Perspectives of fast magic-angle spinning ⁸⁷ Rb NMR of organic solids at high magnetic fields

We report solid-state ⁸⁷ 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 ⁸⁷ Rb quadrupole coupling constants (close to 20 MHz). We have also obtained static ⁸⁷ Rb NMR spectra for these two compounds and determined both ⁸⁷ Rb quadrupole coupling and chemical shift tensors. The experimental ⁸⁷ 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 ⁸⁷ Rb NMR spectra for organic Rb solids at the natural abundance level. We anticipate that, with additional ⁸⁷ Rb isotope enrichment (up to 99%), the sensitivity of solid-state ⁸⁷ Rb NMR will be 400 times higher than ³⁹ K NMR, which makes the former an attractive surrogate probe for studying K⁺ ion binding in biological systems.