Lowest unoccupied molecular orbital separation

Frontier Orbital theory supplies an additional assumption to this calculation. It considers only the interactions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These orbitals have the smallest energy separation, leading to a small denominator in the Klopman-Salem equation. The Frontier orbitals are generally diffuse, so the numerator in the equation has large terms. 

In the simplest frozen orbital approach, both IE and EA values can be approximated as the negative of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies, respectively, following the Koopmans theorem. A better way is to calculate the energies of the system and its cationic and anionic counterparts separately and then estimate fx and 17 from Equations 12.4 and 12.5, respectively. 

Figure 3. Orbital energies of the four highest occupied and four lowest unoccupied molecular orbitals in isolated monomers (MgP, PO) and in face-to-face MgP-P and Cl"-MgP-P dimers with iterring separations of...

The mechanism of the reaction has generally been discussed in terms of a thermally allowed concerted 1,3-dipoIar cycloaddition process, in which control is realized by interaction between the highest occupied molecular orbital (HOMO) of the dipole (diazoalkane) and the lowest unoccupied molecular orbital (LUMO) of the dipolarophile (alkyne).76 In some cases unequal bond formation has been indicated in the transition state, giving a degree of charge separation. Compelling evidence has also been presented for a two-step diradical mechanism for the cycloaddition77 but this issue has yet to be resolved. 

Figure 1.6 Exciton formation and charge separation in a bulk heterojunction, for the case that the donor phase absorbs the incident light. The energies of the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of the donor (D) and acceptor (A) phases are shown. The exciton binding energy Af b is (AEg + AEn).

Some dispute persists as to the size of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap in the closed-shell 24-electron species. This aspect of the molecular orbital diagram is of particular interest for its implications on the photophysical properties of the clusters. Electrochemical measurements are consistent with a large, 2- to 3-eV, separation however, the exact nature of the excited state remains uncertain. 

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