Kinetics of chemically activated ethane

Abstract

AbstractChemically 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) × 109 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 E0 = 85.8 kcal/mole, E* = 114.9 kcal/mole, and kE = 4.6 × 101 sec−1, a thermal A‐factor at 600°K of 1016.6±0.2 sec−1, in approximate agreement with the more recent experimental values. Combining 2 kcal/mole uncertainties in E0 and E* with the uncertainty in our rate constant yields an A‐factor range of 1016.6±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 E0(recombination) = 0) and statistical thermodynamic equilibrium constants, methyl radical recombination rates at 25°C of between 4.4 × 108 to 3.1 × 109 l.‐mole−1‐sec−1, which are 60 to 8 times lower, respectively, than the apparently quite reliable experimental value. A value of E0(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 E0(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.