Highest occupied molecular orbital stability

The thermal stability can be correlated with the energy of the highest occupied molecular orbital of the molecule (HMO approximation) (300). 

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

The above stability order is supported by the results of ab initio calculations, where a correlation has been established between the oxidation limits of these anions and the energy level of their highest occupied molecular orbital (HOMO). However, the gapping discrepancies that exist between intrinsic and solution stability limits remain to be explained. 

Figure 22.9. Disrotatory ring opening of 1,1-difluorocyclopropane (9) to 2,2-difluorocyclo-propane-l,3-diyl (10). The in-phase comhination of 2p-n AOs in the highest occupied molecular orbital (HOMO) is stabilized by a bonding interaction with the 2p AO at C2 in the n combination of low-lying, C—F, antibonding orbitals.

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. 

However, thermodynamic and quantum mechanical calculations indicate that the stability of nitrosyldioxyl radical will be greatly increased by hydrogen bonding with water (Beckman and Koppenol, 1992). TTie reason for this increased stability can be readily visualized in Fig. 7. Nitrosyldioxyl radical is most stable when bent into the cis (or C-shaped) conformation (Boehm and Lohr, 1989). In this conformation, the lobes of the highest occupied molecular orbital from... 

Highest Occupied Molecular Orbital (Frontier MO) Internal (Stability/Instability)... 

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