Gas-Phase Studies of 19F NMR Magnetic Shielding of Simple Isolated Molecules.

19F NMR spectra were recorded in gaseous CFCl3/Xe mixtures on the NMR spectrometer, operated with a standard 2H(D) lock system. These results and shielding values obtained from the best relativistic quantum chemical calculations for single isolated CFCl3 were used to establish the 19F nuclear magnetic moment: μ(19F) = 2.6283258(27) μN and a new absolute shielding scale based on liquid trichlorofluoromethane. The fluorine gas-to-liquid shift for trichlorofluoromethane was measured as Δσgl = -5.28(1) ppm. Next, the 1H, 19F, and 3He NMR spectra were measured in CH3F/3He and CF4/3He gaseous mixtures as density functions. Extrapolating linear relationships to the zero-pressure limit yields frequency parameters for isolated CH3F, CF4, and helium atoms. These results have verified shielding constants for both fluorinated simple molecules. New exact results for the absolute 19F shielding of 16 chemical compounds measured in the gas phase were reported. Based on the new 19F shielding constant of the reference liquid CFCl3, a large set of previously measured shielding constants for fluorine compounds in the gas phase was obtained.