Non-Arrhenius behavior

It is noteworthy that the above rule connects two quite different values, because the temperature dependence of is governed by the rate constant of incoherent processes, while A characterizes coherent tunneling. In actual fact, A is not measured directly, but it is calculated from the barrier height, extracted from the Arrhenius dependence k T). This dependence should level off to a low-temperature plateau at 7 < This non-Arrhenius behavior of has actually been observed by Punnkinen [1980] in methane crystals (see fig. 1). A similar dependence, also depicted in fig. 1, has been observed by Geoffroy et al. [1979] for the radical... 

The non-Arrhenius behavior of the inversion rate constant has been detected by [Deycard et al. 1988] for the oxyranyl radical,... 

The temperature dependence of the reaction rate constant closely (but not exactly) obeys the Arrhenius equation. Both theories, however, predict non-Arrhenius behavior. The deviation from Arrhenius behavior can usually be ignored over a small temperature range. However, non-Arrhenius behavior is common (Steinfeld et al., 1989, p. 321). As a consequence, rate constants are often fitted to the more general expression k = BTnexp( —E/RT), where B, n, and E are empirical constants. 

To see more clearly the temperature effect on ion conduction, the logarithmic molal conductivity was plotted against the inverse of temperature, and the resultant plots showed apparent non-Arrhenius behavior, which can be nicely fitted to the Vogel— Tamman-Fulcher (VTF) equation ... 

Since radical and atom metathesis reactions generally have low activation energies, their rate coefficients are expected to exhibit non-Arrhenius behavior because of the increased importance of the term noted previously. In Fig. 9, rate coefficient data for H-atom abstractions from CH4... 

Grotheer, H.-H., G. Riekert, D. Walter, and Th. Just, Non-Arrhenius Behavior of the Reaction of Hydroxymethyl Radicals with Molecular Oxygen, J. Phys. Chem., 92, 4028-4030 (1988). 

Non-Arrhenius behavior of the isomerization rate constant of sterically hindered aryl radicals 2,4,6-triterbutylphenyl... 

Figure 3-1. Rate constants k(T) for the range 200

Experiments on ILs have generally shown a highly non-Arrhenius behavior that is well described by the VFT equation. Xu et al. [149] report the temperature-dependent viscosity of a series of covalently stable ILs, and note that the VFT equation fits the temperature dependence of the fluidity quite well. A series of studies by Watanabe and co-workers [167-169] on a range of different ILs shows that the VFT provides a good fit to diffusion constants, molar conductivity and viscosity. 

Figure 11 shows a typical example of the temperature-dependent behavior for the reactions of OH radical with aromatic compounds. The measured bimolecular rate constants of OH radical with nitrobenzene showed distinctly non-Arrhenius behavior below 350°C, but increased in the slightly subcritical and supercritical region. Feng a succeeded in modeling these data with a three-step reaction mechanism originally proposed by Ashton et while Ghandi etal. claimed to have developed a so-called multiple collisions model to predict the rates for the reactions of OH radical in sub- and super-critical water. 

The relatively large value of the power n at (T/300) reflects the non-Arrhenius behavior of the calculated rate constants due to the strong negative temperature dependence of the derived tunneling factor. 

Schulz, M., Energy landscape, minimum points, and non-Arrhenius behavior of supercooled liquids. Phys. Rev. B SI, 11319 (1998). 

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