DENSITY-FUNCTIONAL THEORETICAL-STUDY OF OXO(PORPHYRINATO)IRON(IV) COMPLEXES, MODELS OF PEROXIDASE COMPOUND-I AND COMPOUND-II

Local density functional (LDF) calculations, including geometry optimi zation, have been carried out on oxo(porphyrinato)iron(IV), PFeO, and the corresponding cation, [PFeO](+), which have been chosen as simple models of peroxidase compounds II and I, respectively. In the optimize d structure of PFeO, the Fe-O distance was 1.622 Angstrom and the iron atom was positioned 0.215 Angstrom above the plane of the four porphy rin nitrogens. The harmonic Fe-O stretching frequencies of PFeO and [P FeO](+) were 934 and 964 cm(-1), respectively. A three-body (P-Fe-O) v ibrational analysis revealed negligible coupling between the Fe-O stre tch and the displacement of the iron atom out of the porphyrin plane. In both PFeO and [PFeO](+), two unpaired spins, corresponding to a (pi ())(2) configuration, were cleanly localized on the ferryl moiety, be ing divided among Fe and O in the ratio 1.2:0.8. The third unpaired sp in of [PFeO](+) was distributed over the porphyrin ring as an ''A(2u)' '-type cation radical. Overall, these LDF results represent goad agree ment between first-principles theory and experiment. Spin-restricted H artree-Fock theory is known to provide a poor description of both the porphyrin ligand and the ferryl group. The CASSCF method provides a go od description of the ferryl group but is computationally unwieldy for large molecules such as hemes. Density functional theory appears to p rovide an expedient solution to the problem of several configurations with significant contributions to the wave functions of ferryl interme diates and is a practical theoretical tool for studying ferryl species .