Abstract A pulsed NMR study of relaxation times, T 1 and T 1D , and second moments for solid 5,5-dimethyl-5,6,11,12-tetrahydro-5 H-dibenzo[ b,f]silocin is reported. The compound was studied over the temperature range −175 to 50°C. Evidence was obtained for three motions characterized by activation energies, 2.70, 1.2, and ⩾4 kcal/mole, respectively. The first motion is methyl reorientation. The second motion is not assigned but is probably a relatively small amplitude flexing of the central ring. The third motion is also unassigned and may be large amplitude flexing of the central ring, libration, or anisotropic molecular reorientation. The motions was assigned by comparison of X-ray crystallographic data with NMR second moment data.