Isomer Populations in Liquids for 1-Isopropyl-3-methylimidazolium Bromide and Its Iodide and Their Conformational Changes Accompanying the Crystallizing and Melting Processes
2008-08-21T00:00:00Z (GMT) by
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>-C<sub>3</sub>mim]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>-C<sub>3</sub>mim]<sup>+</sup>, 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>-C<sub>3</sub>mim]<sup>+</sup> 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.