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

Abstract

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.

Low-Frequency Spectra of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Poly(ethylene glycol) by Femtosecond Raman-Induced Kerr Effect Spectroscopy

“…However, long-time-window data for over tens of picoseconds may be unnecessary concerning intermolecular vibrational dynamics. The adequacy of the discussion of intermolecular vibrational component obtained from the Kerr transients without long-time-window data was confirmed previously …”

“…The adequacy of the discussion of intermolecular vibrational component obtained from the Kerr transients without long-time-window data was confirmed previously. 86 The Kerr transients were converted to the low-frequency Kerr spectra using the standard Fourier transform deconvolution analysis procedure developed by McMorrow and Lotshaw. 87,88 Figure 6 shows the Fourier transform Kerr spectra for [MOIm][BF 4 ]/PEG400 and OIm/PEG400 with different concentrations of PEG400.…”

An insight into the intermolecular vibrational modes of dicationic ionic liquids through far-infrared spectroscopy and DFT calculations

“…The calculations have been performed at the B3LYP/AUG-CC-PVTZ level with empirical corrections for dispersion. The same functional and basis set has been used by Shirota et al [49][50][51] with good results to compute Raman spectra of similar systems. A direct comparison between our and the above cited papers is not possible.…”

Probing Molecular Interactions in Functionalized Asymmetric Quaternary Ammonium-Based Dicationic Ionic Liquids

“…Dicationic ionic liquids (DILs) composed of two singly charged cations linked through a hydrocarbon linkage chain (spacer) and paired with two singly charged anions have been of growing interest in the recent literature. , DILs possess high thermal stability and larger liquidus ranges, which makes them suitable for applications as solvent in organic reactions, lubricants, gas chromatography, and electrolytes, especially at high temperatures compared to the monocation-based ILs. The works on DILs was initialized with earlier investigations by Armstrong and co-workers who measured thermal stabilities and shear viscosities of germinal DILs based on substituted imidazolium and pyrrolidinium cations.…”