| Abstract
| - The ground-state electronic structure and UV−visible spectra of models for oxyhemocyanin areexamined using the intermediate neglect of differential overlap model and multireference configurationinteraction. The experimental features are interpreted as excitations involving the d orbitals of the Cu atomsand the valence orbitals of the peroxide bridge through which they are antiferromagnetically (AF) coupled.Our model, which replaces the histidine residues with imidazoles, reproduces correctly the higher stability ofthe AF singlet state and the major spectroscopic features. We examine in detail the geometry of the centralpart of the molecule, responsible for the activity, using the experimental electronic spectrum as a guide. Wefurther examine the effect that deprotonation of the chelating imidazoles has on the predicted spectroscopy.The central structure that we assumed to best reproduce the spectroscopy is very similar to that obtained fromBLYP density functional calculations. For comparison with other results, we also examine a model compoundin which NH3 replaces histidine. For both the imidazole- and the NH3-based models, the [Cu2((μ-η2:η2-O2)]2+and [Cu2(η-O)2]2+ isomers have been considered and the results compared with available data. We concludewith the observation that the electronic structure of these compounds does depend on the redox properties ofthe chelating nitrogen ligands.
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