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 i n the framework of the broken symmetry approach. From a quantitative point of view, only hybrid methods containing some exact exchange provide good ag reement with experimental data, which is further improved taking into accou nt solvent effects by a refined continuum model. The magnetic coupling is e ssentially determined by through-bond effects ruled by the cyclohexane spac er, whereas direct through-space interactions between the NO moieties are n egligible. An effective procedure is also introduced and validated, consist ing of single-point quantum mechanical computations at geometries optimized by an extension to nitroxides of standard molecular mechanics approaches. (C) 1999 Elsevier Science B.V. All rights reserved.