13 C NMR investigation of fullerene–cubane C 60 ·C 8 H 8 cocrystals

The rotor–stator molecular cocrystal C 60 ·C 8 H 8 (fullerene–cubane) has been investigated by 13 C nuclear magnetic resonance (NMR). The room‐temperature spectrum obtained using 1 H– 13 C cross‐polarization technique shows two lines with chemical shifts identical with the shifts of the original molecular constituents demonstrating the lack of a strong electronic interaction between C 60 and C 8 H 8 . The temperature dependence of the spin‐lattice relaxation time of the fullerene component confirms the existence of a first‐order orientational ordering transition around 145 K. The activation energies of large‐angle C 60 reorientations above and below the ordering transitions are 260 K and 570 K, respectively. The transition temperature and the activation energies are significantly lower than in other C 60 compounds. The 13 C spectrum remains narrow down to 115 K indicating that similarly to pristine C 60 , the molecular reorientational motion is still fast in the ordered phase on the NMR time scale.