C-13 NMR of the organic ferromagnet TDAE-C-60

The temperature dependence of the C-13 NMR spectra and C-13 spin-lattice relaxation time T-1 has been studied in powdered TDAE-C-60 (TDAE=tetrakis-dimethylaminoethylene). A motionally narrowed C-13 NMR line has been observed at 188 ppm relative to tetramethylsilane (TMS) at room temperature which can be assigned to the C-60(-) ion. A large line broadening has been observed on cooling below 150 K. The second moment of the line increases again below 50 K and reaches 250 kHz(2). This is rather close to the value of the second moment calculated for the static dipolar interaction between the C-13 nuclei and the unpaired electron in the 2p(z) state smeared out over all carbon sites in the C-60(-) ion. The freeze-out of the ''isotropic'' rotation between 150 and 100 K is connected with a high field shift of the C-13 Line by 160 ppm which changes into a low field shift of 87 ppm below 50 K. The change in the sign of the shift may indicate that antiferromagneticlike correlations change into ferromagneticlike correlations at low temperatures.