The spin density in an imino nitroxide free radical: A polarized-neutron study

Imino nitroxide free radicals are potential building blocks for molecular-based magnets. They are purely organic and carry a spin 1/2 which is delocalized over several atoms of the molecule. We have determined the spin density of one of them, the 2-(3-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl(m-NPIM), by polarized-neutron diffraction on a single crystal. The delocalization of the singly occupied molecular orbital (SOMO) and the spin polarization effects, which give rise to negative spin densities, are observed directly.