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) a nd porphyrin (2 and 3) show a fundamental difference. The ground state of 1 is the singlet manganese(v) state, (1)A(Mn-v). in which corrole is a close d shell. In contrast, 2 and 3 have high-spin manganese(iv) states, (3)A(1u) and (3)A(2u), respectively. This difference and the state ordering for eac h system are rationalized based on the competition between the intrinsic te ndency of manganese to prefer high-spin electronic configurations. vis-A-vi s the general tendency to prefer double occupancy in the low-lying orbitals . The outcome of this competition is determined primarily by the identity o f the macrocycle, corrole versus porphyrin. Corrole with a small cavity hol ds the MnO moiety with a high off-plane displacement, and thereby prefers t he 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.