What is the difference between the manganese porphyrin and corrole analogues of cytochrome P450's compound I?

Density functional calculations on oxo-manganese complexes of corrole (1) and porphyrin (2 and 3) show a fundamental difference. The ground state of 1 is the singlet manganese(v) state, ¹A(Mn^V), in which corrole is a closed shell. In contrast, 2 and 3 have high-spin manganese(IV) states, ³A_1u and ³A^2u, respectively. This difference and the state ordering for each system are rationalized based on the competition between the intrinsic tendency of manganese to prefer high-spin electronic configurations, vis-à-vis the general tendency to prefer double occupancy in the low-lying orbitals. The outcome of this competition is determined primarily by the identity of the macrocycle, corrole versus porphyrin. Corrole with a small cavity holds the MnO moiety with a high off-plane displacement, and thereby prefers the low-spin state. On the other hand, porphyrin with the wider cavity holds the MnO moiety closer to the plane, and thereby prefers high-spin states.