Recombination of Methyl Radicals. 2. Global Fits of the Rate Coefficient

The temperature- and pressure-dependent behavior of the cross section for optical absorption by the methyl radical is carefully considered, so we may define a criterion for selecting and correcting measurements of the rate coefficient for the recombination of methyl radicals, CH 3 + CH 3 → C 2 H 6 . The low-temperature data of Slagle et al., Hippler et al., and Walter et al. and the high-temperature data of Glänzer et al., Hwang et al., and our latest results (previous paper in this issue) are used to define a data set which contains 217 points. Subsets of isothermal data show that the temperature dependence of the high-pressure rate coefficient may be described by the simple exponential function A ∞ exp{− T (K)/ T ∞ }. Four different formulations for the pressure-dependent behavior in the falloff region are used for the global fits: (1) the asymmetric Lorentzian broadening function of Gilbert et al.; (2) the Gaussian broadening function of Wang and Frenklach; (3) the empirical “ a equation” introduced by Gardiner; (4) the extension of Lindemann's expression suggested by Oref. All formulations reproduce the data, but Oref's “ J equation” produces the least correlation between the best-fit parameters, the least uncertainty in these parameters, and the smallest uncertainty in the predictions. These results are k ∞ ( T ) = 8.78 × 10 -11 exp{− T (K)/723} cm 3 s -1, k 0 ( T ) = 9.04 × 10 -27 cm 6 s -1, and J exp ( T ) = {exp[ T (K)/268] − 1} 2 .