Solid-State ²³Na 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 23 Na nuclear magnetic resonance study of sodium complexes with crown ethers, cryptands, and naturally occurring antibiotic ionophores. Precise information about 23 Na quadrupole coupling constants and chemical shifts was determined from analysis of 23 Na magic-angle spinning (MAS) NMR spectra. We found that the experimental 23 Na chemical 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 coordination number (CN) around the Na + ion of interest. A reasonably good correlation was also observed between the 23 Na quadrupole parameters measured in CDCl 3 solution and those measured in the solid state, indicating that the cation exchange is slow in CDCl 3 . In MeOH solution, however, neither 23 Na quadrupole parameters nor chemical shifts of the Na−ionophore complexes show correlation with the corresponding solid-state data. Finally, we report the 23 Na chemical shift tensor in Na(12C4) 2 ClO 4: δ 11 = δ 22 = −1 ppm and δ 33 = −15 ppm.