Some of the 4-arylmethyleneamino-TEMPO radicals (TEMPO = 2,2,6,6-tetramethyl-1-piperidyloxyl), such as 4-(4-chlorobenzylideneamino)TEMPO (1) and 4- (benzylideneamino)TEMPO (2), showed intermolecular ferromagnetic interactions at an extremely low temperature. The experimental electron-density distributions of these compounds were investigated by the X-ray diffraction method. Intensity data were measured at 100 K up to 2 theta(max)(Mo K alpha) = 120 degrees and 95 degrees for 1 and 2, respectively. Deformation electron-density distributions were calculated from the structure factors after the refinements by the multipole expansion method. These electron-density distributions suggested that the unpaired electron of the O radical atom was not located on the O atom, but was rather located in the pi*-orbital of the N-O bond. Lone-pair densities of the radical O atom are located in the >N-O plane showing the sp(2) hybridization. From the crystal structures, it has been proposed that the ferromagnetic interactions are transferred through CH ... O interactions between radical O atoms and beta-H atoms of the neighboring TEMPO rings. The beta-H atoms of the neighboring TEMPO rings contact with the O atom in the direction toward the pi*-orbital of the N-O bond, not in the direction of the lone-pair densities of the O atom. The present studies support the mechanism of the intermolecular ferromagnetic interactions through the beta-H atoms. Studies of electron densities from the view point of the intermolecular interactions are important.
