Molecular Motions in Solid [Sb(CH3)4]PF6. A Combined 11H,9F Nuclear Magnetic Resonance and Quasielastic Neutron Scattering Study

Abstract

Abstract The molecular dynamics of tetramethylstibonium hexafluorophosphate, [Sb(CH3)4]PF6, is investigated over a broad temperature range. NMR spin lattice relaxation times T1 and the NMR second moments of the 1H and 19F nuclei were determined in the range 8.6 ≦ T/K ≦ 332.3 for polycrystalline [Sb(CH3)4]PF6. The complex cation undergoes isotropic tumbling for T > 260 K and thermally activated methyl group rotation in the temperature range T < 196 K. The activation energies for the transition from methyl group rotation to cation reorientation, as derived from NMR wideline (18.1 kJ/mol) and relaxation (22.7 kJ/mol) measurements, match. At very low temperatures pseudo classical line narrowing is observed, indicating tunneling motions of the methyl groups. The existence of two crystallographically inequivalent methyl groups is found by X-ray structure analysis at room temperature. The space group is P63mc, Z = 2; a = 738.6 pm, c = 1089.3 pm. It is confirmed by two steps in the temperature dependence of the signal intensity of the quasielastic line in neutron fixed window measurements in the temperature range 2 < T/K <148. The low temperature spin lattice relaxation times can be explained qualitatively by contributions of two crystallographically inequivalent methyl groups. Apparent activation energies for the two crystallographically different methyl groups are estimated. The complex anion undergoes isotropic tumbling in the temperature range 95 < T/K < 330. Above 330 K additionally translational motion is activated. Below 95 K the rotational motion of PF-6 is freezing in via an uniaxial state in range 40 < T/K <80. Activation energies for both isotropical tumbling (10.5 kJ/mol) and uniaxial rotation (5.8 kJ/mol) have been derived from 19F NMR spin lattice relaxation