Effects of aromaticity in cations and their functional groups on the temperature dependence of low-frequency spectrum

In this study, we investigate the temperature dependence of low-frequency spectra in the frequency range of 0.3-200 cm^-1 for ionic liquids (ILs) whose cations possess two systematically different cyclic groups, using femtosecond Raman-induced Kerr effect spectroscopy. The target ILs are bis(trifluoromethylsulfonyl)amide [NTf₂]^- salts of 1-cyclohexylmethyl-1-methylpyrrolidinium [CHxmMPyrr]⁺, 1-cyclohexylmethyl-3-methylimidazolium [CHxmMIm]⁺, N-cyclohexylmethylpyridinium [CHxmPy]⁺, 1-benzyl-1-methylpyrrolidinium [BzMPyrr]⁺, 1-benzyl-3-methylimidazolium [BzMIm]⁺, and N-benzylpyridinium [BzPy]⁺ cations. The aim of this study is to better understand the effects of aromaticity in the cations' constituent groups on the temperature-dependent low-frequency spectral features of the ILs. The low-frequency spectra of these ILs are temperature dependent, but the temperature-dependent spectrum of [CHxmMPyrr][NTf₂] is different from that of other ILs. While [CHxmMPyrr][NTf₂] shows spectral changes with temperature in the low-frequency region below 50 cm^-1, the other ILs also show spectral changes in the high-frequency region above 80 cm^-1 (above 50 cm^-1 in the case of [BzMPyrr][NTf₂]). We conclude that the spectral change in the low-frequency region is due to both the cation and anion, while the change in the high-frequency region is attributed to the red shift of the aromatic ring librations. On the basis of the plots of the first moment of the spectra vs. temperature, we found that the first moment of the low-frequency spectrum of the IL whose cation does not have an aromatic ring is less temperature dependent than that of the other ILs. However, the intrinsic first moment, the first moment at 0 K, of the low-frequency spectrum is governed by the absence or presence of a charged aromatic group, while a neutral aromatic group does not have much influence on determining the intrinsic first moment.