Mechanism for dissociation

Li Y L ef a/1995 Experimentai verification of a new mechanism for dissociative chemisorption atom abstraction Phys. Rev. Lett. 74 2603... 

The Arrhenius-like temperature dependence obtained, which however gives rise to unreasonable Irequency factors, can then be rationalized on the basis of the temperature dependence of the blackbody radiation. At higher temperatures, the energy density per unit wavelength of the blackbody radiation increases with the maximum in the distribution shifted to higher frequency. Also, at a given frequency the intensity of radiation emitted varies approximately as In / oc -T" Therefore, as the temperature increases, so too does the intensity of the radiation and with it the rate of energization of the cluster ion and, consequently, the rate of unimolecular dissociation. Thus the temperature dependence is entirely consistent with a radiative mechanism for dissociation. 

As indicated in Table 7.10, only in the last decade have models considered all three phenomena of heat transfer, fluid flow, and hydrate dissociation kinetics. The rightmost column in Table 7.10 indicates whether the model has an exact solution (analytical) or an approximate (numerical) solution. Analytic models can be used to show the mechanisms for dissociation. For example, a thorough analytical study (Hong and Pooladi-Danish, 2005) suggested that (1) convective heat transfer was not important, (2) in order for kinetics to be important, the kinetic rate constant would have to be reduced by more than 2-3 orders of magnitude, and (3) fluid flow will almost never control hydrate dissociation rates. Instead conductive heat flow controls hydrate dissociation. 

Figure 2-6. An energy level diagram showing the vibrational states of interest in the vibrational predissociation of Ar-C02. The experimental results show that the intramolecular V-V process, indicated by the diagonal arrows, is the dominant mechanism for dissociation. C02 vibrational energy levels designated (a), (b), and (c) are the Fermi diads and Fermi triad discussed in the text.

Fig. 7.27a. Proposed mechanisms for dissociation of Ln-K21DA complexes (coordinated water molecules...

In summary, the mechanism for dissociation (at an on-top site) requires that the cluster has a low lying state with at least... 

Next we consider the methane molecule and its approach to the nickel surface. The main factor determining the mechanism for dissociation of CH is the difficulty of stretching the CH bond. The total energy of a gas phase CH molecule is calculated to increase by 0.8 eV on stretching one of the CH bonds from its equilibrium... 

A NEW MECHANISM FOR DISSOCIATIVE CHEMISORPTION COLLISION-INDUCED DISSOCIATION OF ADSORBED CH4 OR CHEMISTRY WITH A HAMMER... 

We have shown that this mechanism for dissociation does indeed occur by bombarding a monolayer of CH4, physisorbed on the Ni(111) surface at 46 K, with an Ar atom beam (ref. 22). The impact of the Ar atom deforms the molecularly adsorbed methane molecule into the configuration for the transition state that leads to dissociation. 

In a direct mechanism for dissociative chemisorption, Wp well plays a minor role since the important quantity is E. For an indirect mechanism, Wp plays two important roles (1) acceleration of the incoming molecule to... 

A general correlation [65] of sodium flame reaction rates with electron absorption coefficients has been made. This correlation covers a range of rate coefficients varying by a factor of three powers of ten. It is illustrative rather than conclusive and deserves further consideration. A formal similarity could also exist between the mechanisms for dissociative electron attachment of organic halides [66, 67] and the sodium flame reactions. These mechanisms can be either a direct dissociative mechanism... 

Up to now only the simple energy-transfer mechanism for dissociation or recombination has been considered. However, the mechanism may well be different. If radiationless transitions between different electronic states are involved, reaction may proceed in several stages with rate determining steps which are different under different conditions ... 

Molecular orbital studies on fluxional behavior of metal complexes containing monodentate diimines are of obvious relevance to one-end-off mechanisms for dissociation, as in the classic case of [Fe(bipy)3]. In this approach the behavior of the monodentate diimines is treated alongside that of such diaza-ligands as pyridazine (10), phthalazine (11), and naphthyridine (12)/ ... 

These data can be accommodated by a simultaneous addition of the hydride and proton to form an alcohol that is initially hydrogen bonded to the amine and forms the alkoxide without dissociation of free alcohol or by a transition state with a structure resembling those proposed for the direct, simultaneous transfer of the hydride and proton, but leading to the alkoxide complex directly. After this transfer of the hydride to form an alkoxide, a base-assisted elimination of the alkoxide to form the amide complex is proposed to occur. This step parallels the classic conjugate-base mechanism for dissociative Hgand substitution discussed in Chapter 5. 

Even more so than for the case of radiative association, very little experimental information exists for radiative attachment, and the reliability of the simple phase-space treatment has not been tested. One possible problem is that the formation of the negative ion complex may require so-called doorway states known as dipole-bound states, in a similar manner to the Rydberg mechanism for dissociative recombination of positive molecular ions. Such states have been seen in the spectra of negative ions, although a detailed calculation employing them for radiative attachment has not been attempted. 

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