Spin density of a ferromagnetic tempo derivative

We have determined by polarized neutron diffraction the spin density of a f erromagnetic (Tc=0.28K) tempo derivative: the 4(p-chlorobenzylideneamino)-2 ,2,6,6-tetramethylpiperidin-1-oxyl, where the nearest and next nearest neig hbours N-O sites construct a two-dimensional network parallel to the ((a) o ver right arrow,(b) over right arrow) plane. Most of the spin density lies on the N and O atoms of the NO radical and is equally shared between these two atoms. As for the pure tempone, the two carbon atoms neighbouring the n itrogen of the N-O group in the 6-member ring, carry significant spin popul ations of different signs: -0.074(12) for atom C3 and +0.063(17) for atom C 4. This is not consistent with the coupling mechanism proposed previously w hich implied that the populations of both C3 and C4 should be negative. A t entative explanation for the coupling mechanisms, taking into account the s pin density distribution, is discussed.