文摘
One-dimensional crystals of 25% 13C-enriched C60 encapsulated inside highly magnetically purified SWNTs were investigated by following the temperature dependence of the 13C NMR line shapes and the relaxation rates from 300 K down to 5 K. High-resolution MAS techniques reveal that 32% of the encapsulated molecules, so-called the Cclass="stack">60class="stack">α, are blocked at room temperature and 68%, labeled Cclass="stack">60class="stack">β, are shown to reversly undergo molecular reorientational dynamics. Contrary to previous NMR studies, spin−lattice relaxation time reveals a phase transition at 100 K associated with the changes in the nature of the Cclass="stack">60class="stack">β dynamics. Above the transition, the Cclass="stack">60class="stack">β exhibits continuous rotational diffusion; below the transition, Cclass="stack">60class="stack">β executes uniaxial hindered rotations most likely along the nanotubes axis and freeze out below 25 K. The associated activation energies of these two dynamical regimes are measured to be 6 times lower than in fcc-C60, suggesting a quiet smooth orientational dependence of the interaction between Cclass="stack">60class="stack">β molecules and the inner surface of the nanotubes.
