Steric Repulsion and Orbital Interaction

The two components into which we have decomposed the interaction energy act almost always simultaneously and will influence each other. We end this section by giving an account of this mutual influence and of the distinction between donor—acceptor and electron pair bond formation energies. These issues will reappear in the discussions of specific cases in the sections that follow. 

Interactions may occur not only between fully occupied orbitals on one side and empty orbitals on the other side, but also between singly occupied orbitals that form an electron pair bond. It is possible in this case to simply stick to the symmetry decomposition of the orbital interactions, noting only that the irreducible representation r where the electron pair bond is formed will have a large AEr for that reason. It is also possible, at least when the electron pair bond is between two identical fragments, to carry the analysis somewhat further and 

As an example, we take the electron pair bonds of NC—CN and CN— NC in ct symmetry between the 5a singly occupied highest occupied orbitals (SOMOs) of the two CN radicals, two systems treated in the next section. Below the 5ct orbitals, there are fully occupied orbitals, the most important one being the 4a N lone pair orbital (the aHOMo orbital, see next section). The wavefunction VF° is written in this case as follows  

In the sections that follow, we discuss several examples of the energy decomposition analyses we have presented thus far. 

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