest barriers (∼30 kcal/mol relative to the ROȮ well) on alkylperoxy surfaces. However, in speciation measurements, oxetanes are quantitatively significant species. − In the context of chemical kinetics modeling, consumption reactions of cyclic ethers are often truncated, including only disproportionation reactions such as ȮH + cyclic ether → H 2 O + products, or are treated via species lumping . Building on developments from reactions of alkyloxiranes and alkyloxetanes, − recent mechanism development for n -pentane by Liu et al accounts for some of the complexity of cyclic ether consumption reactions including the formation of bicyclic ethers and dicarbonyls.…”

Section: Introductionmentioning

confidence: 99%

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Unimolecular Reactions of 2-Methyloxetanyl and 2-Methyloxetanylperoxy Radicals

2023

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Alkyl-substituted cyclic ethers are intermediates formed in abundance during the low-temperature oxidation of hydrocarbons and biofuels via a chain-propagating step with O ̇H. Subsequent reactions of cyclic ether radicals involve a competition between ring opening and reaction with O 2 , the latter of which enables pathways mediated by hydroperoxy-substituted carbon-centered radicals (Q ̇OOH). Due to the resultant implications of competing unimolecular and bimolecular reactions on overall populations of O ̇H, detailed insight into the chemical kinetics of cyclic ethers remains critical to highfidelity 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 computed 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-firstorder conditions to facilitate R ̇+ O 2 reactions. Photoionization spectra were measured from 8.5 eV to 11.0 eV in 50-meV steps, and relative yields were quantified for species consistent with R ̇→ products and Q ̇OOH → products. Species detected in the MPIMS experiments are linked to specific radicals of 2-methyloxetane. Species from R ̇→ 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 Q ̇OOH-mediated species, which are also low-lying channels on their respective potential energy surfaces. In addition to species common to alkyl oxidation pathways, ring-opening reactions of Q ̇OOH 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), which often serve as proxies for modeling reaction rates of ketohydroperoxides. The experimental and computational results underscore that reactions of cyclic ethers are inherently more complex than currently prescribed in chemical kinetic models utilized for combustion.

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Section: Experimental Methodmentioning

confidence: 99%

Section: Experimental and Computational Approachmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Unimolecular Reactions of 2-Methyloxetanyl and 2-Methyloxetanylperoxy Radicals

2023

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“…Owing to extreme difficulty in experimental detection, − especially in reacting systems, immediate decomposition products are relied upon to infer the presence and concentration of QOOH radicals. , For instance, concentration profiles of cyclic ethers are linked to that of the parent QOOH radicals. While the detection of cyclic ethers using, for instance, VUV photoionization mass spectrometry , and VUV absorption , is well established, the correct inference of QOOH concentrations requires that the loss mechanisms of these cyclic ethers are also well characterized and accounted for in chemical kinetics modeling.…”

Section: Introductionmentioning

confidence: 99%

“… 6 , 7 For instance, concentration profiles of cyclic ethers are linked to that of the parent QOOH radicals. While the detection of cyclic ethers using, for instance, VUV photoionization mass spectrometry 8 , 9 and VUV absorption 10 , 33 is well established, the correct inference of QOOH concentrations requires that the loss mechanisms of these cyclic ethers are also well characterized and accounted for in chemical kinetics modeling.…”

Section: Introductionmentioning

confidence: 99%

Unimolecular Reactions of 2,4-Dimethyloxetanyl Radicals

Doner¹,

Zádor

Rotavera³

2023

J. Phys. Chem. A

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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.

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The role of stereochemistry in combustion processes

Elliott,

Moore,

Mulvihill

et al. 2024

WIREs Comput Mol Sci

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Stereochemical effects significantly influence chemical processes, yet it is not well understood if they are a leading source of uncertainty in combustion modeling. Stereochemistry influences a combustion model (i) at the earliest stage of its construction when mapping the reaction network, (ii) in the computation of individual thermochemical and rate parameters, and (iii) in the prediction of combustion observables. The present work reviews the importance of enumerating stereochemical species and reactions at each of these steps. Further, it analyzes the separate influence of several types of stereochemistry, including geometric, optical, and fleeting transition state diastereomers. Three reaction networks serve to examine which stages of low‐temperature oxidation are most affected by stereochemistry, including the first and second oxidation of n‐butane, the third oxidation of n‐pentane, and the early stages of pyrolysis of 1‐ and 2‐pentene. The 149 reactions in the n‐butane mechanism are expanded to 183 reactions when accounting for diastereomerism. Each of these 183 reactions is parameterized with ab initio kinetics computations to determine that, for the n‐butane mechanism, the median factor of diastereomeric deviation is 3.5 at 360 K for rate constants and as high as 1.6 for mechanism reactivity, in terms of ignition delay times, as opposed to a mechanism without stereochemical expansion.This article is categorized under: Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics Theoretical and Physical Chemistry > Thermochemistry Software > Simulation Methods

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Probing O2 dependence of hydroperoxy-butyl reactions via isomer-resolved speciation

Cited by 6 publications

References 27 publications

Unimolecular Reactions of 2-Methyloxetanyl and 2-Methyloxetanylperoxy Radicals

Unimolecular Reactions of 2-Methyloxetanyl and 2-Methyloxetanylperoxy Radicals

Unimolecular Reactions of 2,4-Dimethyloxetanyl Radicals

The role of stereochemistry in combustion processes

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