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We report the results of $^{13}\mathrm{C}$ NMR measurements on ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ in the normal and superconducting states. Electronic densities of states at the Fermi energy in the normal-state and energy gaps in the superconducting state are estimated from spin-lattice-relaxation data. Implications of the relaxation and spectral data for the electronic properties of these materials are discussed.

We characterize the molecular reorientational dynamics in two phases of solid ${\mathrm{C}}_{60}$ with $^{13}\mathrm{C}$ NMR measurements. A change in the nature of the dynamics, indicated by a change in kinetic parameters extracted from spin-lattice relaxation data, occurs at the phase transition at 260 K. Above the transition, the molecules appear to execute continuous rotational diffusion; below the transition, they appear to jump between symmetry-equivalent orientations. This interpretation is consistent with the x-ray-diffraction results of Heiney et al. as well as our NMR relaxation and spectral data.

Spin dynamics at planar sites in oxygen-deficient ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$ have been investigated by nuclear spin-lattice relaxation measurements using $^{63}$,65Cu nuclear quadrupole resonance and $^{89}\mathrm{Y}$ NMR. A sharp decrease of $^{63}\mathrm{Cu}$ nuclear relaxation below 100 K in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.7}$ suggests the possible onset of spin pairing in individual planes well above the transition to three-dimensional bulk superconductivity. Comparison of the relaxation rates for the 60-K superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.7}$ with those of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7.0}$ in the range 100--300 K shows that the planar Cu(2) spin dynamics are strongly affected by changes in carrier concentration.