Intermolecular vibrational motions of solute molecules confined in nonpolar domains of ionic liquids

In this study, we address the following question about the dynamics of solute molecules in ionic liquids (ILs). Are the intermolecular vibrational motions of nonpolar molecules confined in the nonpolar domains formed by tail aggregation in ILs the same as those in an alkane solvent? To address this question, the optical Kerr effect (OKE) spectrum of CS(2) in the IL 1-pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(5)mim][NTf(2)]) was studied as a function of concentration at 295 K by the use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The OKE spectrum broadens and shifts to higher frequency as the CS(2) concentration is decreased from 20 to 10 mol %; at lower concentrations, no further change in the width of the OKE spectrum is observed. Multicomponent line shape analysis of the OKE spectrum of 5 mol % CS(2) in [C(5)mim][NTf(2)] reveals that the CS(2) and [C(5)mim][NTf(2)] contributions to the spectrum are separable and that the CS(2) contribution is similar to the OKE spectrum of 5 mol % CS(2) in n-pentane with the spectrum being lower in frequency and narrower than that of neat CS(2). These results suggest that, at this concentration, CS(2) molecules are isolated from each other and mainly localized in the nonpolar domains of the IL.