NMR STUDY OF MOLECULAR MOTIONS NEAR SOLID-SOLID TRANSITION IN METAL HEXAFLUORIDES

Fluorine-19 magnetic resonance and relaxation measurements on orthorhombic MoF6 and on cubic MoF6 and WF6 are reported. In the cubic plastic phases the molecules have rapid rotational and slow translational motions. The theory of Resing and Torrey has been applied to interpret the relaxation data and yields a diffusion coefficient of 0.52 × 10-8 cm2sec-1 for MoF6 and 0.36×10-8 cm2 sec-1 for WF6 at the transition point. Slow rotations of the molecules already occur in the orthorhombic phases; this is shown by the study of the shape of the resonance lines in the presence of both motion and anisotropic chemical-shift tensors. Accurate calculations of the contribution of these rotations to the longitudinal relaxation and to the second moments of the resonance line have been made. They show that in MoF6, which exhibits a solid-solid transition, the rotations of the molecules are quasi-isotropic, whereas in UF6, where no transition occurs, the rotations are slower and take place preferentially about a single C3 axis of the molecules. At high Larmor frequencies the relaxation in the plastic phases is probably due to the spin-rotation interaction, as in the liquid. A model for spin-rotational relaxation in a molecular solid is proposed.