Ionic liquid structure:: the conformational isomerism in 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim] [BF4])

As a probe of local structure, the vibrational properties of the 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] ionic liquid were studied by infrared (IR), Raman spectroscopy, and ab initio calculations. The coexistence of at least four Ibmiml' conformers (GG, GA, TA, and AA) at room temperature was established through unique spectral responses. The Raman modes characteristic of the two most stable [bmim](+) conformers, GA and AA, according to the ab initio calculations, increase in intensity with decreasing temperature. To assess the total spectral behavior of the ionic liquid both the contributions of different [bmim](+) conformers and the [bmim](+) - [BF4](-) interactions to the vibrational spectra are discussed. Copyright (c) 2008 John Wiley & Sons, Ltd.