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

Abstract

As a probe of local structure, the vibrational properties of the 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] ionic liquidwere studied by infrared (IR), Raman spectroscopy, and ab initio calculations. The coexistence of at least four [bmim]+ 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.