Solid-state Na-23 nuclear magnetic resonance of sodium complexes with crown ethers, cryptands, and naturally occurring antibiotic ionophores: A direct probe to the sodium-binding sites

We report a systematic solid state Na-23 nuclear magnetic resonance study o f sodium complexes with crown ethers. cryptands, and naturally occurring an tibiotic ionophores. Precise information about Na-23 quadrupole coupling co nstants and chemical shifts was determined from analysis of Na-23 magic-ang le spinning (MAS) NMR spectra. We found that the experimental Na-23 chemica l shifts can be related to an empirical parameter that is a function of the atomic valence of the donor ligand, the Na+-ligand distance, and the coord ination number (CN) around the Nai ion of interest. A reasonably good corre lation was also observed between the Na-23 quadrupole parameters measured i n CDCl3 solution and those measured in the solid state, indicating that the cation exchange is slow in CDCl3. In MeOH solution, however, neither Na-23 quadrupole parameters nor chemical shifts of the Na-ionophore complexes sh ow correlation with the corresponding solid-state data. Finally, eve report the Na-23 chemical shift tensor in Na(12C4)(2)ClO4: sigma (11) = sigma (22 ) = -1 ppm and delta (33) = -15 ppm.