Comparison of Low-Frequency Spectra between Aromatic and Nonaromatic Cation Based Ionic Liquids Using Femtosecond Raman-Induced Kerr Effect Spectroscopy

In this minireview, the features of low-frequency spectra within the frequency range 0200 cm-1 for aromatic and nonaromatic cation based ionic liquids obtained by femtosecond Raman-induced Kerr effect spectroscopy are reviewed. For aromatic cation based ionic liquids, the aromatic group of the cations is largely responsible for the spectral line shape due to their larger polarizability anisotropy volume and more polarizable nature than nonaromatic cations. The low-frequency Kerr spectra are also compared with the bulk properties of the ionic liquids. The differences in the relationship of the first moment of the low-frequency spectrum band and the bulk properties between the aromatic and nonaromatic cation based ionic liquids with respect to the natures of constituent ions and microscopic aspects are discussed. For aromatic cation based ionic liquids, the ionic region is more pronounced than the nonpolar region in their low-frequency Kerr spectra. In contrast, the ionic and nonpolar regions of nonaromatic cation based ionic liquids are likely to be captured due to the weaker signal of the cationic region. Also, the low-frequency Kerr spectra of the nonaromatic analogues are influenced by their weaker segregation as compared to aromatic cation based ionic liquids.