Influence of the alkyl chain on the vibrational structure and interionic interactions in 1-alkyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids

Owing to the structurally simple anion, neat 1-alkyl-3-methylimidazolium trifluoromethanesulfonate (C(n)mimTfO) ionic liquids are potential reference fluids. The present study investigates how the alkyl chain length in the imidazolium cation influences the vibrational spectra and the interactions between the anion and the cation. For this purpose, IR spectroscopy is employed. The spectra reveal that the interionic hydrogen bonding is virtually unaffected by the alkyl chain. Significant frequency changes with increasing chain length, however, were found for the rocking modes involving the imidazolium nitrogen atoms. This was evident as a shape change of a peak that was previously assigned to CF stretching exclusively. Our results show that the NCH rocking signatures are notably strong and overlap to yield a complex spectroscopic behavior. From a spectroscopic viewpoint, TfO-based ILs are more suitable reference fluids compared to the common but much more complex bis(trifluoromethanesulfonyl)imide NTf2 compounds. (C) 2018 Elsevier B.V. All rights reserved.