Ionic liquids are an active area of research due to the unique properties offered by these materials compared to traditional molecular solvents. As such, there is a strong motivation to characterize the liquid structure of ionic liquids to better understand the factors that control these properties. Vibrational spectroscopy is frequently used to investigate interactions among ions, for an ion’s intramolecular vibrational modes are sensitive to its local potential energy environment. Therefore, variations in band intensities and frequencies can provide molecular-level information concerning ion-ion interactions for an ionic liquid. It is frequently the case that strongly absorbing IR bands of ionic liquids exhibit asymmetric shapes; this particularly true for vibrational motions of the anions comprising the ionic liquid. We provide evidence that this band asymmetry originates from long-range coupling of intramolecular vibrational motions across the charge-organized, quasilattice liquid structure of an ionic liquid. According to this model, the local electric field an ion experiences will be different for longitudinal optical (LO) and transverse optical (TO) phonons, leading to band splitting. TO modes may absorb IR radiation, but the sample does not support the propagation of electromagnetic radiation with frequencies between the TO and LO mode frequencies. Accordingly, IR bands are dominated by an intense feature near the TO mode frequency with a high-frequency shoulder whose intensity depends on the degree of TO-LO band splitting. Example infrared spectra for a series of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is provided in the left panel of the figure to illustrate this band splitting. The amount of TO-LO splitting is found to be linearly correlated to the ionic liquid number densities; an observation that is in accord with the predictions of dipolar coupling theory. Temperature- and pressure-dependent IR spectra are presented to support the application of dipolar coupling theory in explaining TO-LO band splitting patterns for ionic liquids. Implications concerning the effect of quasilattice structure on TO-LO splitting are also discussed.

Implications of liquid structure on the vibrational spectra of ionic liquids

255th National Meeting (2018)

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