EXPERIMENTAL SPIN-DENSITY IN THE 1ST PURELY ORGANIC FERROMAGNET - THE BETA-PARA-NITROPHENYL NITRONYL NITROXIDE

A purely organic ferromagnet, the beta form of the para-nitrophenyl nitronyl nitroxide (p-NPNN), was investigated by conventional and polarized neutron diffraction. Several approaches were used to reconstruct the spin density distribution in the radical. Quantitative results in the form of atomic spin populations were obtained, and are discussed in comparison with those previously obtained experimentally for a related phenyl-substituted nitronyl nitroxide. The spin delocalization and/or spin polarization effects are responsible for the transfer of the spin density to the nitrophenyl group. In the framework of the McConnell mechanism these effects play a key role in the intermolecular ferromagnetic interaction, which passes through both the nitro and the phenyl fragments. Hartree-Fock-based ab initio theoretical spin densities found in the literature are inconsistent with the experimental results.