Reaction of Methyl-<i>d</i>3 Radicals with Isobutane, Isobutane-2-<i>d,</i> and Propane

Jackson, William M.; McNesby, J. R.; Darwent, B. deB.

doi:10.1063/1.1733349

Cited by 48 publications

(21 citation statements)

References 11 publications

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“…McNesby and co-workers later used the same technique to study propane and propane-2,2-d 2 . 34, 35 Kerr and Parsonage derived absolute rate constants from these studies using the CD 3 + acetone-d 6 reaction as a reference. In 1966, Tedder and Watson 38 photolyzed azomethane to generate CH 3 in the presence of n-butane, trapped the intermediate butyl radicals by iodination with methyl iodide, and derived relative rates on the basis of the distribution of isomeric butyl iodides.…”

Section: Target Measurements and Constraintsmentioning

confidence: 99%

Evaluated Kinetics of the Reactions of H and CH₃ with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis

2015

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Presented is a combined experimental and modeling study of the kinetics of the reactions of H and CH3 with n-butane, a representative aliphatic fuel. Abstraction of H from n-alkane fuels creates alkyl radicals that rapidly decompose at high temperatures to alkenes and daughter radicals. In combustion and pyrolysis, the branching ratio for attack on primary and secondary hydrogens is a key determinant of the initial olefin and radical pool, and results propagate through the chemistry of ignition, combustion, and byproduct formation. Experiments to determine relative and absolute rate constants for attack of H and CH3 have been carried out in a shock tube between 859 and 1136 K for methyl radicals and 890 to 1146 K for H atoms. Pressures ranged from 140 to 410 kPa. Appropriate precursors are used to thermally generate H and CH3 in separate experiments under dilute and well-defined conditions. A mathematical design algorithm has been applied to select the optimum experimental conditions. In conjunction with postshock product analyses, a network analysis based on the detailed chemical kinetic combustion model JetSurf 2 has been applied. Polynomial chaos expansion techniques and Monte Carlo methods are used to analyze the data and assess uncertainties. The present results provide the first experimental measurements of the branching ratios for attack of H and CH3 on primary and secondary hydrogens at temperatures near 1000 K. Results from the literature are reviewed and combined with the present data to generate evaluated rate expressions for attack on n-butane covering 300 to 2000 K for H atoms and 400 to 2000 K for methyl radicals. Values for generic n-alkanes and related hydrocarbons are also recommended. The present experiments and network analysis further demonstrate that the C-H bond scission channels in butyl radicals are an order of magnitude less important than currently indicated by JetSurf 2. Updated rate expressions for butyl radical fragmentation reactions are provided.

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Section: Target Measurements and Constraintsmentioning

confidence: 99%

Evaluated Kinetics of the Reactions of H and CH₃ with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis

2015

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“…If we first make the assumptioil that the frequencies associated with the activated co~nplex are isotopically independent, then two limiting cases are obtained from equation 2. The first is that cleavage of a C-H bond occurs and the bending frequencies do not change from the initial to the activated state, that is, the only isotopically dependent vibration is the C-H stretch, so that equation (2) can be simplified to k~, ! k~ = exp 4 (UH-LiD).…”

Section: Calculatioksmentioning

confidence: 99%

“…for methyl radical abstractions based on proton tunneling (2). This theory is attractive since it does predict the experimental values of kBlkD with a good degree of accuracy, but the theory does not explain the lower values of kH/kD found for methyl radical abstraction from H1 and Dq.…”

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confidence: 99%

Isotope Effects in Methyl Radical Abstraction Reactions

Salomon¹

1964

Can. J. Chem.

