Spin Density in an Enaminocetone Nitroxide Copper Complex

The copper complex C(18)H(28)O(2)N(2)Cn(NO)(2) is a molecule which carries three spins S=1/2. It crystallizes in the monoclinic space group Pc, with two molecules in the asymetric unit. Its chi T starts to increase when the temperatures decreases, indicating ferromagnetic intramolecular coupling, and then, below T=40K, it completely collapses, showing negative coupling between molecules. We have investigated its spin density with polarized neutrons at T=40K and T=4.10K. At the higher temperature, the spin density is well localized on the Cu atom and the two NO groups of each molecule. At the lower temperature, the spin density exists, for each molecule, on the Cu atom and on one of the two NO groups only. There is a quasi dimerization on two NO groups, belonging to the different molecules, facing each other, and distant only by 3.40 Angstrom.