MAGNETIC-PROPERTIES AND CRYSTAL-STRUCTURE OF THE P-FLUOROPHENYL NITRONYL NITROXIDE RADICAL CRYSTAL - FERROMAGNETIC INTERMOLECULAR INTERACTIONS LEADING TO A 3-DIMENSIONAL NETWORK OF GROUND TRIPLET DIMERIC MOLECULES

The crystal of p-fluorophenyl nitronyl nitroxide [2-(4-fluorophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, abbreviated as p-FPNN] radical has been found to possess a dimeric structure. Each dimer is related to adjacent ones by four-fold screw symmetry to form a three-dimensional network. The temperature dependence of the paramegnetic susceptibility is explained by the formation of a triplet state within the dimer and additional interdimer ferromagnetic interactions. The intra- and inter-dimer exchange interactions are estimated to be J/k(B) = 5.0 K and J'/k(B) = 0.02 K, respectively. The molecular arrangement in the dimer suggests that the intermolecular interactions between the NO group and the phenyl ring are favourable to ferromagnetic coupling. Slight structural changes below ca. 100 K have been studied by use of electron paramagnetic resonance and powder X-ray diffraction measurements.