Ab initio study of the antiferromagnetic coupling in the wheel-shaped [Cu20Cl(OH)(24)(H2O)(12)(P8W48O184)](25-) anion

The most stable spin configuration in the Cu-20 unit embedded in the P8W48O184 POM framework has been analyzed with ab initio computational methods. Because of the large size of the Cu20 unit, a fragment approach has been adopted to compute the four possible magnetic interactions. Difference Dedicated Configuration Interaction results reveal that the magnetic coupling between unpaired copper electrons ranges from ferromagnetic (J = 14.8 cm(-1)) to antiferromagnetic (J = -72.6 cm(-1)). With the four calculated J-values we have performed a simulation of the magnetic susceptibility vs. temperature by using the Ising model and a Monte Carlo algorithm to sample the spin configurations. The most stable spin alignment, governed by the most intense antiferromagnetic interaction, features 12 up and 8 down spins. There is no full cancellation of spins that can comply with the magnetic interactions found for this molecule.