Nuclear Magnetic Resonance and Relaxation of Hexafluoride Molecules in the Solid

Abstract

The 19F magnetic resonance absorption spectra and spin—lattice relaxation times of polycrystalline UF6, PuF6, WF6, OsF6, PtF6, MoF6, and XeF6 have been studied as a function of temperature and magnetic field. In all investigated metal hexafluorides the chemical shifts were found to be quite large, exceeding the dipole—dipole broadening in the solid and allowing an unambiguous determination of the tetragonal distortion of the hexafluoride octahedra in the low-temperature orthorhombic phase. The fluorines can thus occupy either axial or equatorial sites. The chemical shifts of the axial and equatorial fluorines (σ0, axial=−5350 ppm, σ0, equatorial=−2800 ppm) with respect to HF, and the anisotropies of the 19F screening tensors in PtF6, |σ∥—σ⊥ |a=1300±100 ppm and |σ∥—σ⊥ |e=1380±100 ppm seem to be the largest of all known fluorine compounds. A slow, thermally activated motion where each fluorine atom is alternatively axial and equatroial, and which consists of a hindered rotation and subsequent distortion of the octahedron in the crystalline field of its neighbors, produces at higher temperatures a coalescence of the spectrum into a single, narrow line. The same fluorine motion was found to influence the spin—lattice relaxation both by a direct modulation of the resonance fields due to anisotropic chemical shifts tensors as well as by a modulation of the dipole—dipole coupling. The solid—solid transitions to the cubic phase in PtF6, WF6, and MoF6, were found to be accompanied by a very sharp increase in T1 and a lowering of the activation energy for molecular motion.