Molecular orientational dynamics in C70S48: Investigation by 13C MAS NMR

At room temperature the MAS 13C NMR spectrum of C70S48 is identical to that of pure C70 above 323 K, except that the 13C line is shifted by 1.7 ppm compared to that of pure C70. From these results, we deduce that our system is mainly of the van der Waals type. A simulation of the low speed MAS spectrum shows that C70 molecules in C70S48 undergo a uniaxial rotation as in pure C70. This new result contradicts what had been previously published. The chemical shift of the 13C line does not vary with temperature, however the rotation of C70 slows down as the temperature is decreased and stops at ca. 150 K. Moreover the 13C spin-lattice relaxation time, T1, can be described by a single correlation time that follows an Arrhenius law with a 900 K activation energy. By NMR no phase transition is observed at 245 K contrary to dielectric relaxation measurements.