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.

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A new method of obtaining molecular reorientational dynamics from 13C spin-lattice relaxation data of aromatic carbons in viscous solutions is applied to 13C relaxation data of the ionic liquid, 1-methyl-3-nonylimidazolium hexafluorophosphate ([MNIM]PF6). Spin-lattice relaxation times (13C) are used to determine pseudorotational correlation times for the [MNIM]PF6 ionic liquid. Pseudorotational correlation times are used to calculate corrected maximum NOE factors from a combined isotropic dipolar and nuclear Overhauser effect (NOE) equation. These corrected maximum NOE factors are then used to determine the dipolar relaxation rate part of the total relaxation rate for each aromatic 13C nucleus in the imidazolium ring. Rotational correlation times are compared with viscosity data and indicate several [MNIM]PF6 phase changes over the temperature range from 282 to 362 K. Modifications of the Stokes-Einstein-Debye (SED) model are used to determine molecular radii for the 1-methyl-3-nonylimidazolium cation. The Hu-Zwanzig correction yields a cationic radius that compares favorably with a DFT gas-phase calculation, B3LYP/(6-311+G(2d,p)). Chemical shift anisotropy values, delta sigma, are obtained for the ring and immediately adjacent methylene and methyl carbons in the imidazolium cation. The average delta sigma values for the imidazolium ring carbons are similar to those of pyrimidine in liquid crystal solutions.