The Raman and infrared spectra of a series of 1-alkyl-3-methylimidazolium hexafluorophosphate ([C 2-4 MIM]PF 6 ) 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]PF 6 , 1-propyl-3-methyl imidazolium hexafluorophosphate [PMIM]PF 6 , and 1-butyl-3-methyl imidazolium hexafluorophosphate [BMIM]PF 6 using correction factors of 0.964-0.967 with correlation coefficients R 2 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 R 2 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 [C 2-4 MIM]PF 6 ion pairs indicate hydrogen bonding interactions between the fluorine atoms of the PF 6anion and the C2 hydrogen on the imidazolium ring. Additional interactions are observed between PF 6and the H atoms on the adjacent alkyl side chains.

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The reorientational dynamics of the ionic liquid 1-butyl-3-methylimidzolium hexafluorophosphate ([BMIM]PF6) were studied over a wide range of temperatures by measurement of 13C spin-lattice relaxation rates and NOE factors. The reorientational dynamics were evaluated by performing fits to the experimental relaxation data. Thus, the overall reorientational motion was described by a Cole-Davidson spectral density with a Vogel-Fulcher-Tammann temperature dependence of the correlation times. The reorientational motion of the butyl chain was modelled by a combination of the latter model for the overall motion with a Bloembergen-Pur-cell-Pound spectral density and an Arrhenius temperature dependence for the internal motion. Except for C2 in the aromatic ring, an additional reduction of the spectral density by the Lipari-Szabo model had to be employed. This reduction is a consequence of fast molecular motions before the rotational diffusion process becomes effective. The C2 atom did not exhibit this reduction, because the librational motion of the corresponding C2-H vector is severely hindered due to hydrogen bonding with the hexafluorophosphate anion. The observed dynamic features of the [BMIM]+ cation confirm quantum-chemical structures obtained in a former study.