Surface Frontier Molecular Orbitals

As mentioned in Chapter 3, the octahedral models used to describe the active sites on metal surfaces are not compatible with the presence of three different types of saturation sites on a catalyst surface so another model must be developed. On consideration of the fee crystal structure, which is that of most catalytically active metals, it can be seen that the bulk atoms in these metals are bound to twelve nearest neighbor atoms using the lobes of the t2g d orbitals. The octahedraily oriented eg orbitals are directed toward but not bonded to the next nearest neighbors in the crystal lattice as shown in Fig. 4.1.1 This atomic orientation precludes the presence of any octahedral arrangement involving M-M bonds. 

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The most important molecular- orbitals are the so-called frontier molecular- orbitals. These are the highest (energy) occupied molecular- orbital (HOMO), and lowest (energy) unoccupied molecular- orbital (LUMO). The following picture shows the LUMO surface for the hydrogen molecule, H2. The LUMO consists of two separate surfaces, a red... 

A theoretical study based on PM3 frontier molecular orbital (FMO) and potential energy surface (PES) analysis at the RHF/6-31-I-G level was performed to examine the reaction of l-amino-2-ethoxycarbonylpyridinium mesitylenesulfonate and acrylonitrile in the presence of Hiinig s base leading to the formation of 3-cyano-4-hydroxy-l,2-dihydropyrido[l,2-ft]pyridazi-nium inner salt (54) (99JOC9001). The calculations indicated that both the... 

Initial work indicates that dispersed metals may be used to promote a variety of organometallic reactions. The Heck Arylation proceeds smoothly over supported Pd catalysts while diene cyclizations can be catalyzed by dispersed Rh metal. The use of these heterogeneous species facilitates product isolation and permits the application of flow systems rather than batch reactors for these reactions. Frontier Molecular Orbital and mechanistic considerations indicate that these reactions take place on the coordinately unsaturated comer atoms on the metal surface. 

We describe here some of our initial efforts concerned with the use of dispersed metals as catalysts for organometallic reactions and the development of a Frontier Molecular Orbital description of the reactions taking place on the metal surface. 

Figure 7.12 Energy diagrams for surface-molecule interactions in the case of (a) strong electronic coupling, (b) weak electronic coupling, and (c) no electronic coupling. ek stands for an electronic continuum of the metal or semiconductor band, whereas , represents the energy of frontier molecular orbital involved in this interaction...

Figure 6-11. (top) Frontier molecular orbitals, (bottom) potential energy surface for axial and equator-ial attack at 3-substituted cyclohexanones. 

This chapter considers the oxidation of iodide in seawater by natural oxidants (02, H202, and 03). The oxidation of iodide to iodate is considered slow, yet the six-electron T-IOj redox couple normally used to represent the process (or predict stability) is thermodynamically favorable (2). We will discuss both one- and two-electron-transfer processes with these oxidants, focusing on the first step of electron transfer and using the frontier molecular orbital theory approach in conjunction with available thermodynamic and kinetic data. The analysis shows that the chemical oxidation of I to I03 is not a very important process in seawater, except perhaps at the surface microlayer. 

If the two systems A and B approach each other in such a way that 5 = 0, then the PauU deformation is zero. 5 = 0 might occur e.g., if the two molecules would approach along the nodal surfaces of the frontier molecular orbitals. 

This chapter has only scratched the surface of the opportunities and possibilities to demonstrate the natural relationship between chemistry and toxicology. Some of the topics that were not discussed include oxidation and oxidative stress, the influence of electrolytes on calcium homeostasis, immunotoxicology, focusing on biochemistry and applying frontier molecular orbital theory to predicting adverse outcomes. A discussion of catalysis may include the role of enzymes in toxication and detoxication, for example the mechanisms of action of organophosphate pesticides on the function of acetylcholinesterase. 

The adsorption of water on most metal surfaces is typically rather weak and controlled by a balance between the strength of the metal-water bond and the water waterl interactions. Molecular water adsorbs on metal and metal oxide substrates through the donation and back-donation of electrons between the frontier molecular orbitals of water and the states of the metal near the Fermi level. 

In many instances, the relaxation terms, <]>, can be ignored, and the frontier molecular orbital theory as first put forth by Fukui - is recovered. For particular molecules, the identification reactive sites using f(t) has been successful.A convenient way to visualize the reactive sites in a molecule using the above reactivity index is to first display an isosurface of the electron density that just encloses the van der JKials volumes of the individual atoms in the molecule. Typically, the value of this isosurface is between 0.002 and 0.005. Next, the values of the reactivity index are mapped upon this surface and color coded from blue (zero) to ted (most positive). 

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