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Ultrafast solvent dynamics of room-temperature ionic liquids have been investigated by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) by studying the effects of cation and anion substitution on the low frequency librational modes. The spectra of two series of imidazolium salts are presented. The first series is based on the 1-butyl-3-methylimidazolium salts bmim+ containing the anions trifluoromethanesulfate TfO−, bis(trifluoromethanesulfonyl)imide Tf2N−, and hexafluorophosphate PF6−. The second series is based on Tf2N− salts containing the three cations 1-butyl-2,3-dimethylimidazolium bmmim+, 1-methyl-3-octylimidazolium omim+, and bmim+. It is found in all five samples that the signal is due to libration of the imidazolium ring at three frequencies around 30, 65, and 100 cm−1 corresponding to three local configurations of the anion with respect to the cation.
Giraud et al. (Thu,) studied this question.