The electronic structure of ferrous heme with imidazole as the fifth ligand. II. Explanation of MCD spectra from the near IR region to the Soret region J. Chem. Phys. 72, 6059 (1980); 10.1063/1.439062
Nonempirical LCAO MO SCF and CI Studies of the Low Lying Electronic State of the HOO RadicalA semiempirica1 molecular orbital calculation is performed on the porphin-Fe-imidazole system. As basic atomic orbitals. lone pair a orbitals of the N atoms coordinating to Fe are employed in addition to 3d ortibals of Fe and 1/" orbitals of Nand C atoms. Molecular orbitals are obtained by the usual SCF procedure for the closed shell configuration. Then the calculation of the open shell states is carried out by a careful configuration interaction method. The ground state is identified with one component of sE.The energy level scheme suggested by magnetic measurements and by ligand field theory for low-lying quintet, singlet. and triplet states is well reproduced. Corresponding to a broad absorption band in the visible region and weak bands in the near JR. x -and y -polarized electronic transitions. two d-->
On the basis of the ASMO–CI calculation, the MCD associated with x- and y-polarized electronic transitions in the ferrous heme of the high-spin type is studied by computational and analytical methods. The ground state 5Eη is split into five substates by the mixing of the other low-lying quintets, 5Eξ and 5B2, through the spin–orbit interaction. The temperature dependent factor in ellipticity is proportional to the z component of orbital angular momentum averaged over the five substates. For excited states, 28 configurations of one-electron excitations from the three low-lying quintets are considered; 12 π→π (3a2u, 1a1u→4eg), 14 d→π (u,v→4eg; d̄π→2̄b̄1u, 3̄b̄2u), and 2 d→d (d̄π→ū,v̄) configurations. In the configuration interaction among them, the spin–orbit interactions of the 3d electrons are also taken into account. With the use of the energy eigenvalues and eigenfunctions, MCD curves are computed in the range of wavelength 400–1000 nm. Observed MCD spectra of deoxymyoglobin are well reproduced especially at low temperature. The theoretical consideration is also developed for the essential mechanism underlying the complex shapes of the MCD spectra with the use of the perturbation analysis and the group theory. Basically, the present theory is also applicable to ferric compounds of the low-spin type.
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