An imino nitroxide free radical: Experimental and theoretical spin density and electronic structure

Abstract

An imino nitroxide, the 2-(3-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl (m-NPIM), a potential building block for molecular-based magnets, was investigated by conventional and polarized neutron single crystal diffraction. Several methods, including the recently developed Default Model Maximum of Entropy (DMME) technique, were used to reconstruct the spin density distribution in the radical. Quantitative (atomic spin populations) and qualitative (shape of the spin density around the nuclei) results were obtained. Our data provide the basis for the discussion of the electronic configuration of imino nitroxides. The nodes of the singly occupied molecular orbital (SOMO) were directly observed. The spin polarization effect, which gives rise to negative spin densities of several atomic sites in the radical and is liable for the spin density transfer to the nitrophenyl group, is discussed. The spin distribution is compared to that previously observed experimentally in nitronyl nitroxides and with theoretical predictions obtained by density functional first principle calculations.