Weak Collision Effects in the Reaction CH 3 CO → CH 3 + CO

Rate constants for the unimolecular decomposition of CH 3 CO have been obtained as a function of temperature (420 ‐ 500 K) and helium density (3 ‐ 18 · 10 16 atom cm −3 ), conditions which are in the second order region of the fall‐off curve. An Arrhenius expression for the low‐pressure limit unimolecular rate constant was obtained from the results, k 1 ±(He) = (6.7 ± 1.8) · 10 −9 exp[(‐6921 ± 126 K)/ T ] cm 3 molecule −1 s −1 . Using a Master Equation formalism to calculate values of k 1 , a set of the two energy parameters needed in the calculations, E ± and Δ E ) down (including its temperature dependence), was found that is within the range of expected values (including a temperature dependence in the case of Δ E down ), which, when incorporated into the Master Equation, provides calculated rate constants which agree well with the measured ones. They are E ± = 65.3 ± 4.0 kJ mol −1 and Δ E down = 65.6 + 0.271 T cm −1 (the latter, a parameterized expression, is valid only in the temperature range of this study). A transition state model for the unimolecular decomposition of CH 3 CO was produced which provides high‐pressure limit rate constants for this reaction ( k 1 ±(CH 3 CO ± CH 3 + CO) = 2.50 · 10 13 exp(‐8244 K/ T ) s −1 and k −1 ±(CH 3 + CO ± CH 3 CO) = 7.64 · 10 −13 exp(‐3073 K/ T ) cm 3 molecule −1 s −1 ) and k ( E ) values for solving the Master Equation for reaction conditions that are in the fall‐off region. Fall‐off behavior of k 1 and k −1 reported by others for several different bath gases was reproduced within the uncertainty limits of the experimental results using the Master Equation formalism incorporating the transition state model, the energy parameter E ± given above, and reasonable values for Δ E down for the different bath gases used. This Master Equation formalism and transition state model should provide unimolecular rate constants for reaction (1, ‐ 1) in the fall‐off region for additional bath gases using reasonable estimates of Δ E down (e. g., values obtained for this energy‐transfer parameter for collisions between other polyatomic radicals and the bath gases of interest).