Effect of cation symmetry on the low-frequency spectra of imidazolium ionic liquids: OKE and Raman spectroscopic measurements and DFT calculations

This article reports a study of the low-frequency optical Kerr effect (OKE) and Raman spectra of 1-propyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([C3C1im][NTf2]) and 1,3-diethylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(C2)2im][NTf2]). These ionic liquids differ only in the symmetry of the alkyl substitution on the imidazolium ring of the cation. DFT calculations on the isolated ions provide guidance in the assignment of the OKE and Raman spectra in the 0–200 cm−1 region to intermolecular and intramolecular vibrational modes of the liquid.