Chemical anisotropy

Layered sulfides predominate in Groups IV to VII except for MnS, which is isotropic. The main structural feature of this class is a strong chemical anisotropy that greatly affects their catalytic properties. In the molybdenite structure (MoS2), the metal is in trigonal prismatic coordination. Other layered sulfides are octahedrally coordinated or distorted octahedrally coordinated (ReS2) with the metal surrounded by six sulfur atoms. It should also be noted that some TMS sulfides have structures that fall in between isotropic and layered sulfides. Rh2S3 is an example and others are completely amorphous, such as IrSt and OsSr. 

First, the diffusional radical reactions in solutions whose rates are proportional to I) sometimes have rate constants that are much smaller than their contact estimate for the isotropic black sphere, ki> = 4naD. It was proved that this is the result of chemical anisotropy of the reactants. Partially averaged by translational and rotational diffusion of reactants, this anisotropy manifests itself via the encounter efficiency w < 1, which enters the rate constant kn = w4nak) [249]. Even the model of white spheres with black spots is more appropriate for such reactions than the conventional Smoluchowski model. 

Both chemical anisotropy and finite jump size are the important features of the pseudocontact reactions of electron and especially proton transfer. They should be incorporated in IET and MET in approximately the same way as in DET. 

The complexity of carbon materials together with the surface chemical anisotropy and the modifying effects of defects creates a highly complex situation for surface chemistry even when only one heteroelement is considered to be actively bonded. It is a characteristic of this branch of chemistry to describe surfaces in terms of distributions of properties rather than in sharp numbers. 

A new approach of the bimolecular reaction theories is presented, which is based on the averaging of chemical anisotropy by translational and rotational Brownian motion of the particles.The effective steric factor change in reactions with only one anisotropic reagent was found. It is shown, that it can fall down to the values experimentally observed, only if the hopping mechanism of molecules approach and reorientation is realized. But if the motion is diffusive, then both particles should be chemically anisotropic to explain the experiment. 

When m > /Ji, the contacts fail to be independant and such a simple interpretation loses its power together with the MMDCR. However the kinematic approximation is still true and using it the efficiency of the diffusion encounters with on anisotropic reagent has been calculated in /20,21/ for hopping rotation and in /20/ for the rational diffusion. As it is shown in the fig.6, rotational diffusion averages the chemical anisotropy faster and more efficiently then large angles reorientation. 

We first searched for a nitrate-aluminum bound acetyl nitrate structure as proposed by Prins and coworkers. Prins and coworkers have used chemical anisotropy broadening arguments in their N NMR results to claim that the NO2 moiety is bound to the aluminum site. " We, however, were unsuccessful in finding such a species with our All-03 acid site model. As mentioned previously, A1 sites 1 and 2 are not as flexible as A1 sites 3-9. Thus, stable nitrate coordination with All would not be expected. More of an interaction between the nitrate moiety of the acetyl nitrate and A1 sites 3-9 may be possible because these sites participate in octahedral coordination because of their flexibility, which we are currently pursuing." Prins and coworkers have noted that ZSM-5 also exhibits similar broadening, but octahedral aluminum coordination occurs negligibly in ZSM-5. Thus, there appears to be more than one way to explain the broadening they observed. We show that chemical... 

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