Relative rates of radical–radical processes in the pyrolysis of ethane

Pacey, Philip D.; Wimalasena, Jayantha H.

doi:10.1139/v84-050

Cited by 12 publications

(9 citation statements)

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“…CH 3 + C 2 H 6 → C 2 H 5 + CH 4 . Although experimental data ,− are only available in the temperature range 1000−1400 K, Baulch 27 et al have extended their recommendation to the 300−1500 K range based on low-temperature data on isotopic variations. This resulted in an Arrhenius plot with very high curvature (the temperature exponent in their three-parameter expression is 6.0).…”

Section: Resultsmentioning

confidence: 99%

Use of Quantum Methods with Transition State Theory: Application to H-Atom Metathesis Reactions

2001

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The use of quantum chemical methods to determine rate constants for some H-atom metathesis reactions using transition state theory and tunneling corrections is explored. Comparisons are made among several methods (DFT, MP2, QCISD), all of which yield similar structures and frequencies for the transition states, but quite different barrier heights. Tunneling corrections are made using either the well-known Eckart method or one based on the WKB approach. We find that we can fit the extant data by varying the barrier heights using either tunneling approach, although the WKB method is both more accurate and more labor intensive. Values of the barrier heights obtained this way are not in good agreement with those obtained from any of the quantum methods. † Part of the special issue "Harold Johnston Festschrift". The authors would like to express their gratitude to Harold Johnston for his invaluable contributions to our science and his continued inspiration.

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Section: Resultsmentioning

confidence: 99%

Use of Quantum Methods with Transition State Theory: Application to H-Atom Metathesis Reactions

2001

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“…The rate constant for the disproportionation of ethyl radicals has also been found to increase sharply at higher temperatures. 45 Such behavior can be interpreted in terms of the participation of excited vibrational states in the transition species.…”

Section: Discussionmentioning

confidence: 99%

Properties of Transition Species in the Reactions of Hydroxyl with Ammonia and with Itself

1996

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Quantum transition state theory was used to model two hydrogen atom abstraction reactions of hydroxyl radicals. Moments of inertia for the transition species were calculated from geometries obtained by ab initio or bond energy bond order methods. The transition state equation was fit by nonlinear least squares to experimental rate constants to determine parameters of the transition species. For the reaction of hydroxyl with ammonia, the geometric mean, ω b , of four transitional vibrational frequencies was found to be 660 ( 20 cm -1 , the activation barrier height, V b , including zero point energy, to be 8.5 ( 0.5 kJ mol -1 , and the barrier thickness, at half its height, to be 65 ( 30 pm. For the disproportionation reaction of hydroxyl radicals, the corresponding values of the first two parameters, based on the most recent experimental data, were 890 ( 20 cm -1 and 2.7 ( 0.8 kJ mol -1 . Ab initio calculations on the latter reaction gave ω b ) 746 cm -1 at the HF/6-31G(d,p) level and V b ) 13.6 kJ mol -1 at the MP4/6-311G(d,p) level. The curvature of the Arrhenius plots can be explained by the contributions of the low-frequency vibrations and, for the disproportionation reaction, of the low-lying electronic states.

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“…The most direct attempt to measure rate constants for reaction , k 1 , has been reported by Möller et al In their work, gas mixtures of azomethane (CH 3 N) 2 and C 2 H 6 in Ar were prepared, and CH 3 -radical depletion was measured behind incident shock waves using UV absorption at λ = 216.5 nm to probe [CH 3 ]. Additional high temperature rate constants for reaction have been reported, − but none of these experimental studies were designed to measure k 1 . Instead, the observations of CH 3 -radical depletion in some of these studies were most likely complicated by contributions from the CH 3 –CH 3 self-reaction and potentially other unimolecular and bimolecular processes.…”

