AbstractThe structure of the acetone complex of tri‐o‐thymotide was solved by single‐crystal x‐ray diffraction. The acetone molecule is situated on the twofold axis of the cage and shows no sign of unusual disorder. Solid‐state 2H and 13C NMR were used to show that, in addition to fast methyl group rotation, there is a rotation of the acetone molecule about the carbonyl bond. The motion is best described as a twofold flip plus the temperature‐dependent population of a secondary site 63 ± 10° away from the minimum energy position. Molecular modelling calculations were used to confirm the potential shape, which was remarkably sensitive to methyl group orientation. 2H spin–lattice relaxation was used to derive an activation energy of 13 ± 6 ± 0 ± 8 kJ mol−1 for the twofold flips, and the secondary site was shown to be energetically less favourable by 4.4 ± 0.5 kJ mol−1 from the temperature dependence of the line shape. Neither motion is seen in the diffraction experiment.