The electronic structure of the conjugated π systems in porphyrins and corroles are frequently discussed in terms of the so-called Gouterman orbitals (Figure ); 101 a four-orbital model that, in the porphyrin, includes occupied π a 1u and π a 2u orbitals and a degenerate set of empty π eg orbitals. The corrole has analogous orbitals, albeit with lower C 2V symmetry, and can be described with the electron configuration