Optical Kerr effect spectroscopy of CS₂ 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 ([C_nC₁im][NTf₂] for n = 3-5) and 1,2n-bis(3-methylimidazolium-1-yl) alkane bis[(trifluoromethane)sulfonyl]amide ([(C₁im)₂C_2n][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 [(C₁im)₂C_2n][NTf₂]₂ (DIL(2n)) is the same as that of a mixture with [C_nC₁im][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.