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

► The intermolecular part of the OKE spectrum of 1-propyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([C3C1im][NTf2]) is higher in frequency and broader than that of 1,3-diethylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(C2)2im][NTf2]). ► The high-frequency tail of the OKE spectrum of [(C2)2im][NTf2] is anomalously higher in intensity and more structured than that of [C3C1im][NTf2]. ► Comparison of the OKE spectra with the Raman spectra of these ionic liquids indicates that the excess intensity arises from intramolecular vibrations of the [(C2)2im]+ ion. ► On the basis of the DFT calculated Raman spectra, the cation modes that underlie the high-frequency tail of the OKE spectrum of [(C2)2im][NTf2] are associated with low-frequency scissoring and shearing modes of the [(C2)2im]+ ion.