Article pubs.acs.org/JPCA
Kinetic (T = 201−298 K) and Equilibrium (T = 320−420 K) Measurements of the C3H5 + O2 ⇆ C3H5O2 Reaction Matti P. Rissanen,† Damien Amedro,†,‡ Arkke J. Eskola,† Theo Kurten,§,∥ and Raimo S. Timonen*,† †
Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55 (A.I. Virtasen aukio 1), FIN-00014 Helsinki, Finland § Department of Physics, University of Helsinki, P.O. Box 64, FIN-00014 Helsinki, Finland ∥ Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 København ø, Denmark S Supporting Information *
ABSTRACT: The kinetics and equilibrium of the allyl radical reaction with molecular oxygen have been studied in direct measurements using temperature-controlled tubular flow reactor coupled to a laser photolysis/ photoionization mass spectrometer. In low-temperature experiments (T = 201−298 K), association kinetics were observed, and the measured time-resolved C3H5 radical signals decayed exponentially to the signal background. In this range, the determined rate coefficients exhibited a negative temperature dependence and were observed to depend on the carrier-gas (He) pressure {p = 0.4−36 Torr, [He] = (1.7−118.0) × 1016 cm−3}. The bimolecular rate coefficients obtained vary in the range (0.88−11.6) × 10−13 cm3 s−1. In higher-temperature experiments (T = 320−420 K), the C3H5 radical signal did not decay to the signal background, indicating equilibration of the reaction. By measuring the radical decay rate under these conditions as a function of temperature and following typical secondand third-law procedures, plotting the resulting ln Kp values versus 1/T in a modified van’t Hoff plot, the thermochemical parameters of the reaction were extracted. The second-law treatment resulted in values of ΔH298 ° = −78.3 ± 1.1 kJ mol−1 and −1 −1 ΔS298 ° = −129.9 ± 3.1 J mol K , with the uncertainties given as one standard error. When results from a previous investigation were taken into account and the third-law method was applied, the reaction enthalpy was determined as ΔH298 ° = −75.6 ± 2.3 kJ mol−1.
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INTRODUCTION Reactions of free radicals with molecular oxygen constitute key reaction steps in hydrocarbon oxidation.1 Small unsaturated hydrocarbon radicals with resonance-stabilized structures are thermodynamically more stable than similar saturated radicals lacking resonance stabilization. Consequently, they reach higher concentrations under combustion conditions and are identified as precursors for polycyclic aromatic hydrocarbons (PAHs) and, further, for soot formation.2−4 Allyl radical (C3H5) is the simplest conjugated, resonancestabilized alkenyl radical. Alkenyl radicals are formed, for example, in hydrogen abstractions from carbon atoms at the β-position to the double bond in alkenes by reactive species (e.g., OH or other radicals) and by pyrolysis and cracking of larger hydrocarbons at elevated temperatures.5−7 The delocalization of an electron in the radical formed lowers the activation energy for the abstraction reaction considerably.5,6 In peroxy radicals (RO2), formed from resonance-stabilized radicals such as allyl radicals, the R−O2 bond is significantly weaker than for similarly sized alkyl radicals. This arises as a consequence of breaking the resonance in the radical when the R−O2 bond is formed and causes the reverse dissociation reaction back to reactants (R + O2) to become important at lower temperatures.1,6,8−10 © 2012 American Chemical Society
