Ultrafast dynamics in 1-butyl-3-methylimidazolium-based ionic liquids: a femtosecond Raman-induced Kerr effect spectroscopic study.

We investigated the ultrafast dynamics in 1-butyl-3-methylimidazolium-based ionic liquids with two series of anions, (1) cyano-group substituted anions (thiocyanate [SCN](-), dicyanamide [N(CN)(2)](-), and tricyanomethide [C(CN)(3)](-)) and (2) trifluoromethylsulfonyl-group substituted anions (trifluoromethanesulfonate [OTf](-), bis(trifluoromethylsulfonyl)amide [NTf(2)](-), and tris(trifluoromethylsulfonyl)methide [CTf(3)](-)). This was done by femtosecond Raman-induced Kerr effect spectroscopy. From the Fourier transform Kerr spectra of the ionic liquids, the low-frequency spectrum of 1-butyl-3-methylimidazolium tricyanomethide shows a low-frequency shift compared to the ILs with the other cyano-group substituted anions due to the planar structures of the cation and the anion. The relative amplitude of the low-frequency band at approximately 20 cm(-1) compared to the entire broad spectrum for the ionic liquids with trifluoromethylsulfonyl-group substituted anions becomes larger with the order [OTf](-) < [NTf(2)](-) ≈ [CTf(3)](-). This vibrational band can be attributed to the librational motion of anions and/or the coupling of the translational and reorientational motions.