The challenge of paramagnetism in two-dimensional 6,7Li exchange NMR

Li-6,Li-7 fast magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy is used to study LiMn2O4 and Li3V2(PO4)(3). The presence of paramagnetic transition metal centers in these materials has a profound effect on the resulting NMR spectra. Lithium ion mobility has been studied by two-dimensional (2-D) exchange spectroscopy (EXSY) in Li3V2(PO4)(3) but an absence of lithium ion exchange was observed for LiMn2O4. Several differences between the two materials are explored to explain these results. LiMn2O4 experiences a greater donation of electron spin density to the Li nucleus via the Fermi-contact interaction when compared with Li3V2(PO4)(3). This contributes to a greater hyperfine chemical shift and a larger dependence of chemical shift on temperature. The delocalized electrons in LiMn2O4 cause temperature-independent T (1) relaxation rates and shorter relative T (2) values. The relative rates of ionic conductivity and spin-lattice or spin-spin relaxation in LiMn2O4 and Li3V2(PO4)(3) are contrasted to illustrate the constraints on the use of 2-D EXSY to characterize ion dynamics in paramagnetic materials.