ABSTRACT:Oxo-or hydroxo-bridged diiron centers are ubiquitous in metalloenzymes such as hemerythrin (Hr), ribonucleotide reductase, methane monooxygenase, and rubrerythrin. In each enzyme the diiron core plays a central role in the highly specific reaction. To elucidate mechanisms of these reactions, many experimental studies have been carried out, and bioinorganic model compounds have also been synthesized for the purpose. In this study electronic structures of diiron centers for Hr model compounds are investigated from the viewpoint of magnetic interactions. To this end, the Hubbard model for the three-center four-electron bond is analytically solved to elucidate an important role of electron correlation and the resulting superexchange interaction between localized spins. The hybrid density functional theory (DFT) calculations also are performed for Hr model compounds to provide the natural orbitals and their occupation numbers, which are crucial for computations of several chemical indices, such as effective bond order, information entropy, and unpaired electron density. These indices are useful for characterization and understanding of chemical bonds in FeOFe cores. The calculated effective exchange integrals (J ab ) are wholly consistent with the available experiments. The orbital interactions in the FeOFe cores are reconsidered in relation to recent work by other groups. It is found that magnetic interactions are sensitive to the hydrogen bonds in the systems and are related to effective regulation of the activity. Implications of the Correspondence to: M. Shoji;
ABSTRACT:Hybrid density functional theory (HDFT) and post Hartree-Fock CCSD(T) methods are applied to elucidate the binding energies and the optimized MOO distances of transition metal oxides: MO (M ϭ Cr, Mn, Fe, Co, Ni, Cu). The HDFT method can reproduce the CCSD(T) results, in agreement with the experimental ones. The nature of the manganese-oxygen bonds in the Mn(VI)OO, Mn(IV)OO porphyrin (PP), and Mn(V)OO PP systems are examined in relation to possible mechanisms of oxygen evolution from H 2 O 2 and H 2 O in native and non-native manganese complexes. It is found that the radical character of the high-valent (PP)Mn(V)OO bond is remarkable, showing the strong potential to generate molecular oxygen because of its high reactivity. The electronic structure and magnetic property of -oxo-bridged manganese porphyrin dimer (PPMn(III)OMn(III)PP) are investigated for further discussion of structure and reactivity of PPMn(X)O (X ϭ II-IV). The potential curve for release of molecular oxygen from PPMn(II)O 2 is also examined to show weak affinity of O 2 in the Mn complex where the oxidation number (X) of Mn is low. Implications of the computational results are also discussed in relation to oxygen evolution reactions.
New schemes for effective exchange integrals ( J ab ) and approximate spin-projected energies for low spin states (E APLS ) were examined on simple two-site radical models. These schemes are based on Hubbard model and include ionic term in wavefunction. By estimation of expectation values of spin correlation function, those schemes could directly calculate J ab and E APLS values without the approximation that on-site spins for high-spin (HS) state was equivalent to one of low-spin (LS) state. Calculated J ab values by new scheme were larger than conventional ones in short spinspin distance, suggesting an importance of the on-site spins and ionic term. On the other hand, new E APLS values were similar to conventional ones. Estimation methods for new J ab and E APLS also were derived by using HS state energy, LS state energy, and ͗Ŝ 2 ͘ LS analytically. To correct dynamical correlation, magnetic effective density functional (MEDF) method was successfully applied to calculate new J ab and E APLS .
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