The Molecular Orbitals and transitions of Zn porphine

The transitions of metalloporphyrins have been described by Gouterman's four orbital model. Gouterman noted that the LUMO of porphyrins was double degenerate (e_g p* in D_4h symmetry). This model is shown below.

Using a semi-empirical calculation we obtain the following images of MOs for the relevant states.

e_g doubly degenerate orbitals with 5 nodes.

a_1u and a_2u orbitals form a nearly degenerate pair with 4 nodes each.