Spectroscopic Measurements of Ionic Association in Solutions of LiPF6

Ionic association in nonaqueous electrolytes containing LiPF6 was investigated with infrared absorption spectroscopy. The spectral intensity of the nondegenerate ν1 mode of the PF6 - anion was found to be sensitive to ion pairing. Although the ν1 mode of an isolated PF6 - anion is only Raman active, coordination of Li+ to PF6 - destroys the octahedral symmetry of the anion and results in ν1 becoming simultaneously IR and Raman active. When the dielectric constant of the solvent is increased, the IR-intensity of the ν1 band decreases because ion pairing is not favored in high dielectric solvents. Spectroscopic studies of solutions containing LiPF6 in diglyme show that ion pairing is also affected by specific cation−solvent interactions. The diglyme-containing solutions contain significantly fewer ion pairs than expected on the basis of the solvent's dielectric constant. It is thought that diglyme:LiPF6 solutions consist mostly of “spectroscopically free” PF6 - anions because the sixfold coordination of Li+ by two diglyme molecules hinders Li+···PF6 - ion pairing.