6Li magic angle spinning nuclear magnetic resonance spectroscopy: a powerful probe for the study of lithium-containing materials

Abstract

A narrow chemical-shift range has been established for a variety of lithium compounds via study of their ⁶Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. Comparative studies of ⁶Li and ⁷Li spectra (MAS, and ‘off-angle’ spinning) establish that, for solid-state (and even solution) analytical purposes, ⁶Li is the preferred nucles, since the advantage of narrow absorption lines outweighs the poorer sensitivity of ⁶Li relative to ⁷Li.

⁶ Li MAS NMR spectra have been obtained for laponite clay, which had been thermally treated at 200, 400, 600, 800 and 1300 °C; at the lower temperatures (<800 °C) both dehydrated and rehydrated specimens were considered. The data are consistent with mobility of lithium ions from the trioctahedral clay sites at 600 °C.

Both conventional and microwave methods were used to prepare lithium-exchanged laponite. The superior resolution achievable in ⁶Li MAS NMR in comparison with ⁷Li MAS NMR is demonstrated with the microwave specimen where use of ⁶Li spectroscopy revealed two lithium sites. On storage of the sample for 3 months, the two sites give way to a single lithium environment. Possible causes are discussed.