Xα multiple scattering calculations on iron(II) porphine

Spin-restricted and spin-unrestricted Xα multiple scattering calculations are reported for iron porphine with D4h symmetry in the absence of axial ligands. A 3A2g ground state is predicted when the radius of the porphyrin ‘‘hole’’ is less than 2.0 Å, whereas a quintet ground state is expected for larger Fe–N distances. A large asymmetry is found in the occupations of the three Fe 4p orbitals. This has important implications for the interpretation of Mössbauer quadrupole coupling constants. Magnetic susceptibility and NMR results can be understood in terms of a model that involves spin-orbit mixing of an excited 3Eg state with the ground 3A2g state; an essential element in explaining the NMR data is that the proton hyperfine constants are not the same in these two states. Tentative assignments are given for observed absorptions in the optical and near-infrared regions. It is shown that spin-unrestricted wave functions are particularly suited for the qualitative interpretation of electronic structure of metallophorphyrins.