CH₃ + Cl = CH₃Cl: RRKM/master equation modeling

Abstract

Data for the title reaction have been fitted using an RRKM/master equation approach. Energy transfer was modeled using an exponential decay with downward step sizes, E d , as a fitting parameter. The low temperature (200 < T (K) < 300) combination of CH 3 with Cl atoms in He can be accommodated with E d (cm −1 ) = 400. Higher temperature (1600 < T (K) < 2100) decomposition in Ar required E d (T ) (cm −1 ) = 694(T /300) 0.46 . Previous analysis of the analogous system CH 4 = CH 3 + H required E d (T ) (cm −1 ) = 100(T /300) for He and E d (T ) (cm −1 ) = 150(T /300) for Ar. Understanding of the magnitudes and temperature dependence of E d remains the greatest detriment to quantitative calculation, extrapolation, and prediction of unimolecular rate constants.

Theoretical Unimolecular Kinetics for CH4 + M ⇄ CH3 + H + M in Eight Baths, M = He, Ne, Ar, Kr, H2, N2, CO, and CH4

“…Likewise, the present results confirm the accuracy of the recommended experimental values, with the possible exception of the falloff rate coefficients for M = Ar, where the recommended and experimental values are somewhat higher than the theoretical rate coefficients. We note that Golden recently discussed difficulties consistently modeling the experimental results for Ar using the ME and empirical values for α.…”

“…Occasionally, however, surprising values of α 300 and n are required to reproduce experimental data. For example, in a recent study of CH 3 Cl decomposition the values of α required to fit the results of an ME calculation empirically to measured decomposition rate coefficients were found to be four times as large as those obtained in a similar study of CH 4 . As another example, low-pressure decomposition rate coefficients for water were calculated using the ME approach, and a relatively weak temperature dependence in α ( n = 0.4–0.5) was required to fit the experimental data.…”

Evaluation of data for atmospheric models: Master Equation/RRKM calculations on the combination reaction ClO + NO2 → ClONO2, a recurring issue

“…(These become even tighter when negative temperature dependences of k ∞,comb are taken into account). These are not the relatively unencumbered transition states of alkyl radical combination with H‐atoms 33,34. The hindered‐Gorin approach has no difficulty characterizing either type of transition state since the choice of hindrance parameter is purely empirical.…”