Effect of nitrogen substitution in porphyrin ring on Mössbauer parameters of iron ions

Electron configuration changes of Fe(III) ions in porphyrin complexes with chloride axial ligands caused by the successive nitrogen substitution of CH methine bridges at meso positions of the porphyrin ring is discussed on the basis of Mossbauer spectroscopy results. It was shown that increase of a number of nitrogen atoms at the meso positions changes the character of quantum-mechanically mixed spin state of Fe(III) ions (S = 5/2 + 3/2) by the increase of the intermediate-spin (S = 3/2) contribution. This feature is reflected in Mossbauer spectra by an increase of quadrupole splitting values and the decrease of the asymmetry of quadrupole doublets, when the number of nitrogen atoms at the meso positions increases. Isomer shifts remain practically unchanged. These peculiarities are discussed in the light of spin relaxation mechanisms and the occupancy of d orbitals in Fe(III) ions coordinated to the porphyrin ring and chloride ligand. It has been noticed that Mossbauer parameters correlate qualitatively with EPR data.