Structure of an ionic liquid, 1-n-butyl-3-methylimidazolium iodide, studied by wide-angle X-ray scattering and Raman spectroscopy

The structure of a room temperature ionic liquid (IL), 1-n-butyl-3-methylimidazolium iodide ([bmim]I), is studied by wide-angle X-ray scattering (WAXS) and Raman spectroscopy. The radial distribution function obtained by WAXS shows prominent peaks ascribable to a structure constructed by iodide anions; this means that the structure of the anions has a long-range correlation. The Raman spectrum of liquid [bmim]I agrees with those of [bmim]Cl and [bmim]Br in the molten phase and in crystals. The results show that the cation structures in the liquids of three [bmim] halides are similar to those in the crystals. However, the arrangement of the iodide anions in liquid [bmim]I differs slightly from those in crystalline [bmim]Cl and [bmim]Br, although it partly retains the hydrophilic channel structure characteristic of the crystals.