Raman Spectroscopic Study on Alkyl Chain Conformation in 1-Butyl-3-methylimidazolium Ionic Liquids and their Aqueous Mixtures

Ionic liquids of 1-butyl-3-methylimidazolium ([BMIM]) cation with different anions (Cl-, Br-, I-, and BF4-), and their aqueous mixtures were investigated by using Raman spectroscopy and dispersion-included density functional theory (DFT). The characteristic Raman bands at 600 and 624 cm(-1) for two isomers of the butyl chain in the imidazolium cation showed significant changes in intensity for different anions as well as in aqueous solutions. The area ratio of these two bands followed the order I- > Br- > Cl- > BF4- (in terms of the anion X in [BMIM]X), indicating that the butyl chain of [BMIM]I tends to adopt the trans conformation. The butyl chain was found to adopt the gauche conformation upon dilution, irrespective of the anion type. The Raman bands in the butyl C-H stretch region for [BMIM]X (X= Cl-, Br-, and I-) blueshifted significantly with the increase in the water concentration, whereas that for [BMIM]BF4 changed very little upon dilution. The blueshift in the C-H stretch region upon dilution also followed the order: [BMIM]I> [BMIM]Br>[BMIM]Cl>[BMIM]BF4, the same order as the above trans conformation preference of the butyl chain in pure imidazolium ionic liquids, which suggested that the cation-anion interaction plays a role in determining the conformation of the chain.