Structure and properties of a model of deoxyheme,an ab initio SCF calculation

Ab initio SCF calculations have been used to study the structure and the electronic properties of four- and five-coordinate Fe(II) porphyrins. The following systems have been considered: FeP (P = porphine dianion) (S = 1 and S = 2) and Fe(NH₂)₄ (S = 1) as four-coordinate systems. FePNH₃, Fe(NH₂)₄NH₃ and Fe(NH₂)₄1m as models of the deoxyheme (S = 2). Fe(NH₂)₄SH (S = 2) as a model of the reduced cytochrome P450. The basis sets used are either of the split-valence type or of double-zeta quality. The ground-state electronic configurations have been assigned. The potential energy curve of FePNH₃, as a function of the out-of-plane displacement of the iron atom, has a minimum for a displacement of 0.32 Å. a value significantly smaller than the accepted value of 0.6 Å in human deoxyhemoglobin. The out-of-plane displacement is larger, by ≈ 0.2 Å, when a mercaptide ligand replaces the ammonia or imidazole ligand. The electric field gradient tensor, the quadrupole splitting and the asymmetry parameter have been calculated and compared with the experimental values derived from the ⁵⁷Fe Mössbauer spectra.