7Li nuclear magnetic spin–lattice relaxation investigation of lithium trifluoromethanesulfonate and its complex with poly(ethylene oxide)

Abstract

A ⁷Li NMR spin–lattice relaxation investigation over the temperature range 200–340 K is reported for a polycrystalline sample of LiCF₃SO₃ and a sample of poly(ethylene oxide)(PEO)–LiCF₃SO₃ with an O: Li molar ratio of 3.5:1. In both cases the ⁷Li magnetization recoveries were found to be single exponential and so the spin–lattice relaxation could be characterized by a single time constant, T₁(⁷Li). The ⁷Li spin–lattice relaxation in polycrystalline LiCF₃SO₃ is shown to be dominated by a mechanism that involves the modulation of ⁷Li,¹⁹F magnetic dipolar interactions due to the reorientational motions of the CF₃ groups about their threefold symmetry axes. The Arrhenius parameters derived for this motion are shown to be in good agreement with those determined in an earlier ¹⁹F spin–lattice relaxation study. The ⁷Li spin–lattice relaxation time measurements in the 3.5:1 PEO–LiCF₃SO₃ sample are taken to reflect the properties of the single crystalline complex, which is known to be present in PEO–LiCF₃SO₃ mixtures. The extent to which magnetic dipolar relaxation processes contribute to ⁷Li spin–lattice relaxation in this complex is discussed in some detail. It is demonstrated that modulation of ⁷Li, ¹H magnetic dipolar interactions, due to backbone motions of the PEO chains, probably makes a significant contribution. There also appear to be strong links between the motional processes responsible for both ¹H and ⁷Li spin–lattice relaxation.