Recent Applications of Concepts

At low temperature and voltage, the current is directly proportional to the local density of states at each point on the surface. But this DOS is just equal 

There is a theory explaining the stronger adsorption of H2 on the early transition metals of each series, and the non-adsorption on Au, Ag and Cu. As the H2 molecule approaches the metal surface, the electron in the s orbital of the metal is pushed into an empty d orbital to avoid repulsion. This detailed mechanism has been confirmed by DFT calculations. The local DOS as well as orbital symmetry are the determinants. These also determine the surface sites where a hydrogen atom will be bound. 

A rather remarkable paper has appeared on catalytic reactions in zeolites. It combines ab-initio calculations on zeolites with the results of the following reaction, catalyzed by the same zeolites 

Both ortho- and / 3ra-xylenes are formed as products. In toluene, the ortho position is more negative than the para, but the local softness is greater as the para position. If coulombic effects are dominant, we expect a low pjo ratio. If electron transfer, or orbital control, is dominant, we should have a high pjo ratio. 

Both ab-initio and semi-empirical MO calculations were made for model clusters of zeolites, in which Al/Si and Al/Ga/B ratios were changed. The HOMO-LUMO gap was found in each case. The experimental pjo ratio, indeed, was found to vary inversely with the size of the gap. Softer zeolites favored orbital control, leading to more para-xylene, as would be predicted. 

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