Ab initio study of the O2 binding in dicopper complexes
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- Rode, M. & Werner, HJ. Theor Chem Acc (2005) 114: 309. doi:10.1007/s00214-005-0692-6
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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.