The experimental deformation electron-density distributions of 4-arylmethyleneamino-2,2,6,-tetramethyl-1-piperidyloxyl (TEMPO) radicals: Relations between electron densities and intermolecular interactions

Some of the 4-arylmethyleneamino-TEMPO radicals (TEMPO = 2,2,6,6-tetramethy l-1-piperidyloxyl), such as 4-(4-chlorobenzylideneamino)TEMPO (1) and 4- (b enzylideneamino)TEMPO (2), showed intermolecular ferromagnetic interactions at an extremely low temperature. The experimental electron-density distrib utions 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) = 12 0 degrees and 95 degrees for 1 and 2, respectively. Deformation electron-de nsity distributions were calculated from the structure factors after the re finements by the multipole expansion method. These electron-density distrib utions suggested that the unpaired electron of the O radical atom was not l ocated 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 pla ne showing the sp(2) hybridization. From the crystal structures, it has bee n proposed that the ferromagnetic interactions are transferred through CH . .. O interactions between radical O atoms and beta-H atoms of the neighbori ng TEMPO rings. The beta-H atoms of the neighboring TEMPO rings contact wit h the O atom in the direction toward the pi*-orbital of the N-O bond, not i n the direction of the lone-pair densities of the O atom. The present studi es support the mechanism of the intermolecular ferromagnetic interactions t hrough the beta-H atoms. Studies of electron densities from the view point of the intermolecular interactions are important.