Rotational Isomerism and Structure of the 1-Butyl-3-methylimidazolium Cation in the Ionic Liquid State

Abstract The crystal structures of 1-butyl-3-methylimidazolium chloride [bmim]Cl and 1-butyl-3-methylimidazolium bromide [bmim]Br show that two rotational isomers, the TT form and the GT form, of the [bmim]+ cation exist in the crystalline state. A vibrational analysis based on a DFT calculation indicates that two characterstic Raman bands of crystalline [bmim]Cl and three of crystalline [bmim]Br can be used as marker bands of the rotational isomerism around the C7–C8 bond of the n-butyl group. The Raman spectra of liquid [bmim]BF4, in which both sets of marker bands are simultaneously observed, then prove that at least two rotational isomers of the [bmim]+ cation coexist in the ionic liquid state.