The diagram below is a Frost-Hückel
circle mnemonic. A polygon representing the ring system, in
this case a regular hexagon for benzene, is inscribed in a
circle with a vertex at the bottom. The energy levels of the
six-π-MO's are placed at each vertex. The energy of an
isolated p-orbital is taken as passing through the center of
the circle. From the geometry of a regular hexagon, there
should be equal separation of the three energy levels.
Compare with the energies on the right. There are three
bonding MO's and three anti-bonding MO's. MO's
Ψ2
and Ψ3 are degenerate (same energy) bonding orbitals while Ψ4* and Ψ5*
are anti-bonding degenerate orbitals. Bonding MO's are green/blue; anti-bonding MO's are red/blue.
