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

Abstract

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, CH3 + CH3 → C2H6. 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} cm3 s-1, k0(T) = 9.04 × 10-27 cm6 s-1, and Jexp(T) = {exp[T(K)/268] − 1}2.