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

• Published in: Chemical physics letters 2010-09, Vol.497 (1-3), p.37-42
• Main Authors: Xiao, Dong, Hines, Larry G., Holtz, Mark W., Song, Kihyung, Bartsch, Richard A., Quitevis, Edward L.
• Format: Article
• Language: English
• ISSN: 0009-2614; 1873-4448
• DOI: 10.1016/j.cplett.2010.07.085

[Display omitted] ► 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. 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 0–200cm−1 region to intermolecular and intramolecular vibrational modes of the liquid.