19F nuclear magnetic shielding scale from gas phase studies

By using results of density and temperature studies, 19F nuclear magnetic resonance frequencies measured in 20 gases of known densities are reduced to the zero-density limit at 300 K. From these we obtain differences in shielding between rovibrationally averaged isolated molecules at this temperature. The magnitudes of the corrections linking the shielding in the liquid phase to the isolated molecule at its equilibrium configuration are discussed. These corrections are not necessarily small and are required in deriving an accurate absolute shielding of any molecule from the known absolute shielding of another molecule by observing the NMR chemical shift between the two.