Accurate Determination of NMR Chemical Shifts in Alkali Halides and Their Correlation with Structural Factors

High-resolution solid-state NMR spectra of alkali halides have been measured using the magic angle spinning (MAS) of the sample, and chemical shifts of 7Li, 23Na, 87Rb, 133Cs, 35Cl, 79Br, and 127I have been determined accurately. The 39K chemical shifts have been determined by NMR without MAS. As the ratio of the cation radius to the anion radius (Rm) increases, the chemical shifts of halogen nuclei show an upfield shift when Rm<0.6, while they show a downfield shift when Rm>0.6. The chemical shifts of the halogen nuclei are explained in terms of the first and the second nearest neighbor interactions, both of which cause a downfield shift. On the other hand, the chemical shifts of alkali nuclei show a downfield shift for the ratio of the anion radius to the cation radius (Rx) less than 1.6, while they show an upfield shift for Rx>1.6. The chemical shifts of the alkali nuclei are expressed by the sum of the nearest neighbor interaction in the absence of the halogen-halogen interaction and the “isolation effect” caused by the halogen-halogen interaction. The “isolation effect” makes the chemical shifts approach to the values of the isolated ions.