Interionic Vibrational Spectra of Electrolytes in Nonaqueous Solvents

Abstract

Far-infrared spectroscopy is shown to provide a powerful new method for studying ion aggregates in solution. The interionic vibration frequency of Na+BP14-is at 175 cm-1 in pyridine 1 4-dioxane piperidine and tetrahydrofuran, indicating that bare, rather than solvated ions, produce the' absorption. The isotop~ shift of the 198-cm-1 band in ~+BP14to 183 cm-lin ND4+ BP14-also fits a bare-ion model. The Li+BP14salt gives a band at 410 cm-1 in THF, which is probably due to a Li+ THF vibration, while the K+BP14band at 133 cm-1 arises from interionic vibration. The Na+, NH4+, ND4+, and K+ salt stretching force constants, based on a bare ion-pair model, are nearly identical, 3.85±0.11 X 10 4 dyn cm-1 . These force constants differ markedly from those predicted from the Pettit and Bruckenstein ion-pair potential used to interpret conductivity data of strong electrolyte solutions. Present observations indicate that the dominating terms in the ion-pair potential are -(ill R) + (BI Rn), where n»7 (Pettit and Bruckenstein) and may approach 30 for a pair of polyatomic ions. The dependence of the band intensities on solvent: concentration, and the nature of the cation appear to be primarily due to variations in the proportion of absorbing systems, rather than in the intrinsic absorption coefficient, and support the assignment of the bands to ion pairs, rather than higher aggregates. Intensity measurements in a mixed benzene-pyridine solvent demonstrate the phenomenon of preferential solvation by pyridine. 4

Spectroscopic studies of ionic solvation in pyridine solutions

“…Phys., 21,1170 (1953). 6 The shifted 1581-cm-1 band lies too close to the 1598-cm-1 pyridine band to be resolved; consequently, only a single band at 1597 cm-1 is observed.…”

Spectroscopic studies of ionic solvation. VIII. Alkali metal salts in acetone solutions

“…Figure5. The 789-cm-1 acetone and the 935-cm-1 C104" Raman bands: A, 3.5 M acetone in nitromethane; B, acetone/LiC104 ratio = 6:1; C, acetone/LiC104 ratio = 4:1; D, acetone/LiC104 ratio = 3:1; E, acetone/LiC104 ratio = 2:1; F, acetone/LiC104 ratio = ~1:1; band intensity semiquantitative.…”