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Kinetics of chemically activated ethane
Author(s) -
Growcock F. B.,
Hase W. L.,
Simons J. W.
Publication year - 1973
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550050108
Subject(s) - chemistry , reaction rate constant , radical , excited state , methylene , thermodynamics , singlet state , rrkm theory , transition state theory , kinetics , computational chemistry , atomic physics , organic chemistry , physics , quantum mechanics
Abstract Chemically activated ethane, with an excitation energy of 114.9 ± 2 kcal/mole, was formed by reaction with methane of excited singlet methylene radicals produced by the 4358 Å photolysis of diazomethane. A decomposition rate constant of (4.6 ± 1.2) × 10 9 sec −1 was measured for the chemically activated ethane. This result agrees, via RRKM theory, with most other chemically activated ethane data, and the result predicts, via RRKM and absolute rate theory for E 0 = 85.8 kcal/mole, E * = 114.9 kcal/mole, and k E = 4.6 × 101 sec −1 , a thermal A ‐factor at 600°K of 10 16.6±0.2 sec −1 , in approximate agreement with the more recent experimental values. Combining 2 kcal/mole uncertainties in E 0 and E * with the uncertainty in our rate constant yields an A ‐factor range of 10 16.6±0.7 sec −1 . It is emphasized that this large uncertainty in the A ‐factor results from an improbable combination of uncertainty limits for the various parameters. These decomposition results predict, via absolute rate theory (with E 0 (recombination) = 0) and statistical thermodynamic equilibrium constants, methyl radical recombination rates at 25°C of between 4.4 × 10 8 to 3.1 × 10 9 l.‐mole −1 ‐sec −1 , which are 60 to 8 times lower, respectively, than the apparently quite reliable experimental value. A value of E 0 (recombination) greater than zero offers no improvement, and a value less than zero would be quite unusual. Activated complexes consistent with the experimental recombination rate and E 0 (recombination) = 0 greatly overestimate the experimental chemical activation and high pressure thermal decomposition rate data. Absolute rate theory as it is applied here in a straightforward way has failed in this case, or a significant amount of internally consistent data are in serious error. Some corrections to our previous calculations for higher alkanes are discussed in Appendix II.