Toward the Development of a Fundamentally Based Chemical Model for Cyclopentanone: High-Pressure-Limit Rate Constants for H Atom Abstraction and Fuel Radical Decomposition

Low-Temperature Oxidation Reaction Processes of Cyclopentanone Unraveled by In Situ Mass Spectrometry and Theoretical Study

Although cyclopentanone (CPO) is a promising bio-derived fuel, thermodynamic data of its low-temperature oxidation under high-pressure conditions are lacking. In this work, the low-temperature oxidation mechanism of CPO is investigated in a flow reactor in the temperature range of 500-800 K and at a total pressure of 3 atm by a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer. The electronic structure and pressure-dependent kinetic calculations are carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+G(d,p) level to explore the combustion mechanism of CPO. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPO radicals with O₂ is HO₂ elimination, yielding 2-cyclopentenone. The hydroperoxyalkyl radical (^•QOOH) generated by 1,5-H-shifting is easily reacted with second O₂ and forms ketohydroperoxide (KHP) intermediates. Unfortunately, the third O₂ addition products are not detected. In addition, the decomposition pathways of KHP during the low-temperature oxidation of CPO are further assessed, and the unimolecular dissociation pathways of CPO radicals are confirmed. The results of this study can be used for future research on the kinetic combustion mechanisms of CPO under high pressure.

Cyclic Ketones as Future Fuels: Reactivity with OH Radicals

For a sustainable energy future, research directions should orient toward exploring new fuels suitable for future advanced combustion engines to achieve better engine efficiency and significantly less harmful emissions. Cyclic ketones, among bio-derived fuels, are of significant interest to the combustion community for several reasons. As they possess high resistance to autoignition characteristics, they can potentially be attractive for fuel blending applications to increase engine efficiency and also to mitigate harmful emissions. Despite their importance, very few studies are rendered in understanding of the chemical kinetic behavior of cyclic ketones under engine-relevant conditions. In this work, we have conducted an experimental investigation for the reaction kinetics of OH radicals with cyclopentanone and cyclohexanone for the first time over a wide range of experimental conditions ( T = 900-1330 K and p ≈ 1.2 bar) in a shock tube. Reaction kinetics was followed by monitoring UV laser absorption of OH radicals near 306.7 nm. Our measured rate coefficients, with an overall uncertainty (2σ) of ±20%, can be expressed in Arrhenius form as (in units of cm³ molecule^-1 s^-1): k1(CPO+OH)=1.20×10-10exp(-2115KT) (902-1297 K); k2(CHO+OH)=2.11×10-10exp(-2268KT) (935-1331 K). Combining our measured data with the single low-temperature literature data, the following three-parameter Arrhenius expressions (in units of cm³ molecule^-1 s^-1) are obtained over a wider temperature range: k₁(CPO + OH) = 1.07×10-13(T300K)3.20exp(1005.7KT) (298-1297 K); k2(CHO+OH)=3.12×10-13(T300K)2.78exp(897.5KT) (298-1331 K). Discrepancies between the theoretical and current experimental results are observed. Earlier theoretical works are found to overpredict our measured rate coefficients. Interestingly, these cyclic ketones exhibit similar reactivity behavior to that of their linear ketone counterparts over the experimental conditions of this work.

The Unimolecular Reactions of CF3CHF2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions, and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules

The recombination of CF₃ and CHF₂ radicals in a room-temperature bath gas was used to prepare vibrationally excited CF₃CHF₂* molecules with 101 kcal mol^-1 of vibrational energy. The subsequent 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions were observed as a function of bath gas pressure by following the CHF₃, CF₃(F)C: and C₂F₄ product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF₃(F)C: concentration was measured by trapping the carbene with trans-2-butene. The experimental rate constants are 3.6 × 10⁴, 4.7 × 10⁴, and 1.1 × 10⁴ s^-1 for the 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions, respectively. These experimental rate constants were matched to statistical RRKM calculated rate constants to assign threshold energies (E₀) of 88 ± 2, 88 ± 2, and 87 ± 2 kcal mol^-1 to the three reactions. Pentafluoroethane is the only fluoroethane that has a competitive H atom transfer decomposition reaction, and it is the only example with 1,1-HF elimination being more important than 1,2-HF elimination. The trend of increasing threshold energies for both 1,1-HF and 1,2-HF processes with the number of F atoms in the fluoroethane molecule is summarized and investigated with electronic-structure calculations. Examination of the intrinsic reaction coordinate associated with the 1,1-HF elimination reaction found an adduct between CF₃(F)C: and HF in the exit channel with a dissociation energy of ∼5 kcal mol^-1. Hydrogen-bonded complexes between HF and the H atom migration transition state of CH₃(F)C: and the F atom migration transition state of CF₃(F)C: also were found by the calculations. The role that these carbene-HF complexes could play in 1,1-HF elimination reactions is discussed.