AN IMINO NITROXIDE FREE-RADICAL - EXPERIMENTAL AND THEORETICAL SPIN-DENSITY AND ELECTRONIC-STRUCTURE

An imino nitroxide, the 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 Defaul t Model Maximum of Entropy (DMME) technique, were used to reconstruct the spin density distribution in the radical. Quantitative (atomic spi n populations) and qualitative (shape of the spin density around the n uclei) results were obtained. Our data provide the basis for the discu ssion of the electronic configuration of imino nitroxides. The nodes o f the singly occupied molecular orbital (SOMO) were directly observed. The spin polarization effect, which gives rise to negative spin densi ties of several atomic sites in the radical and is liable for the spin density transfer to the nitrophenyl group, is discussed. The spin dis tribution is compared to that previously observed experimentally in ni tronyl nitroxides and with theoretical predictions obtained by density functional first principle calculations.