Electronic States of Dioxygen Heme Complex Revealed from ab Initio LCAO-SCF-MO Calculations

Ab initio LCAO-SCF-MO calculations of a model oxyheme were carried out on several singlet, triplet and quintet states arising from the three types of configurations, (dε)6(πga)2, (dε)5(dγ)1(πga)1(πgb)1, and (dε)5(πga)2(πgb)1. The lowest states in the SCF energy are of the configuration (dε)5(πga)2(πgb)1 whose electronic structures can be also expressed as Fe(III)–O2− from the results of Mulliken’s population analysis. Calculated Mössbauer parameters of those states are in good agreement with experiments for oxyhemoglobin, oxymyoglobin and model oxyheme, while those of the closed-shell singlet state (dε)6(πga)2 are inconsistent with experiments. The Fe–O1 overlap populations indicate a bonding nature for the former states and a non-bonding nature for the latter state. These results all predict singlet states of Fe(III)–O2− structure as the main configuration of the ground state in the model oxyheme.