19F nuclear magnetic resonance spin–lattice relaxation study of polycrystalline lithium trifluoromethanesulfonate, including comments on the influence of cross-correlation

Abstract

A ¹⁹F nuclear magnetic resonance spin–lattice relaxation investigation of polycrystalline lithium trifluoromethanesulfonate, CF₃SO₃Li, as a function of temperature, and at resonance frequencies of 24.05 and 55.93 MHz, is reported. The ¹⁹F spin–lattice relaxation is found to be non-exponential, to varying degrees, over the whole of the temperature range studied, 205–340 K, and this is attributed to the influence of cross-correlation effects. Values of T₁(¹⁹F) are determined from the initial gradients of appropriate magnetisation recovery plots: this method of measurement is discussed in some detail. The ¹⁹F spin–lattice relaxation is attributed to the reorientational motion of the CF₃ groups about their threefold symmetry axes and the activation energy for this type of motion is found to be 24.40 ± 0.69 kJ mol^–1. The form of the experimental ¹⁹F magnetisation recovery at 24.05 MHz and at a temperature where cross-correlation effects are particularly marked, i.e. close to the T₁(¹⁹F) minimum, is shown to compare favourably with that predicted on the basis of the theoretical treatment of Hilt and Hubbard.