Temperature Dependence of Low-Frequency Spectra in Molten Bis(trifluoromethylsulfonyl)amide Salts of Imidazolium Cations Studied by Femtosecond Raman-Induced Kerr Effect Spectroscopy.

In this study, the temperature dependence of the low-frequency spectra of liquid bis(trifluoromethylsulfonyl)amide salts of the monocations 1-methyl-3-propylimidazolium and 1-hexyl-3-methylimidazolium and the dications 1,6-bis(3-methylimidazolium-1-yl)hexane and 1,12-bis(3-methylimidazolium-1-yl)dodecane has been investigated by means of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The intensity in the low-frequency region below 20 cm(-1) in the spectra of the four ionic liquids increases with rising temperature. From a line-shape analysis of the broadened low-frequency spectra of the ionic liquids, it is clear that the lowest-frequency component, which peaks at approximately 5 cm(-1), contributes to the temperature dependence of the spectra. This implies that the activity of the intermolecular translational vibrational motion is increasing with rising temperature. It is also possible that decoupling in the crossover process between intermolecular vibrational motion and structural relaxation occurs as a result of a deterioration of the non-Markovian feature or the loss of memory caused by the higher temperature. The peak of the highest-frequency component, which is due mainly to the imidazolium ring libration, shifts to lower frequency with increasing temperature. This is attributed to weaker interactions of the ionic liquids at higher temperatures. Temperature-dependent viscosities from 293 to 353 K of the four ionic liquids have also been characterized.