Vibrational spectra of imidazolium tetrafluoroborate ionic liquids

Abstract

The Raman and infrared spectra of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids ([C2-4MIM][BF 4]) 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][BF 4], 1-propyl-3-methyl imidazolium tetrafluoroborate [PMIM][BF 4] and 1-butyl-3-methyl imidazolium tetrafluoroborate [BMIM][BF 4] using correction factors of 0.963–0.967 (DFT) and 0.913–0.916 (RHF) with correlation coefficients R 2 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 [C 2–4MIM][BF 4] ion pairs suggest hydrogen bonding interactions between the fluorine atoms of the BF 4 − anion and the C2 hydrogen on the imidazolium ring. Additional interactions are observed between [BF 4] and H atoms on the adjacent alkyl side chains in all polymorphic structures.