Vibrational spectra of imidazolium tetrafluoroborate ionic liquids

The Raman and infrared spectra of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids ([C2-4MIM][BF4]) are reported and analyzed using Density Functional Theory (DFT) and RHF methods at the 6-311+G(2d,p) computational level. The B3LYP (DFT) and RHF calculations reproduce the vibrational spectra of 1-ethyl-3-methyl imidazolium tetrafluoroborate [EMIM][BF4], 1-propyl-3-methyl imidazolium tetrafluoroborate [PMIM][BF4] and 1-butyl-3-methyl imidazolium tetrafluoroborate [BMIM][BF4] using correction factors of 0.963-0.967 (DFT) and 0.913-0.916 (RHF) with correlation coefficients R2 of 0.999 using the fully optimized structures. Theoretical (DFT) alkyl side chain conformational changes in the 1-alkyl-3-methyl tetrafluoroborate ionic liquids have only a limited effect on the theoretical gas phase vibrations. The gas phase molecular structures of the [C2-4MIM][BF4] ion pairs suggest hydrogen bonding interactions between the fluorine atoms of the BF4− anion and the C2 hydrogen on the imidazolium ring. Additional interactions are observed between [BF4] and H atoms on the adjacent alkyl side chains in all polymorphic structures.