CF₃CH₃ → HF + CF₂CH₂:  A Non-RRKM Reaction?

Abstract

The experimental shock tube data recently reported by Kiefer et al. [J. Phys. Chem. A 2004, 108, 2443-2450] for the title reaction at temperatures between 1600 and 2400 K have been compared to master equation simulations using three models: (a) standard RRKM theory, (b) RRKM theory modified by local random matrix theory, which introduces dynamical corrections arising from slow intramolecular vibrational energy randomization, and (c) an ad hoc empirical non-RRKM model. Only the third model provides a good fit of the Kiefer et al. unimolecular reaction rate data. In separate simulations, all three models accurately reproduce the experimental 300 K chemical activation data of Marcoux and Setser [J. Phys. Chem. 1978, 82, 97-108] when the energy transfer parameters are freely varied to fit the data. When experimental energy transfer parameters for a geometrical isomer (1,1,2-trifluoroethane) are used, the standard RRKM model fits the chemical activation data better than the other models, but if energy transfer in the 1,1,1-trifluoroethane is significantly reduced in comparison to the 1,1,2 isomer, then the empirical ad hoc non-RRKM model also gives a good fit. While the ad hoc empirical non-RRKM model can be made to fit the data, it is not based on theory, and we argue that it is physically unrealistic. We also show that the master equation simulations can mimic the Kiefer et al. vibrational relaxation data, which was the first shock tube observation of double-exponential relaxation. We conclude that, until more data on the trifluoroethanes become available, the current evidence is insufficient to decide with confidence whether non-RRKM effects are important in this reaction, or whether the Kiefer et al. data can be explained in some other way.

Collisional energy transfer in polyatomic molecules at high temperatures: Master equation analysis of vibrational relaxation of shock-heated alkanes

“…Their hypothesis assumed that the fast stage corresponds to buildup of the torsional excitation and that the IVR transfer occurs when the excitation reaches a certain level. However, no indication of the slow IVR has been found in subsequent computational studies [25,44,45]. Also, collisions with bath gas should induce IVR, which makes the assumption further improbable [44].…”

“…However, no indication of the slow IVR has been found in subsequent computational studies [25,44,45]. Also, collisions with bath gas should induce IVR, which makes the assumption further improbable [44]. Barker et al proposed an alternative explanation for the double relaxation [44].…”

Energy transfer in master equation simulations: A new approach

“…Because of this problem, it has been necessary to limit the number of iterations to, e.g., 2-5, so that normalization at high energy converges sufficiently, while normalization at low energy does not diverge too much. In previous work, more iterations were sometimes used to achieve better performance in certain energy ranges [34]. As the number of iterations is increased, results at higher energies systematically become more accurate, while those at low energies become less so.…”

“…However, in shock tube simulations [34][35][36] and in simulating reactions with very low energy barriers [26] the relevant energies extend all of the way to the bottom of the energy ladder. Furthermore, in 2D (E,J ) master equation simulations, the normalization problems are exacerbated because the 2D density of states is even more sparse and hence exhibits even greater fluctuations than in the one-dimensional (1D) version.…”

“…Vibrational frequencies for ClOOCl were taken from Jacobs et al [50] with vibrational anharmonicities calculated using density functional theory (B3LYP/6-311+G(3df)) [Jingyao Liu, private communication, 2008]. The lowest vibrational frequency (127 cm −1 ) Table AI. Vibrational frequencies, moments of inertia, and other parameters can be found in previous papers on 1,1,1-trifluoroethane [34] and norbornene [36]. Lennard-Jones parameters for all of the collision partners are summarized in Table AII. BIBLIOGRAPHY…”