Higher Occupied Molecular Orbital

For OH and SH, the NOF EAs are larger than the experimental values. This trend is due to the expected underestimation of the correlation energy for open-shell states with our approach. In fact, we fix the unpaired electron in the corresponding HF higher-occupied molecular orbital (HOMO) of the neutral molecule, and then this level does not participate in the correlation. Note that for these molecules the total spin of the neutral molecule is greater than the total spin of the anion (S > Sa)- The underestimation of the total energy is for neutral molecules larger than for anions and therefore the NOF vertical EAs are overestimated. 

Table 1. Higher occupied molecular orbital energy and 7i-electron localization energy in model compounds...

It is now possible to "see" the spatial nature of molecular orbitals (10). This information has always been available in the voluminous output from quantum mechanics programs, but it can be discerned much more rapidly when presented in visual form. Chemical reactivity is often governed by the nature of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Spectroscopic phenomena usually depend on the HOMO and higher energy unoccupied states, all of which can be displayed and examined in detail. 

By virtue of the definition of Eq. (16) it is recognized that this expression represents the sum of the self-energies of all occupied molecular orbitals. The higher the self-energy of a particular orbital, the smaller the space to which this orbital is confined. The localization sum of Eq. (28) represents, therefore, an average measure for the degree of concentration of all orbitals in the set. 

The features in Figure 15.2 are believed to be due to electron transitions from the highest occupied molecular orbital states to lowest unoccupied molecular orbital states (HOMO-LUMO). Specifically, for single molecules and dilute solutions, the absorption in the blue part of the spectrum is proposed to be caused by the aromatic structure of TNT [4], probably involving it to it transitions.1 In the solid state, where the molecules are stacked up on top of each other, interactions between the molecules occur causing the energy levels to split into higher... 

The discussion of the distortion of the water molecule from a linear to a bent shape allows a tentative general conclusion to be reached. This is that if a distortion of a molecule from a particular symmetry allows two MOs to mix, so that the lower occupied orbital is stabilized at the expense of the higher vacant orbital, such a distortion will occur and will confer stability on the distorted molecule. A gain of stability will only occur if the two orbitals concerned in the stabilization process are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).. If both orbitals are doubly occupied, interaction between them does not lead to any change in stability. The generality of this conclusion is explored further in the next sections of this chapter and in Chapter 6. 

Two main mechanisms were proposed for the supersensitization effect. One is the hole-trapping mechanism in which the electron from SS fills the hole in the highest occupied molecular orbital (HOMO) of the excited sensitizing dye, since the HUMO level of SS is chosen to be higher than that of the sensitizer (Fig. 5) [2,10,11]. The resultant ionic state gives up an electron to the conduction band of silver halide with much higher quantum yield. 

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