Zusammenfassung
The structures and stabilities of ［Cu₂(μ-η2:η2- peroxo)］2+ (A) and ［Cu₂(μ-oxo)］2+ (B) complexes with three NH₃ ligands per copper are investigated using DFT and high-level ab initio methods. These are model systems for active centers in enzymes like hemocyanine and tyrosinase. Previous studies have shown that at the DFT/B3LYP level the peroxo form A is more stable than the μ-oxo form B, while the opposite was found using CASPT2 (Flock M, Pierloot K (1999) J Phys Chem 103:95). At the two computational levels, the energy difference of the isomers differed by more than 30 kcal/mol. In this work this problem is reinvestigated using a localized orbital description and multireference configuration interaction (MRCI) methods. It is found that CASPT2 strongly over-corrects the correlation effect and MRCI predicts structure A to be energetically lower than B, in qualitative agreement with B3LYP and experiment. However, B3LYP seems to stabilize the biradicalic structure A too much, and this effect depends approximately linearly on the amount of exact exchange in the B3LYP density functional. Reducing the amount of exact exchange to 10–15% yields good agreement between MRCI and B3LYP.
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