AB-INITIO CALCULATION AND NMR SPIN-ECHO MEASUREMENT OF F-19 CHEMICAL SHIELDING IN THE ALKALI-METAL FLUORIDES

Experimental values of F-19 chemical shielding in the series of anhydr ous alkali-metal fluorides LiF to CsF are reported. An NMR spin-echo t echnique is used for the measurements and the results are shown to com pare favourably with others, in particular those determined by magic-a ngle spinning (MAS) techniques, reported in the literature. Ab initio methods of calculating F-19 chemical shielding in these primarily ioni c materials are then discussed in terms of simplified model systems. F or all the alkalimetal fluorides, values of absolute F-19 chemical shi elding are calculated which are significantly less than the free fluor ide ion value. In the specific cases of LiF and NaF reasonable agreeme nt between ab initio calculation and experiment is achieved.