Ab initio SCF + CI direct calculation of the exchange interaction in aromatic N-oxide bridged copper(II) dimers. Comparison of the structural parameters' effects on modelled and real geometries

Abstract

The singlet–triplet (S–T) splitting energy of aromatic N-oxide bridged copper(II) dimers has been calculated using an ab initio SCF + CI treatment. Modelled molecules have been used to point out the chemical and geometrical factors which are suspected to influence the metal–metal interaction in this series of compounds. The direct substituent effects are very weak, but they govern the geometry of the complexes owing to the intramolecular interactions. The bridging angle and copper–copper distance have an important, but not decisive influence on the exchange coupling. The variation of the copper–oxygen bond length has a drastic effect on the 2J constant. Its shortening in the models leads to very strong antiferromagnetic couplings comparable to the experimental ones. The deviation of the halogen atoms out of the Cu₂O₂ plane leads to a very strong decrease of the antiferromagnetic interaction. Both effects reproduce a shift of 2J comparable to the experimental range (|2000 cm^–1|). Using real geometries the calculated S–T energy gaps are in agreement with the experimental ones and with the calculations on modelled molecules.