Liquid Structure of Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids Assessed by FT-IR Spectroscopy

Ionic liquids are a fertile and active area of research, in part, due to the unique properties these solvents offer over traditional molecular solvents. Because these properties are rooted in the fundamental ion-ion interactions that govern their liquid structure, there is a strong motivation to characterize the liquid structure of ionic liquids. Infrared spectroscopy is a standard analytical tool for assessing liquid structures, for the intramolecular vibrational modes of the ions composing the materials are often quite sensitive to their local potential energy environment. In this work, we demonstrate that the band asymmetry for the ν_a(SNS) anion mode of N(Tf)₂^--based ionic liquids originates from the dynamic coupling of vibrationally induced dipole moments of anions across a quasilattice. The magnitude of TO-LO splitting is linearly correlated with the number densities of the ionic liquids; an observation that is in accord with the predictions of dipolar coupling theory. Dipole moment derivatives of ν_a(SNS) calculated from dipolar coupling theory, (∂μ/∂q)_DCT, are lower than those obtained from independent measurements of (∂μ/∂q). The most likely explanation for this disparity is that although ionic liquids possess sufficient long-range structure to support TO-LO splitting of infrared-active modes, there is enough orientational and translational disorder in the quasilattice to partially disrupt the coupling of vibrationally induced dipole moments across the quasilattice. This will result in diminished amounts of TO-LO splitting than would be expected if the ionic liquid were a perfect crystal at 0 K. Impacts of cation molecular structure and the formation of a binary solution on the liquid structure are also explored.