The electronic structure of ferrous heme with imidazole as the fifth ligand. I. SCF–LCAO–ASMO and CI calculation on the low-lying multiplets and optically allowed excited states

Abstract

A semiempirical molecular orbital calculation is performed on the porphin–Fe–imidazole system. As basic atomic orbitals, lone pair σ orbitals of the N atoms coordinating to Fe are employed in addition to 3d ortibals of Fe and π orbitals of N and 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 5E. 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 IR, x- and y-polarized electronic transitions, two d→d transitions and six d→π (porphin) transitions as well as π→π transitions, are calculated to be in a range of excitation energy 9–20 kK. Two z-polarized transitions, d→4dgx (porphin) and d→π′ (imidazole), also become fairly low in excitation energy, 2 and 11 kK, respectively.