NA-23 NMR-SPECTROSCOPY OF SOLIDS - INTERPRETATION OF QUADRUPOLE INTERACTION PARAMETERS AND CHEMICAL-SHIFTS

Quadrupole coupling constants (QCC), asymmetry parameters of the electric field gradients (eta), and isotropic chemical shifts (delta(cs)) of sodium-23 were determined by computer simulation of the central transition line shapes of the Na-23 MAS NMR spectra of a series of crystalline sodium compounds in which sodium has exclusively oxygen atoms as nearest neighbors. The relations between the crystal structure and the above parameters were discussed qualitatively for typical examples. The bond-valence method is applied to derive empirical models for the calculation of a ''shift parameter'' (A) and of oxygen atomic charges (model I) from which QCC and eta were calculated by a simple point charge model. A linear correlation between A and delta(cs) is obtained, indicating that delta(cs) can be characterized by the total valence and the Na-O distances of all oxygen atoms located in a sphere of 3.4 Angstrom around the sodium cation. The correlation obtained between calculated and experimental QCC's shows that QCC can be estimated by point charges at the crystallographic oxygen sites determined from the total covalences of the respective oxygen atoms present in the 3.4-Angstrom sphere. QCC and eta were further calculated using oxygen atomic charges obtained from the electronegativity difference of oxygen and the cations (model II) or from quantum chemical ab inito calculations (model III). QCC calculated using charges from models II and III correlate, in general, less well with the experimental data than those of model I. However, the best correlation for eta were further calculated using oxygen atomic charges obtained from the electronegativity difference of oxygen and the cations (model II) or from quantum chemical ab inito calculations (model III). QCC calculated using charges from models II and III correlate, in general, less well with the experimental data than those of model I. However, the best correlation for eta is observed for model III while models II and I yield considerable scatter.