Effect of anions on the intermolecular vibrational dynamics of lithium salt–acetamide deep eutectic solvents

In this study, we investigated the intermolecular vibrational dynamics of five deep eutectic solvents (DESs) composed of a 4:1 molar ratio mixture of acetamide and lithium salts, namely, lithium nitrate, perchlorate, trifluoromethanesulfonate, bis(fluorosulfonyl)amide, and bis(trifluoromethylsulfonyl)amide, using femtosecond Raman-induced Kerr effect spectroscopy. The low-frequency spectra below ∼200 cm-1 revealed that the peak position and first moment of the intermolecular vibrational band depended on the anion species, while acetamide molecules mainly determined the spectral intensity in most cases. However, the librational motion associated with the planar nitrate ion resulted in an intense low-frequency spectrum. The first-moment value was larger for the nitrate DES than for the other four DESs, indicating stronger intermolecular interactions in the former. A similar trend was observed for surface tension values. Comparison with previous results for lithium bis(trifluoromethylsulfonyl)amide-organic amide DESs showed that variation in the first moment of the intermolecular vibrational band and in the bulk parameter (square root of surface tension divided by liquid density) is larger upon changing the anion species than upon changing the organic amide species. Accordingly, the microscopic intermolecular interactions probed by the intermolecular vibrational band are significantly influenced by the anion species. Furthermore, a plot of the first moment of the intermolecular vibrational band vs the bulk parameter displayed a linear correlation, indicating a strong relationship between microscopic intermolecular interactions and macroscopic properties in the DES systems.