Comparison of interionic/intermolecular vibrational dynamics between ionic liquids and concentrated electrolyte solutions

In this study, we have compared the interionic/intermolecular vibrational dynamics of ionic liquids (ILs) and concentrated electrolyte solutions measured by femtosecond optically heterodyne-detected Raman-induced Kerr effect spectroscopy. A typical anion in ILs, bis(trifluoromethanesulfonyl)amide ([NTf2](-)), has been chosen as the anion for the sample ILs and concentrated electrolyte solutions. ILs used in this study are 1-butyl-3-methylimidazolium, 1-butylpyridinium, N-butyl-N,N,N-triethylammonium, and 1-butyl-1-methylpyrrolidinium with [NTf2](-). Li[NTf2] solutions (similar to 3.3 M) of water, methanol, propylene carbonate, and poly(ethylene glycol) have been selected as control samples. Kerr transients of the ILs and electrolyte solutions show intra- and interionic/intermolecular vibrational dynamics followed by slow picosecond overdamped relaxation. Fourier transform Kerr spectra have shown a difference in the relative intensities of intraionic vibrational bands of [NTf2](-) (280-350 cm(-1)) between the ILs and electrolyte solutions. The origin of the difference is attributed to the change in the conformational equilibrium between cisoid and transoid forms of [NTf2](-), which is caused by a favorable stabilization of dipolar cisoid form due to Li+ and dipolar solvent molecules in the electrolyte solutions. Low-frequency Kerr spectra (0-200 cm(-1)) exhibit unique features with the variation of cation and solvent species. The aromatic ILs have a prominent high-frequency librational motion at about 100 cm(-1) in contrast to the case for the nonaromatic ones. The common structure of the spectra observed at about 20 cm(-1) likely comes from an interionic motion of [NTf2](-). The nonaromatic ILs allow a fair comparison with the electrolyte solutions of propylene carbonate and poly(ethylene glycol) because of the structural similarities. The comparison based on the first moment of the interionic/intermolecular vibrational spectrum suggests the stronger interionic/intermolecular interaction in the concentrated electrolyte solutions than the ILs.