Raman and Infrared Spectra and ab Initio Calculations of C2-4MIM Imidazolium Hexafluorophosphate Ionic Liquids

Abstract

The Raman and infrared spectra of a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ([C2-4MIM]PF6) ionic liquids have been recorded and analyzed using density functional theory (DFT) and RHF methods at the 6-311+G(2d,p) computational level. The DFT calculations reproduce the vibrational spectra of 1-ethyl-3-methyl imidazolium hexafluorophosphate [EMIM]PF6, 1-propyl-3-methyl imidazolium hexafluorophosphate [PMIM]PF6, and 1-butyl-3-methyl imidazolium hexafluorophosphate [BMIM]PF6 using correction factors of 0.964−0.967 with correlation coefficients R2 of 0.999. The vibrational spectra calculated at the RHF/6-311+G(2d,p) level require a correction factor of 0.89 and a correlation coefficient R2 of 0.999 using the fully optimized structures. The 1-alkyl-3-methyl hexafluorophosphate ionic liquids have common Raman C−H stretching frequencies that may serve as possible probes in studies of ionic liquid interactions. The DFT (B3LYP) and RHF gas-phase molecular structures of the [C2-4MIM]PF6 ion pairs indicate hydrogen bonding interactions between the fluorine atoms of the PF6- anion and the C2 hydrogen on the imidazolium ring. Additional interactions are observed between PF6- and the H atoms on the adjacent alkyl side chains.