Through-bond and through-space effects in the magnetic properties of nitroxide biradicals by an integrated QM/MM approach including solvent effects

The structure and magnetic properties of a typical nitroxide biradical have been studied by different models rooted in the density functional theory in the framework of the broken symmetry approach. From a quantitative point of view, only hybrid methods containing some exact exchange provide good agreement with experimental data, which is further improved taking into account solvent effects by a refined continuum model. The magnetic coupling is essentially determined by through-bond effects ruled by the cyclohexane spacer, whereas direct through-space interactions between the NO moieties are negligible. An effective procedure is also introduced and validated, consisting of single-point quantum mechanical computations at geometries optimized by an extension to nitroxides of standard molecular mechanics approaches.