To our knowledge, the rate coefficient of the C3H5 + O2 addition reaction has been measured only by Jenkin et al.11 at room temperature and 1 atm of N2 using laser photolysis for C3H5 radical generation and UV absorption spectroscopy for direct detection, resulting in the value k296 = (6 ± 2) × 10−13 cm3 s−1. The high-temperature kinetics of the forward reaction was studied by Walker and co-workers.12−14 They studied the oxidation of allyl radicals by decomposing 4,4-dimethylpent1-ene in the presence of oxygen between 400 and 500 °C (673−773 K)12,13 and also in conjunction with the oxidation chemistry of propene in the autoignition region14 using gaschromatographic end-product analysis to indirectly determine the kinetic parameters. Ruiz et al.15 were the first to perform direct measurements to determine the equilibrium constant of the C3H5 + O2 ⇆ C3H5O2 reaction. Equilibrium was studied at around 348 K and 2.8 Torr (He) by following real-time decays of the produced allyl concentrations in the presence and absence of excess O2 using laser photolysis/photoionization mass spectrometer. Received: October 17, 2011 Revised: March 21, 2012 Published: April 13, 2012 3969
dx.doi.org/10.1021/jp209977h | J. Phys. Chem. A 2012, 116, 3969−3978
The Journal of Physical Chemistry A
Article
From the equilibrium constant, ΔG° was calculated, and when combined with an estimation for ΔS°, the reaction enthalpy was deduced as ΔH300 ° = −72.0 ± 4.2 kJ mol−1 [i.e., −ΔH300 ° (C3H5−O2)]. Morgan et al.16 were the first to measure the temperature dependence of the equilibrium constant. They employed laser photolysis and direct UV-absorption detection in the 382−453 K temperature range and the 50−400 Torr pressure range to obtain the thermodynamic parameters of the reaction from the second-law treatment: ΔH298 ° = −76.2 ± 2.1 kJ mol−1 and ΔS298 ° = −122 ± 5 J mol−1 K−1. Slagle et al.17 measured the C3H5 + O2 equilibrium between 352 and 413 K at about 3 Torr (He) using a laser photolysis/photoionization mass spectrometry (LP/PIMS) setup similar to that used in the current study. Knyazev and Slagle later reanalyzed the data5,9 to account for possible further reactions of the allyl peroxy adduct (C3H5O2) and obtained the values ΔH298 ° = −77.9 ± 3.2 kJ mol−1 and ΔS298 ° = −126.8 ± 8.6 J mol−1 K−1 from the second law and ΔH298 ° = −76.8 ± 2.6 kJ mol−1 from the third law. The C3H5 + O2 reaction has also been the subject of computational studies. Bozzelli and Dean10 used energized complex/ QRRK (quantum Rice−Ramsperger−Kassel) theory described by Dean18 to analyze the kinetics of different reaction channels and reported −75 kJ mol−1 as the enthalpy of the reaction. More recently, Lee and Bozzelli8 studied the equilibrium reaction at the CBSQ//B3LYP/6-31G(d,p) composite and at density functional levels and calculated the reaction enthalpy as −79.7 kJ mol−1. They also predicted rate coefficients and product yields using the QRRK/master equation approach. Despite the interest the C3H5 + O2 reaction has received, the low-temperature kinetics (201−298 K) described in this article, together with the lowest-temperature equilibrium constants (320−346 K), have not been presented previously.
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Other photolytic products observed4,21 were allene (C3H4); propene (C3H6); propane (C3H8); cyclopropenylidene (C3H2); ethylene (C2H4); ethane (C2H6); 1,3-butadiene (C4H6); butene (C4H8); vinylacetylene (C4H4) 1,5-hexadiene (C6H10); and C5H7, C4H7, vinyl (C2H3), and methyl (CH3) radicals. However, allyl radical accounts for more than 80% of all radical products and about 93% of the primary radical products at 193 nm.4 The initial allyl radical concentrations in the experiments for 1,5-hexadiene precursor were estimated from the gas flow rates, precursor vapor pressure, laser fluence, and absorption cross section at 193 nm [σ193 nm = (4.1 ± 0.5) × 10−18 cm2 molecule−1].22 For allyl bromide, the estimation of initial concentrations involved more unknown variables (i.e., absorption cross section at 193 nm and exact vapor pressure at the precursor temperature) and consequently have larger uncertainties. In all of the measurements, the estimated initial radical concentration was lower than 2 × 1012 cm−3. In addition to the radicals produced, the flowing gas mixture contained the molecular reagent (O2 from 0.01% to about 7%) in great excess over the radical concentration, with the radical precursor at
