Reaction of Propargyl with Oxygen

Probing the Reaction of Propargyl Radical with Molecular Oxygen by Synchrotron VUV Photoionization Mass Spectrometry

Self-reaction of propargyl (C3H3) radical is the main pathway to benzene, the formation of which is the rate-controlling step toward the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. Oxidation of C3H3 is a promising strategy to inhibit the formation of hazardous PAHs and soot. In the present study, we studied the C3H3 + O2 reaction from 650 to 1100 K in a laminar flow reactor and identified the intermediates and products by synchrotron VUV photoionization mass spectrometry. 2-Propynal, ethenone, formaldehyde, CO, CO2, C2H2, C2H4, and C3O2 were identified. Among them, 2-propynal, ethenone, and formaldehyde provided direct evidence for the branching reaction of C3H3 + O2 → HCCCHO + OH, C3H3 + O2 → H2CCO + CHO, and C3H3 + O2 → H2CO + CHCO, respectively. Potential energy surface calculation and mechanistic analysis of the C3O2 formations implied that C3H3 + O2 → CCCHO + H2O and C3H3 + O2 → HCCCO + H2O could occur, despite lacking direct observations of CCCHO and HCCCO radicals. The formation of ethenone and CO suggested the occurrence of the two CO elimination channels. We incorporated these validated reactions and the corresponding rate coefficients in the kinetic model of NUIGMech1.3, and the simulation showed obvious improvements toward the measured mole fractions of C3H3 and H2CCO, suggesting that the new C3H3 + O2 reaction channels were crucial in the overall combustion modeling of the important intermediate propyne (C3H4).

Temperature and Pressure-Dependent Rate Constants for the Reaction of the Propargyl Radical with Molecular Oxygen

Ab initio CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) calculations have been conducted to map the C₃H₃O₂ potential energy surface. The temperature- and pressure-dependent reaction rate constants have been calculated using the Rice-Ramsperger-Kassel-Marcus Master Equation model. The calculated results indicate that the prevailing reaction channels lead to CH₃CO + CO and CH₂CO + HCO products. The branching ratios of CH₃CO + CO and CH₂CO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300-2000 K, whereas CCCHO + H₂O (0-10%) and CHCCO + H₂O (0-17%) are significant minor products. The desirable products OH and H₂O have been found for the first time. The individual rate constant of the C₃H₃ + O₂ → CH₂CO + HCO channel, 4.8 × 10^-14 exp[(-2.92 kcal·mol^-1)/(RT)], is pressure independent; however, the total rate constant, 2.05 × 10^-14 T^0.33 exp[(-2.8 ± 0.03 kcal·mol^-1)/(RT)], of the C₃H₃ + O₂ reaction leading to the bimolecular products strongly depends on pressure. At P = 0.7-5.56 Torr, the calculated rate constants of the reaction agree closely with the laboratory values measured by Slagle and Gutman [Symp. (Int.) Combust.1988, 21, 875-883] with the uncertainty being less than 7.8%. At T < 500 K, the C₃H₃ + O₂ reaction proceeds by simple addition, making an equilibrium of C₃H₃ + O₂ ⇌ C₃H₃O₂. The calculated equilibrium constants, 2.60 × 10^-16-8.52 × 10^-16 cm³·molecule^-1, were found to be in good agreement with the experimental data, being 2.48 × 10^-16-8.36 × 10^-16 cm³·molecule^-1. The title reaction is concluded to play a substantial role in the oxidation of the five-member radicals and the present results corroborate the assertion that molecular oxygen is an efficient oxidizer of the propargyl radical.

Theoretical Study on the Reaction of Nitric Oxide with Propargyl Radical

The reaction of nitric oxide (NO) with propargyl radical (C₃H₃) was investigated at the CCSD(T)/cc-pVTZ//B3LYP/6-311++G(df, pd) level of theory. The rate coefficients of the system were determined by using the RRKM-CVT method with Eckart tunneling correction over a temperature range of 200-800 K and a pressure range of 1.0 × 10^-4 to 10.0 bar. Eight channels proceeding via the barrierless formation of excited intermediate ONCH₂CCH or CH₂CCHNO at the first step were explored. Three favorable channels (i.e., channels producing adduct of ONCH₂CCH and CH₂CCHNO and products of HCN and H₂CCO) were confirmed. The rate coefficients of channels producing adduct of ONCH₂CCH and CH₂CCHNO are comparable and have weak negative temperature dependence and positive pressure dependence. Channel producing products of HCN and H₂CCO is more important at low pressure and high temperature and less important after pressure greater than 1.0 × 10^-2 bar (with a branching ratio less than 6% at 0.1 bar).

Theoretical Modeling of Hydroxyl-Radical-Induced Lipid Peroxidation Reactions

The OH-radical-induced mechanism of lipid peroxidation, involving hydrogen abstraction followed by O2 addition, is explored using the kinetically corrected hybrid density functional MPWB1K in conjunction with the MG3S basis set and a polarized continuum model to mimic the membrane interior. Using a small nonadiene model of linoleic acid, it is found that hydrogen abstraction preferentially occurs at the mono-allylic methylene groups at the ends of the conjugated segment rather than at the central bis-allylic carbon, in disagreement with experimental data. Using a full linoleic acid, however, abstraction is correctly predicted to occur at the central carbon, giving a pentadienyl radical. The Gibbs free energy for abstraction at the central C11 is approximately 8 kcal/mol, compared to 9 kcal/mol at the end points (giving an allyl radical). Subsequent oxygen addition will occur at one of the terminal atoms of the pentadienyl radical fragment, giving a localized peroxy radical and a conjugated butadiene fragment, but is associated with rather high free energy barriers and low exergonicity at the CPCM-MPWB1K/MG3S level. The ZPE-corrected potential energy surfaces obtained without solvent effects, on the other hand, display considerably lower barriers and more exergonic reactions.