Are 19F NMR Shifts a Measure for the Nakedness of Fluoride Ions

摘要

The solvent dependency of the 19F NMR shifts of the fluoride anion in CH3OH, H2O, CH3OCH3 CHCl3, CHCl2, CHF3 CH3CN, CH3NO2, (CH3)2SO and CH3COCH3 solutions was studied by theoretical calculations at the MP2/6-31++G(d,p) and B3LYP/6-31++G(d,p) levels of theory and compared to the experimental values. It is shown that the free gaseous fluoride anion is most shielded. The stepwise build-up of a solvation sphere was modeled for the F/nH2O system and results in a progressive deshielding of the F nucleus with an increasing number of water ligands in the first solvation sphere. Theoretical calculations predict the first solvation sphere of F to be comprised of six or seven monodentate water molecules. The F-H bond distances increase from 1.42 A in the monohydrate to 1. 69 - 1.87 A and 1.82 A in the penta- and hexa-hydrates, respectively, and the transfer of negative charge from F to the water ligands reaches its maximum for the tetrahydrate.