Structure and spin-state energetics of an Iron Porphyrin Model: an assessment of theoretical methods

D. A. SCHERLIS, D. A. ESTRIN

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY

Año: 2002 vol. 87 p. 158 - 166

0020-7608

The ability of unrestricted Hartree–Fock (UHF), Møller–Plesset (MP2),density functional theory (DFT), and hybrid density functional/Hartree–Fockmethodologies to describe the structure and spin-state energetics of iron porphyrins wasassessed. In the first place, these techniques have been applied to Fe, Fe+, Fe2+, and Fe3+for which HF calculations overestimate energy gaps, favoring stabilization of highermultiplicity states. DFT shows the opposite trend at the GGA level, with someimprovement using the hybrid schemes B3LYP and half-and-half. We use the hybridfunctionals to explore the dependence of the spin state with the iron displacement out ofthe porphyrin plane in the five-coordinate system, for which a high-spin ground state hasbeen experimentally determined. The possibility of spin crossover, proposed in previousstudies, is examined. Finally, the hybrid methodologies are applied to the computation ofthe oxyhemoglobin model. The B3LYP description of the electronic structure of both pentaand hexa coordinate model systems is consistent with previous theoretical calculationsand with experimental information of deoxy and oxy hemoglobin.