Femtosecond Raman-Induced Kerr Effect Study of Temperature-Dependent Intermolecular Dynamics in Molten Bis(trifluoromethylsulfonyl)amide Salts: Effects of Cation Species.

Abstract

In this study, we have investigated the effects of cation structures on the temperature dependence of the intermolecular vibrational dynamics of ionic liquids using femtosecond Raman-induced Kerr effect spectroscopy. The ionic liquids used in this study are bis(trifluoromethylsulfonyl)amide [NTf2]- salts of the cations 1-butyl-3-methylimidazolium [C4MIm]+, 1-butyl-1-methylpyrrolidinium [Pyrr14]+, 1-butylpyridinium [C4Py]+, butyldiethylmethylammonium [N1224]+, triethyloctylammonium [N2228]+, and triethyloctylphosphonium [P2228]+. All of the ionic liquids show temperature-dependent low-frequency spectra. A difference in the temperature dependence between the spectra of the aromatic and nonaromatic cation based ionic liquids is especially significant. In the case of the aromatic cation based ionic liquids [C4MIm][NTf2] and [C4Py][NTf2], the spectral intensities in the low-frequency region below ca. 50 cm-1 increase and the high-frequency components at ca. 80 cm-1 shift to lower frequencies with rising temperature. In contrast, the ionic liquids based on nonaromatic cations only exhibit an increase in the low-frequency region below ca. 50 cm-1 with increasing temperature, while the high-frequency region of the spectra above ca. 50 cm-1 shows little change with variation of the temperature. These results suggest that the presence or absence of aromatic rings is the main factor in determining the temperature-dependent spectral features, particularly in the high-frequency region. We also found that the alkyl chain length and central atoms of the nonaromatic quaternary cations do not have much influence on the temperature-dependent spectral features. The first moments of the aromatic cation based ionic liquids are a little more sensitive to temperature than those of the nonaromatic cation based ionic liquids. The temperature-dependent viscosities and fragilities of the ionic liquids have also been examined.