Ahmed S. Ellaboudy

Lineshapes in sodium-23 NMR spectra of crystalline sodides

Abstract High-resolution MAS-NMR of 23 Na nuclei in polycrystalline samples of sodium anion salts (M + L n · Na − ) were studied at three sodium Larmor frequencies corresponding to magnetic field strengths of 4.7, 8.5, and 11.7 T. In all cases, Na − NMR lines were narrowed substantially by proton decoupling and had frequency-independent chemical shifts within a few ppm of that of gaseous Na − . These results confirm the high symmetry of Na − and the effective shielding of the 2 p orbitals by the filled 3 s orbital. Dipolar coupling of the methylene protons of the complexant to Na − quantitatively accounts for the static linewidth. The chemical shift and lineshape of the 23 Na NMR line of the cryptated sodium cation, Na + C222 depend on frequency. Detailed analysis of the proton-decoupind signal yielded a quadrupolar coupling constant, e 2 qQ / h , of 1.2 ± 0.1 MHz, an asymmetry parameter, η, of about 0.1, and an isotropic chemical shift of −6.5 ± 0.5 ppm relative to Na + (aq). The combination of quadrupolar and dipolar (from 1 H and 14 N) contributions satisfactorily accounts for the static linewidth of the complexed Na + .