Abstract

4(p-Chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidine-N-oxyl is a ferromagnetic TEMPO derivative (T_c = 0.28 K) where the nearest and next nearest neighbour NO sites construct a two-dimensional network. We have determined the spin density in this compound by polarized neutron diffraction on a single crystal at low temperature. Most of the spin density (80%) has been found on the NO group, with an equal repartition between the two atoms. 20% of the spin density is delocalized on the rest of the molecule. As for the pure TEMPONE, the two carbon atoms neighbouring the nitrogen of the NO group carry significant spin populations of different signs: −0.074(12) for C3 and +0.069(17) for C4. This implies that the coupling mechanism is rather complex: it concerns many contacts including methyl hydrogens, but also methylene hydrogens, the imino nitrogen N2 and the terminal chlorine atom. Ab initio (DFT) calculations account correctly for the repartition of the spin density between nitrogen and oxygen, but they underestimate the part of the spin density which is delocalized and also the effects of the magnetic interactions on the spin populations of adjacent molecules.