Optical Kerr Effect Spectroscopy of a Nonpolar Solute in Dicationic versus Monocationic Ionic Liquids*

A comparison of the intermolecular dynamics of small nonpolar solute molecules in monocationic and dicationic ionic liquids (ILs) was performed using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The optical Kerr effect (OKE) spectrum of CS$_{2}$ in 1-methyl-3-propylimidazolium bis(trifluoromethane-sulfonyl)amide [C$_{3}$mim][NTf$_{2}$] and 1,6-bis(3-methylimidazolium-1-yl) hexane bis(trifluoromethane-sulfonyl)amide [C$_{6}$(mim)$_{2}$][NTf$_{2}$]$_{2}$ was investigated as a function of concentration at 295 K. An additivity model with components from the subpicosecond dynamics of IL and CS$_{2}$ was used to interpret the OKE spectra of the mixtures. The spectrum of CS$_{2}$ in the two ILs is lower in frequency and narrower than that of neat CS$_{2}$. The spectrum of CS$_{2}$ in the dicationic IL is higher in frequency than in the monocationic IL. This result shows that CS$_{2}$ molecule experiences a stiffer potential in dicationic ILs as compared to monocationic ILs. Higher stiffness in C$_{6}$(mim)$_{2}$][NTf$_{2}$]$_{2}$ might be due to a more ordered arrangement and lower mobility of the alkyl chains linking the imidazolium rings.