Temperature- and pressure-dependent infrared spectroscopy of 1-butyl-3-methylimidazolium trifluoromethanesulfonate: A dipolar coupling theory analysis

Christopher M Burba; Hai-Chou Chang

doi:10.1016/j.saa.2017.12.026

Temperature- and pressure-dependent infrared spectroscopy of 1-butyl-3-methylimidazolium trifluoromethanesulfonate: A dipolar coupling theory analysis

Spectrochim Acta A Mol Biomol Spectrosc. 2018 Mar 15:193:338-343. doi: 10.1016/j.saa.2017.12.026. Epub 2017 Dec 6.

Authors

Christopher M Burba¹, Hai-Chou Chang²

Affiliations

¹ Department of Natural Sciences, Northeastern State University, 611 N. Grand Ave., Tahlequah, OK, United States. Electronic address: burba@nsuok.edu.
² Department of Chemistry, National Dong Hwa University, Taiwan.

PMID: 29268233
DOI: 10.1016/j.saa.2017.12.026

Abstract

Continued growth and development of ionic liquids requires a thorough understanding of how cation and anion molecular structure defines the liquid structure of the materials as well as the various properties that make them technologically useful. Infrared spectroscopy is frequently used to assess molecular-level interactions among the cations and anions of ionic liquids because the intramolecular vibrational modes of the ions are sensitive to the local potential energy environments in which they reside. Thus, different interaction modes among the ions may lead to different spectroscopic signatures in the vibrational spectra. Charge organization present in ionic liquids, such as 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([C₄mim]CF₃SO₃), is frequently modeled in terms of a quasicrystalline structure. Highly structured quasilattices enable the dynamic coupling of vibrationally-induced dipole moments to produce optical dispersion and transverse optical-longitudinal optical (TO-LO) splitting of vibrational modes of the ionic liquid. According to dipolar coupling theory, the degree of TO-LO splitting is predicted to have a linear dependence on the number density of the ionic liquid. Both temperature and pressure will affect the number density of the ionic liquid and, therefore, the amount of TO-LO splitting for this mode. Therefore, we test these relationships through temperature- and pressure-dependent FT-IR spectroscopic studies of [C₄mim]CF₃SO₃, focusing on the totally symmetric SO stretching mode for the anion, ν_s(SO₃). Increased temperature decreases the amount of TO-LO splitting for ν_s(SO₃), whereas elevated pressure is found to increase the amount of band splitting. In both cases, the experimental observations follow the general predictions of dipolar coupling theory, thereby supporting the quasilattice model for this ionic liquid.

Keywords: Dipolar coupling theory; Infrared spectroscopy; Ionic liquids; Pressure; Quasilattice structure; Temperature.