Metal-porphyrin orbital interactions in paramagnetic iron complexes having planar and deformed porphyrin ring

¹H and ¹³C NMR chemical shifts of iron porphyrin complexes are determined mainly by the spin densities at the peripheral carbon and nitrogen atoms caused by the interaction between paramagnetic iron 3d and porphyrin molecular orbitals. This review describes how the half-occupied iron 3d orbitals such as d_π(d_xz, d_yz), d _xy, d_z2, and d_x2-y2 interact with a specific porphyrin molecular orbital and affect the ¹H and ¹³C NMR chemical shifts in planar, ruffled, saddled, and domed complexes. Revealing the relationship between the orbital interactions and NMR chemical shifts is quite important to determine the fine electronic structures of synthetic iron porphyrin complexes as well as naturally occurring heme proteins.