Interionic Vibrational Spectra of Electrolytes in Nonaqueous Solvents

Far‐infrared spectroscopy is shown to provide a powerful new method for studying ion aggregates in solution. The interionic vibration frequency of Na⁺BPh₄⁻ is at 175 cm⁻¹ in pyridine, 1,4‐dioxane, piperidine, and tetrahydrofuran, indicating that bare, rather than solvated ions, produce the absorption. The isotope shift of the 198‐cm⁻¹ band in NH₄⁺BPh₄⁻ to 183 cm⁻¹ in ND₄⁺BPh₄⁻ also fits a bare‐ion model. The Li⁺BPh₄⁻ salt gives a band at 410 cm⁻¹ in THF, which is probably due to a Li⁺ THF vibration, while the K⁺BPh₄⁻ band 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 × 10⁴ 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 ${\mathrm{-(e}}^2{\mathrm{\hspace{0.17em}/\hspace{0.17em}R)+(B\hspace{0.17em}/\hspace{0.17em}R}}^n)$ , where $\text{n≫7}$ (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.