Rb-87 NMR was used to investigate the appearance and disappearance of
the T' line in Rb3C60. One-dimensional and two-dimensional experiments
together with T-2(-1) relaxation data reveal an exchange process only
between T and T' lines with an activation energy of 580 meV. Differen
t models are discussed in order to explain the data by orientational d
isorder, vacancy diffusion, and the off-center position of O sites as
well as C-60(3-) reorientations.