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

A comparative study of the intermolecular dynamics of CS 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 CS in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CCim][NTf] for n = 3-5) and 1,2n-bis(3-methylimidazolium-1-yl) alkane bis[(trifluoromethane)sulfonyl]amide ([(Cim)C][NTf] for n = 3-5) were investigated as a function of concentration at 295 K. An additivity model was used to obtain the CS contribution to the RSD of a mixture in the 0-200 cm region. One of the aims of this study is to show how CS can be used as a probe of intermolecular/interionic interactions in ILs. The concentrations were chosen such that the CS-to-imidazolium ring mole fraction of a mixture with [(Cim)C][NTf] (DIL(2n)) is the same as that of a mixture with [CCim][NTf] (MIL(n)). As found previously for CS in monocationic ILs, the intermolecular spectrum of CS in dicationic ILs is lower in frequency and narrower than that of neat CS. The new result is that the intermolecular spectrum of CS is higher in frequency in DIL(2n) than in the corresponding MIL(n), indicating that CS molecules experience a stiffer potential in dicationic ILs than in monocationic ILs. The intermolecular dynamics of CS 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 CS 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.