Quantum-chemical analysis of the features of the electronic structure of model bis(imidazole)iron porphyrins as compared to data from Mössbauer spectroscopy

A quantum-chemical analysis of the electronic structure of two low-spin complexes, viz., bis(imidazole)hemichrome [Fe(III)P]2Im (S = 1.2) and bis(imidazole)hemochrome [Fe(II)P]·2Im (S = 0), which simulate hemi- and hemoproteins (P denotes porphyrin, Im denotes imidazole, and S is the spin of the iron ion), has been carried out by the extended Hückel method with SCF convergence with respect to the charges of the atoms. The data on the electronic structure of the complexes have been used for calculations of the quadrupole splitting in the⁵⁷Fe Mössbauer spectra of these compounds. An analysis of the contributions of the valence electrons of the Fe atom and the ligands to the quadrupole splitting as a function of the structure of the iron ion, its electronic environment, and the parameters of the first coordination sphere has been carried out. Good agreement between the calculated and experimental values of the quadrupole splitting has been obtained. An analysis of the orbital interactions has been carried out, and data on the classification and composition of the MO's have been presented.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 146–153, March–April, 1985.