Complementing Crystallography with Ultralow-Frequency Raman Spectroscopy: Structural Insights into Nitrile-Functionalized Ionic Liquids

Abstract

Functionalized ionic liquids are a subclass of ionic liquids that are tailored for a specific application. Structural characterization in both solid and liquid phases is central to understanding their physical properties. Here, we used ultralow-frequency Raman spectroscopy, which can measure Raman spectra down to approximately 5cm-1, to study the structures and physical properties of 1-(4-cyanobenzyl)-3-methylimidazolium salts with five different anions. A comparison of the observed low-frequency Raman spectral patterns enabled us to predict the crystal symmetry of one of the synthesized salts for which single-crystal X-ray diffraction data were unobtainable. Real-time tracking of the low-frequency Raman spectral changes during melting revealed peak shifts indicative of different degrees of microscopic heterogeneity in the ionic liquids. The results show that our method provides a facile means that is complementary to X-ray crystallography, for obtaining structural information of ionic liquids. Low frequency, high potential: Fast ultralow-frequency Raman spectroscopy is used to study the structures, intermolecular interactions, and melting processes of a new series of nitrile-functionalized ionic liquids. This method not only provides information on crystal symmetry that is complementary to crystallography, but it also reveals the distinct nature of microscopic heterogeneity in the ionic liquids studied. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.