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(I1) 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 Cu,Oz plane leads to a very strong decrease of the antiferromagnetic interaction. Both effects reproduce a shift of 2J comparable to the experimental range (12000 cm-'l). Using real geometries the calculated S-T energy gaps are in agreement with the experimental ones and with the calculations on modelled molecules.

Toward the Prediction of Magnetic Coupling in Molecular Systems: Hydroxo- and Alkoxo-Bridged Cu(II) Binuclear Complexes

“…The first work in this field was reported by Daudey et al, who used ab initio CI calculations with a perturbational scheme to calculate the magnetic coupling constant for Cu(II) binuclear systems with acetato, hydroxo, alkoxo, and oxalato bridges as well as for a Cu(II)−V(IV) heterodinuclear system . The approach proposed by Daudey was subsequently employed by Haase et al to study N -oxide, hydroxo, alkoxo/acetato and terephthalato-bridged Cu(II) dimers, and hemocyanin models to calculate small energy differences called difference-dedicated configuration interaction (DDCI), employing it to estimate coupling constants for dihalide-bridged Cu(II) dimers, , as well as for dichloride and azido-bridged Ni(II) binuclear complexes .…”

Highly efficient perturbative + variational strategy based on orthogonal valence bond theory for the evaluation of magnetic coupling constants. Application to the trinuclear Cu(ii) site of multicopper oxidases

“…a¼1 o a;i;j ln o a;i;j (11) This value quantifies the entanglement between the pair of orbitals i and j, and the other orbitals. If i and j are not entangled with each other, one has the equality s(1) i + s(1) j = s(2) i,j .…”

“…They have proposed an expression of J based on second-order perturbation theory (PT), which only takes into account the differential effects playing a role on the energy difference between the states involved in the coupling. The procedure has been useful for rationalizing the magnetostructural behaviour of several Cu(II) binuclear compounds, 6,11,12 and it has been quickly surpassed by a variational version, the difference dedicated configuration interaction (DDCI) approach by Malrieu and coworkers, 13,14 which ensures the introduction of higher-order effects and avoids the intrinsic convergence problems of the perturbation expansion. The first DDCI calculation was carried out by Broer and Maaskant 15 with the aim of analyzing magnetostructural correlations in the [Cu 2 Cl 6 ] À2 complex.…”