Perspectives of fast magic-angle spinning Rb NMR of organic solids at high magnetic fields.

We report solid-state Rb NMR spectra from two Rb-ionophore complexes obtained with fast magic-angle spinning (MAS) (up to 60 kHz) at 21.1 T. These Rb-ionophore complexes containing macrocycles such as benzo-15-crown-5 and cryptand [2.2.2] are typical of organic Rb salts that exhibit very large Rb quadrupole coupling constants (close to 20 MHz). We have also obtained static Rb NMR spectra for these two compounds and determined both Rb quadrupole coupling and chemical shift tensors. The experimental Rb NMR tensor parameters are compared with those obtained by quantum chemical computations. Our results demonstrate that the combination of fast MAS (60 kHz or higher) and a high magnetic field (21.1 T or higher) is sufficient to produce high-quality solid-state Rb NMR spectra for organic Rb solids at the natural abundance level. We anticipate that, with additional Rb isotope enrichment (up to 99%), the sensitivity of solid-state Rb NMR will be 400 times higher than K NMR, which makes the former an attractive surrogate probe for studying K ion binding in biological systems.