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The high k~/ k~ ratio foundexperimentally for methyl radicalabstraction reactions is explained on the basis of classical rate theory. I t is shown that if the bending frequencies in the activated complex approach zero, good agreement is found between experimental and calculated kn/ko values. For abstraction of H and D from hydrocarbons, both the tunneling and classical rate theories predict acceptable k~/ k o values. Abstraction of H and D from Ha and Ds, however, can best be explained on the basis of a classical process. ISTRODUCTIONThe ratio of rate constants, kH/kD, for the abstraction of H and D from hydrocarbons by methyl radicals has been found to be higher than that calculated from classical rate theory when the bending frequencies of the RC-H bond is assumed to remain coilstant from the initial to final states (1-6). T o explain these high values for kHlkD, several workers (1, 2) have proposed that the rates are largely determined by appreciable participation of a quantum mechanical tunneling process. The basis for the tunneling mechanism is the theory of Johnston and Rapp (3), who give the relation for methyl radical abstractions based on proton tunneling (2). This theory is attractive since it does predict the experimental values of kBlkD with a good degree of accuracy, but the theory does not explain the lower values of kH/kD found for methyl radical abstraction from H1 and Dq. In Table I , the kH/kD values for various reactions are given together with the ratio of frequency factors, AH/AD, and the difference in activation energies, A E , found experimentally. I t will now be shown that these observed values can be correlated with classical rate theory and that quantum mechanical tunneling need not be invoked. CALCULATIOKSFrom absolute reaction rate theory (9), the ratio of rate constants is given by k~ 3 8 -7 3n-6

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“…been measured [8] for reaction (4). Using these data we find that log A, (mole-' cm3 sec-l) = 11.95 f 0.23 and E, (kcal/mole) = 5.7 f 0.4.…”

Section: 3mentioning

confidence: 88%

The addition of methyl radicals to tetrafluoroethylene

Sangster

Thynne

1969

Int J of Chemical Kinetics

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The addition of methyl radicals to tetrafluoroethylene in the gas phase has been studied over the temperature range 8O-18O0C, using a material balance method.CH, + C,F4 -+ CH,C,F, Arrhenius parameters of 1011.95*0.23 (mole-l cm3 sec-l) and 5.7 f 0.4 (kcal/mole) have been measured for the addition reaction.Electrophilic reagents such as 0 or CF, appear to react almost equally readily with ethylene and tetrafluoroethylene but methyl radicals add much more rapidly to tetrafluoroethylene than to ethylene, the difference in reactivity being principally due to an activation energy difference of -2 kcal/mole.The addition of methyl radicals to a variety of olefins has been studied by several workers [ 1-41, in general methods involving either mass-balance relationships or reaction product analysis being used. Recently, Cvetanovic and Irwin [5] have developed a technique based on the methane deficiency to study such reactions; this involves the use of small amounts of biacetyl as the methyl radical source in the presence of large concentrations of a reference hydrocarbon (isobutane) and the olefin. Determination of the carbon monoxide and methane yields at different olefin/hydrocarbon ratios enables a rate constant to be deduced for the addition reaction.No study of the reaction of methyl radicals with tetrafluoroethylene has been reported and accordingly we have applied Cvetanovic's technique to the study of this addition reaction in the temperature range 80-180°C. I t is clearly of interest to compare the methyl radical addition data with those for the analogous reaction involving trifluoromethyl radicals. Szwarc [6] has studied the latter reaction using a competitive technique in which abstraction of hydrogen from 2,3-dimethylbutane was the competing reaction so that values of A l / A , and El -E, are known.

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Reaction of Methyl-d3 Radicals with Isobutane, Isobutane-2-d, and Propane

Cited by 48 publications

References 11 publications

Evaluated Kinetics of the Reactions of H and CH₃ with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis

Evaluated Kinetics of the Reactions of H and CH₃ with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis

Isotope Effects in Methyl Radical Abstraction Reactions

The addition of methyl radicals to tetrafluoroethylene

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Reaction of Methyl-d3 Radicals with Isobutane, Isobutane-2-d, and Propane

Cited by 48 publications

References 11 publications

Evaluated Kinetics of the Reactions of H and CH3 with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis

Evaluated Kinetics of the Reactions of H and CH3 with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis

Isotope Effects in Methyl Radical Abstraction Reactions

The addition of methyl radicals to tetrafluoroethylene

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Product

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About

Evaluated Kinetics of the Reactions of H and CH₃ with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis

Evaluated Kinetics of the Reactions of H and CH₃ with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis