Morse Potential Energy Curves Antibonding

Figure 7.17 Current best Morse potential energy curves for benzene and its anions. There are two additional antibonding curves going to each dissociation limit that are not shown. The adiabatic electron affinity corresponds to the polarization curve.

Since the real energy of the system is T plus V, we must add the T and V curves for both the g and u states. Upon addition for the g state, you find there are certain intemuclear distances that the T term dominates, causing a bond to form. At shorter distances the V term dominates, giving the typical shape found for a Morse potential curve (see Sections 2.1.4 and 8.1.2). For all intemuclear distances, the T term dominates in the u state, and this state is always repulsive. This is the origin of the repulsion between atoms that arises when populating an antibonding orbital with electrons. 

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