Magnetic properties of nitroxide multiradicals

Abstract: After a discussion of previous results on nitroxide mono- and multiradicals, magnetic properties of new bi- and tetraradicals are presented. Magnetic properties of a molecule may refer to the isolated species or to its condensed phase (1). Typical individual properties of monoradicals are g values and hyperfine coupling constants. In multiradicals (reserving polyradicals for polymers obtainedfrom stable radicals as monomer), dipolar B dnd exchange J interactions must be added. Collective properties may be obtained from magnetic susceptibilities (x) measurements as a function of temperature T. Deviation from Curie-Law (x.T = Cte) indicate exchange interactions. At high temperature (T>>I 8 1) ferro and antiferromagnetic substances follow Curie-Weiss law[x(T-9) = C C i, their Weiss constant 9 being respectively positive and negative (2)(3). However, a whole taxonomy of magnetism exists (3)(4). Let us mention metamagnets (3)(5), substances undergoing transi-tion from an antiferromagnetic state to a ferromagnetic state when the applied magnetic field becomes larger than a critichb value. Organic stable free-radicals are paramagnetic at high temperature and follow Curie-Weis