19F Nuclear Magnetic Resonance Chemical Shift of Alkali Fluorides in Light- and Heavy-Water Solutions

Fluorine‐19 chemical shifts of alkali fluorides dissolved in mixtures of light and heavy water are reported. The ¹⁹F isotopic shift of the fluoride ion is relatively insensitive to the cation, salt concentration, and temperature. There is a linear variation of shift with isotopic composition of the solvent and a total shift of approximately 3 ppm to high field on going from light‐ to heavy‐water solutions. It is found that the results can be interpreted in terms of alterations in the vibrational energy of the fluoride ion, F⁻ (H₂0)_x, resulting from isotopic substitution. This leads to a change of the average excitation energy $\mathrm{〈\Delta E〉}$ which occurs in the paramagnetic term of the nuclear magnetic shielding constant and results in a high‐field shift with the heavier isotope. Similar effects were found for solutions of potassium fluoride in deuterated alcohols.