posted on 2015-07-16, 00:00authored byM. A. Blitz, N. J.
B. Green, R. J. Shannon, M. J. Pilling, P. W. Seakins, C. M. Western, S. H. Robertson
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.