Isomer Populations in Liquids for 1-Isopropyl-3-methylimidazolium Bromide and Its Iodide and Their Conformational Changes Accompanying the Crystallizing and Melting Processes

Most ionic liquids (ILs) are characterized by the presence of some kind of isomer for component ions. We investigated the liquid and crystalline structures of 1-isopropyl-3-methylimidazolium halides ([<i>i</i>-C₃mim]X, X = Br, I) by means of Raman scattering measurements backed up by density functional theory (DFT) calculations. The DFT calculations indicated that there are two stable rotational isomers of [<i>i</i>-C₃mim]⁺, one of an asymmetric form (<b>Asym</b>) and the other a symmetric one (<b>Sym</b>). For both salts, it was revealed that <b>Asym</b> and <b>Sym</b> coexist in liquid states, in contrast to the presence of only <b>Asym</b> in crystalline states. We determined the isomer populations and showed that <b>Asym</b> is dominant in the liquid states of both salts (approximately 7/3). Conformational changes in [<i>i</i>-C₃mim]⁺ during the crystallizing and melting processes were observed by conducting simultaneous measurements using the techniques of Raman spectroscopy and calorimetry. We have revealed that the conformational change from <b>Asym</b> to <b>Sym</b> occurs gradually, with linking to melting in the premelting region. This is a conclusive observation of the structural changes of ILs in the premelting region and demonstrates the conformational changes between isomers during the crystallizing and melting processes.