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