Reanalysis of Rate Data for the Reaction CH3 + CH3 → C2H6 Using Revised Cross Sections and a Linearized Second-Order Master Equation

Rate coefficients for the CH3 + CH3 reaction, over the temperature range 300–900 K, have been corrected for errors in the absorption coefficients used in the original publication ( Slagle et al., J. Phys. Chem. 1988, 92, 2455−2462 ). These corrections necessitated the development of a detailed model of the B̃2A1′ (3s)–X̃2A2″ transition in CH3 and its validation against both low temperature and high temperature experimental absorption cross sections. A master equation (ME) model was developed, using a local linearization of the second-order decay, which allows the use of standard matrix diagonalization methods for the determination of the rate coefficients for CH3 + CH3. The ME model utilized inverse Laplace transformation to link the microcanonical rate constants for dissociation of C2H6 to the limiting high pressure rate coefficient for association, k ∞(T); it was used to fit the experimental rate coefficients using the Levenberg–Marquardt algorithm to minimize χ2 calculated from the differences between experimental and calculated rate coefficients. Parameters for both k ∞(T) and for energy transfer ⟨ΔE⟩down(T) were varied and optimized in the fitting procedure. A wide range of experimental data were fitted, covering the temperature range 300–2000 K. A high pressure limit of k ∞(T) = 5.76 × 10–11(T/298 K)−0.34 cm3 molecule–1 s–1 was obtained, which agrees well with the best available theoretical expression.