Molecular reorientational dynamics of the neat ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate by measurement of 13C nuclear magnetic relaxation data

The reorientational dynamics of the ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM]PF6) were studied over a wide range of temperatures of measurement of C-13 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-Purcell-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. The 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.