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12. G. G. Dyadyusha, A. A. Ishchenko, N. A. Derevyanko, et al., "Investigation of the solvato- chromism of cyanine dues by the method of moments," Teor. Eksp. Khim., 15, No. 4, 412-416 (1979). 13. V. A. Mostovnikov, A. N. Rubinov, M. A. Al'perovich, et al., "Dependence of the lumin- escence and lasing properties of solutions of polymethine dyes on their structure," Zh. Prikl. Spektrosk., 20, No. i, 42-47 (1974). 14. A.I. Tolmachev, N. A. Derevyanko, and A. A. Ishchenko, "Pyrylocyanines. 16. Tetrapheny- lated 2- and 4-pyrylocyanines," Khim. Geterotsikl. Soedin., No. 9, 1178-1183 (1982). 15. K. B. Wiberg, "Application of the Pople--Santry--Segal CNDO method to the cyclopropylcar- binyl and cyclobutyl cation and to bicyclobutane," Tetrahedron, 24, No. 3, 1083-1096 (1968). QUANTL~-CHEMICAL ANALYSIS OF THE FEATURES OF THE ELECTRONIC STRUCTURE OF MODEL BIS(IMIDAZOLE)IRON PORPI~RINS AS COMPARED TO DATA FROM MOSSBAUER SPECTROSCOPY V. I. Khleskov, B. N. Burykin, and R. E. Garibov UDC 539.19 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)hemo- chrome [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 car- ried out by the extended HHckel 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 the 57Fe M~ssbauer spec- tra 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 cal- culated 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. Complexes of iron prophyrins with peptides in which the iron atom is axially coordinated to the nitrogen atoms of a histidine imidazole group serve as models of heme-protein systems and have attracted considerable interest on the part of investigators [1-3]. Some character- istics of the electronic structure of the iron atoms in h~stidine-containing hemin peptides, particularly the quadrupole splitting (AEQ) in the Fe 57 Mossbauer spectra, were calculated in the ligand-field-theory approximation in [4]. A more complete understanding of the electronic properties of complexes of iron prophyrins and an adequate interpretation of the data from M~ssbauer spectrscopy on their basis calls for more exact quantum-chemical methods of cal- culation than the ligand-field-theory approximation. In this communication we shall carry out an analysis of the electronic structure and calculate the parameters of the quadrupole splitting of the 57Fe MSssbauer spectra of two model low-spin complexes, viz., bis(imidazole)- hemichrome [Fe(III)P].2Im (S = 1/2) (complex A) and bis(imidazole)hemochrome [Fe(II)P]-2im (S = 0) (complex B), where P denotes porphyrin,Im denotes imidazole, and S is the spin of the iron ion, in the framework of the MO method. QUADRUPOLE SPLITTING The quadrupole splitting for the STFe nucleus with spin 1/2 is determined by the expres- sion [5-7] Institute of Biophysics, Ministry of Public Health, Moscow. Translated from Teoretiches- kaya i Eksperimental'naya Khimiya, Vol. 21, No. 2, pp. 146-153, March-April, 1985. Original article submitted June 8, ]984. 0040-5760/85/2102-0139509.50 O 1985 Plenum Publishing Corporation 139