A b i n i t i o MO studies of electronic states and Mössbauer spectra of high-, intermediate-, and low-spin Fe(II)-porphyrin complexes

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Abstract
Ab initio LCAO SCF MO calculations were carried out on Fe‐porphine, Fe‐porphine‐pyridine, and Fe‐porphine‐pyridine‐carbonmonoxide which are typical intermediate‐, high‐, and low‐spin complexes, respectively. Mössbauer spectra of ferrous porphyrins were analyzed theoretically by the evaluation of isomer shift (IS), quadrupole splitting (QS), sign and direction of the principal component of electric field gradient tensor, and asymmetry parameter for the complexes. The calculated values have a good agreement with the experimental ones for the low‐ and high‐spin complexes. The ground state of Fe‐porphine, which remains undetermined so far, is assigned as the 3Eg state which is described by mixing of the two configurations, (dxy)2 (dxz,dyz)3 (dz2)1 and (dxy)1 (dxz,dyz)3 (dz2)2. A new relation between Mössbauer spectra and electronic structure is found. The field gradient along the perpendicular axis to the porphyrin plane is determined by two factors, the d‐electron configuration and a constant term depending on axial ligands.

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