We investigated the structures of ionic liquids (1-butyl-3-methylimidazolium iodide [BMIM][I] and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF 4 ]) and their aqueous mixtures using attenuated total reflection (ATR) infrared absorption and Raman spectroscopy. The ATR spectrum in the CH x (x ) 1, 2, 3) vibration region from 2800 to 3200 cm [BF 4 ]. These differences are considered to come from the variation in the position of the anion, where I -is expected to be closer to the C(2) hydrogen of the imidazolium cation and interacting more specifically as compared to BF 4 -.

Mixtures of ionic liquid (IL, 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4]) and water with varying concentrations were studied by attenuated total reflection infrared absorption and Raman spectroscopy. Changes in the peak intensities and peak positions of CHx (x = 1, 2, 3) vibration modes of the cation of the IL and OH vibration modes of the water molecules were investigated. Peaks from normal-mode stretch vibrations of CH bonds belonging to the imidazolium ring of the cation did not change their positions, while those from the terminal methyl group of the butyl chain blueshifted by approximately 10 cm-1 with the addition of water. On the other hand, change in the spectral shape in the OH stretch vibration region shows hydrogen-bonding network of water molecules breaking down rapidly as the IL is added. Trends in the change of the peak positions and the peak intensities suggested qualitative change of the intermolecular structure in the [BMIM][BF4] + H2O mixture at 32 +/- 2 and 45 +/- 2 mol/L of water concentration.