Optical Kerr effect spectroscopy of CS 2 in monocationic and dicationic ionic liquids: insights into the intermolecular interactions in ionic liquids.

A comparative study of the intermolecular dynamics of CS2 in monocationic and dicationic ionic liquids (ILs) was performed using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The reduced spectral densities (RSDs) of mixtures of CS2 in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([Cn C1 im][NTf2 ] for n = 3-5) and 1,2n-bis(3-methylimidazolium-1-yl) alkane bis[(trifluoromethane)sulfonyl]amide ([(C1 im)2 C2n ][NTf2 ]2 for n = 3-5) were investigated as a function of concentration at 295 K. An additivity model was used to obtain the CS2 contribution to the RSD of a mixture in the 0-200 cm-1 region. One of the aims of this study is to show how CS2 can be used as a probe of intermolecular/interionic interactions in ILs. The concentrations were chosen such that the CS2 -to-imidazolium ring mole fraction of a mixture with [(C1 im)2 C2n ][NTf2 ]2 (DIL(2n)) is the same as that of a mixture with [Cn C1 im][NTf2 ] (MIL(n)). As found previously for CS2 in monocationic ILs, the intermolecular spectrum of CS2 in dicationic ILs is lower in frequency and narrower than that of neat CS2 . The new result is that the intermolecular spectrum of CS2 is higher in frequency in DIL(2n) than in the corresponding MIL(n), indicating that CS2 molecules experience a stiffer potential in dicationic ILs than in monocationic ILs. The intermolecular dynamics of CS2 being higher in frequency in DIL(2n) than in MIL(n) is consistent with recent molecular dynamics simulations (Lynden-Bell and Quitevis, J. Chem. Phys., 2018, 148, 193844) that show the stiffer potential is the result of greater confinement of CS2 in DIL(2n) than in MIL(n). We also show in this study how effects due to dilution and the intermolecular potential seen by a solute molecule in solution are unraveled.