Experimental spin density in a purely organic free radical: Visualisation of the ferromagnetic exchange pathway in p-(methylthio)phenyl nitronyl nitroxide, Nit(SMe)Ph

The P2(1)/a form of the 2-(4-methylthiophenyl)-4,4,5,5-tetramethylimidazoli ne-1-oxyl-3-oxide [Nit(SMe)Ph] ferromagnet (T-c = 0.2 K) was investigated b y conventional and polarised neutron diffraction. The data was analysed by both direct methods (3D maximum entropy) and indirect methods (wave functio n modelling). In both cases, we found that the major part of the spin densi ty is located on the O-N-C-N-O fragment with a negative contribution on the bridging sp(2) carbon atom. In addition, the magnetic molecular orbital of the two oxygen atoms of the O-N-C-N-O fragment is rotated and hybridised. Delocalisation of the unpaired electron onto the methylthio fragment is sig nificant; it suggests that this group is involved in the intermolecular exc hange pathway. The experimental results were compared to those obtained by density functional theory (DFT) calculations on an isolated molecule as wel l as on a pair of molecules which have the same short intermolecular contac t as those observed in the structure. The main conclusion is that DFT fails to predict, in terms of quantitative results, the transfer and the delocal isation of the spin density outside of the O-N-C-N-O fragment.