Existence of optical phonons in the room temperature ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate

Abstract

The technologically important properties of room temperature ionic liquids (RTILs) are fundamentally linked to the ion-ion interactions present among the constituent ions. These ion-ion interactions in one RTIL (1-ethyl-3-methylimidazolium trifluoromethanesulfonate, [C(2)mim]CF(3)SO(3)) are characterized with transmission FTIR spectroscopy and polarized attenuated total reflection (ATR) FTIR spectroscopy. A quasilattice model is determined to be the best framework for understanding the ionic interactions. A novel spectroscopic approach is proposed to characterize the degree of order that is present in the quasilattice by comparing the dipole moment derivative calculated from two independent spectroscopic measurements: (1) the TO-LO splitting of a vibrational mode using dipolar coupling theory and (2) the optical constants of the material derived from polarized ATR experiments. In principle, dipole moment derivatives calculated from dipolar coupling theory should be similar to those calculated from the optical constants if the quasilattice of the RTIL is highly structured. However, a significant disparity for the two calculations is noted for [C(2)mim]CF(3)SO(3), indicating that the quasilattice of [C(2)mim]CF(3)SO(3) is somewhat disorganized. The potential ability to spectroscopically characterize the structure of the quasilattice, which governs the long-range ion-ion interactions in a RTIL, is a major step forward in understanding the interrelationship between the molecular-level interactions among the constituent ions of an ionic liquid and the important physical properties of the RTIL.

Using FT-IR Spectroscopy to Measure Charge Organization in Ionic Liquids

“…Dipolar coupling theory 14 has been previously used to relate the magnitude of the TO-LO splitting to the dipole moment derivatives of vibrational modes in a variety of isotropic and anisotropic crystals 15–17 , disordered salts 18,19 , and molten salts 8,19 . A simplified form of dipolar coupling theory results in the following expression for the TO-LO splitting of a normal mode in a unimolecular unit cell: 16 …”

“…The dipole moment derivative for a particular vibrational mode may be determined by integrating the extinction coefficient data, 8 …”

“…Burba and Frech 8 proposed a solution to this problem by quantifying quasilattice structure with simple FT-IR spectroscopic experiments. The heart of the approach is to compare two independent measurements of the dipole moment derivative for select vibrational modes of the ions composing the ionic liquid.…”

Di‐ureasil Hybrid Electrolytes Incorporating a New Proton Ionic Liquid

“…PILs with more delocalized chargesa nd fewer ionic interactions are expected to exhibit higher conductivities. [35,36] In ILs, ion-ion interactions (strong short-range cohesive forces) give rise to long-range, correlated ion motion,w hich is responsible for ion transport. As ac onsequence, PILs with shorter alkyl chains (lowerm olecular weights) exhibit higherc onductivity.…”

“…The ionic conductivity of PILs is also known to decreasea st he size of the cation is increased and becomes less mobile. Attempts to shed some light onto this issue by means of infrared spectroscopy were proposed by Burba et al [35,36] These authors concluded that in the presence of large anions and asymmetrical imidazolium cations, ILs exhibit low degrees of charge ordering. Although the literature suggestst hat the trendso bserved for anions are less clear, [31] in the present study this effect is not relevant,a st he two PILs under analysis contain the same anion.…”