Vibrational Corrections to the Nuclear-Magnetic Shielding and Spin–Rotation Constants for Hydrogen Fluoride. Shielding Scale for 19F

Abstract

The theory of vibrational effects on the nuclear shielding and spin–rotation constants for a diatomic molecule is outlined. Corrections for these effects are calculated theoretically for both 1H and 19F nuclei in the HF molecule. The theoretically calculated isotopic shift for 19F shielding between DF and HF is in good agreement with the zero pressure experimental value, 2.5 ± 0.5 ppm. The shielding for the vibrating molecule at 300°K is computed from the theoretically calculated value for σFd and the experimentally measured spin–rotation constant, as 〈σF(HF)〉 = 410 ± 6 ppm. The measured chemical shift between the HF monomer and SiF4(gas) then provides a practical 19F reference of known shielding, 〈F(SiF4)〉 = 363 ± 6 ppm. Vibrational corrections are applied to the measured proton shielding in HF gas at zero pressure to obtain the shielding constant as σH(HF) = 29.2 ± 0.5 ppm for the nonvibrating molecule at the equilibrium internuclear distance. Combination of this with the measured spin–rotation constant leads to a separate determination of experimental values for σHd and σHp, with arbitrary choice of gauge origin, for the non-vibrating molecule.