C-13 CHEMICAL SHIELDING PARAMETERS IN LIQUID HEXAFLUOROBENZENE DETERMINED BY NMR RELAXATION MEASUREMENTS

Simple pulse sequences have been used to determine and isolate most ca rbon-13 and fluorine-19 longitudinal relaxation parameters in pure hex afluorobenzene (carbon-13 in natural abundance). The derived parameter s include fluorine-19 and carbon-13 specific relaxation rates (at diff erent field values), dipolar fluorine-carbon cross-relaxation rate, an d dipolar (C-13-F-19)-chemical shift anisotropy (C-13 Or F-19) cross-c orrelation rates. From these experimental parameters, it was possible to estimate the reorientational anisotropy of liquid hexafluorobenzene and to extract the shielding tensor elements of fluorine-19 and carbo n-13. Comparison with those elements as determined from solid-state NM R shows a good agreement for fluorine-19 but one important difference (for one element) in the carbon-13 shielding tensor. This feature is c onfirmed by quantum chemistry calculations and can be explained by the particular ordering in pure solid.