Promotion energy gap

Reaction series where reactivity obeys the promotion energy gap abound [7,11], and a few more examples are discussed below. 

Figure 2 Avoided crossing situations in the SCD. The adiabatic states are in bold lines, (a) A large promotion energy gap leads to a TS on the lower surface, (b) A very small promotion energy gap leads to a stable intermediate...

Figure 3 The effects of the curvature in (a) and the reaction driving force in (b) on the height of the crossing point for a given promotion energy gap. Compare crossing points 1 and 2...

Promotion energy gap patterns were observed also for allowed cycloadditions and their bond exchange analogs. An interesting example is the trimerization of acetylene to benzene, equation (13), which possesses a very large barrier (rj62 kcal mol at the MP3/6-31G //HF/6-31G level ), despite the fact that the reaction is symmetry allowed, and can potentially proceed with an enormous thermodynamic advantage of A7f Rs —140 kcal mol . 

The energy of the frontier orbitals determines the reactivity. The small energy gap between the HOMO of electron donors and the LUMO of electron acceptors promotes the interaction and stabilizes the transition states. Electron donors react fast as the HOMO energy is high. Electron acceptors reacts fast as the LUMO energy is low. 

There are another type of photocells known as barrier layer photocells which work on an entirely different principle. They are semiconductor devices in which impinging photons promote the electrons from the valence band to the conduction band across the energy gap. A photovoltagc is generated which can be measured by a voltmeter. Such photovoltaic devices can have a large surface area and are easy to operate. They are commonly used in many simple colorimeters and fluorimeters and as light Otters for cameras. 

The promotion gap is determined by two basic excitations. For -electron pairs, the promotion energy is k-fold singlet—triplet excitation of the A—A dimer, while for mixed-valence cases where the number of electrons exceeds the number of centers (e.g., H3 ) and vice versa, the promotion energy is a charge-transfer excitation, (e.g., from Hr to H2 in H i ). Let us now use the promotion gap quantity to pattern the data concerning delocalized species. 

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