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.