Section: Introductionmentioning

confidence: 99%

Direct Measurements of Rate Constants for the Reactions of CH₃ Radicals with C₂H₆, C₂H₄, and C₂H₂ at High Temperatures

et al. 2013

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The shock tube technique has been used to study the reactions CH3 + C2H6 → C2H4 + CH4 + H (1), CH3 + C2H4 → Products + H (2), and CH3 + C2H2 → Products + H (3). Biacetyl, (CH3CO)2, was used as a clean high temperature thermal source for CH3-radicals for all the three reactions studied in this work. For reaction 1, the experiments span a T-range of 1153 K ≤ T ≤ 1297 K, at P ~ 0.4 bar. The experiments on reaction 2 cover a T-range of 1176 K ≤ T ≤ 1366 K, at P ~ 1.0 bar, and those on reaction 3 a T-range of 1127 K ≤ T ≤ 1346 K, at P ~ 1.0 bar. Reflected shock tube experiments performed on reactions 1-3, monitored the formation of H-atoms with H-atom Atomic Resonance Absorption Spectrometric (ARAS). Fits to the H-atom temporal profiles using an assembled kinetics model were used to make determinations for k1, k2, and k3. In the case of C2H6, the measurements of [H]-atoms were used to derive direct high-temperature rate constants, k1, that can be represented by the Arrhenius equation k1(T) = 5.41 × 10(-12) exp(-6043 K/T) cm(3) molecules(-1) s(-1) (1153 K ≤ T ≤ 1297 K) for the only bimolecular process that occurs, H-atom abstraction. TST calculations based on ab initio properties calculated at the CCSD(T)/CBS//M06-2X/cc-pVTZ level of theory show excellent agreement, within ±20%, of the measured rate constants. For the reaction of CH3 with C2H4, the present rate constant results, k2', refer to the sum of rate constants, k(2b) + k(2c), from two competing processes, addition-elimination, and the direct abstraction CH3 + C2H4 → C3H6 + H (2b) and CH3 + C2H4 → C2H2 + H + CH4 (2c). Experimental rate constants for k2' can be represented by the Arrhenius equation k2'(T) = 2.18 × 10(-10) exp(-11830 K/T) cm(3) molecules(-1) s(-1) (1176 K ≤ T ≤ 1366 K). The present results are in excellent agreement with recent theoretical predictions. The present study provides the only direct measurement for the high-temperature rate constants for these channels. Lastly, measurements of H-atoms from the reaction of CH3 with C2H2 provided direct unambiguous determinations of the rate constant for the dominant process under the present experimental conditions, the addition-elimination, CH3 + C2H2 → p-C3H4 + H (3b). Experimental rate constants for k(3b) can be represented by the Arrhenius equation k(3b)(T) = 5.16 × 10(-13) exp(-3852 K/T) cm(3) molecules(-1) s(-1) (1127 K ≤ T ≤ 1346 K). The present determinations for k(3b) represent the only direct measurements for this reaction and are also in good agreement with recent theoretical predictions. The present experimental k(3b) values were also used to derive rate constants, k(-3b), for the more extensively studied back-process, the reaction of H-atoms with propyne. The best fit Arrhenius equation, combining the presently derived k(-3b) values with a recent experimental determination for k(-3b), can be represented by k(-3b)(T) = 3.87 × 10(-11) exp(-1313 K/T) cm(3) molecules(-1) s(-1) (870 K ≤ T ≤ 1346 K). The present studies represent a novel implementation of the sensitive H-ARAS technique to measure...

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Relative rates of radical–radical processes in the pyrolysis of ethane

Cited by 12 publications

References 10 publications

Use of Quantum Methods with Transition State Theory: Application to H-Atom Metathesis Reactions

Use of Quantum Methods with Transition State Theory: Application to H-Atom Metathesis Reactions

Properties of Transition Species in the Reactions of Hydroxyl with Ammonia and with Itself

Direct Measurements of Rate Constants for the Reactions of CH₃ Radicals with C₂H₆, C₂H₄, and C₂H₂ at High Temperatures

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Relative rates of radical–radical processes in the pyrolysis of ethane

Cited by 12 publications

References 10 publications

Use of Quantum Methods with Transition State Theory: Application to H-Atom Metathesis Reactions

Use of Quantum Methods with Transition State Theory: Application to H-Atom Metathesis Reactions

Properties of Transition Species in the Reactions of Hydroxyl with Ammonia and with Itself

Direct Measurements of Rate Constants for the Reactions of CH3 Radicals with C2H6, C2H4, and C2H2 at High Temperatures

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Direct Measurements of Rate Constants for the Reactions of CH₃ Radicals with C₂H₆, C₂H₄, and C₂H₂ at High Temperatures