Spectroscopic studies of ionic solvation. XIX. Fluorine-19 and sodium-23 NMR studies of ionic interactions in nonaqueous solutions of hexafluorophosphate salts

Abstract

Fluorine-19 and sodium-23 NMR measurements were carried out on sodium hexafluorophosphate solutions in a number of solvents. In solvents of medium polarity and donicity (e.g., propylene carbonate, acetone, acetonitrile) the 19 F chemical shift moved upfield with increasing concentration of the salt. This behavior is indicative of anion-cation interactions which may be of long-range type, i.e., formation of solvent-separated ion pairs; the possibility of contact ion pair formation, however, cannot be excluded. In solvents of low polarity and donicity (acetic acid, tetrahydrofuran), the salt is essentially completely associated in the 0.1–1.0M concentration range. On the other hand, in solvating solvents with high dielectric constants, such as dimethyl-formamide, dimethylsulfoxide, and formamide, there is very little ionic association in the same concentration range. The above conclusions are supported by 23 Na chemical shift measurements. Potassium hexafluorophosphate solutions do not show any concentration dependence of the 19 F chemical shifts, while for tetra-n-butylammonium solutions the 19 F resonance moves downfield with increasing concentration of the salt.