Abstract
The rotational dynamics of the hexafluorophosphate anion (PF(6)(-)) in the crystalline and liquid states of the archetypal room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF(6)) are investigated using (31)P NMR spectroscopy line shape analyses and spin-lattice relaxation time measurements. The PF(6)(-) anion performs isotropic rotation in all three polymorphic crystals phases α, β, and γ as well as in the liquid state with a characteristic time scale that ranges from a few ps to a few hundred ps over a temperature range of 180-280 K. The rotational correlation time τ(c) for PF(6)(-) rotation follows the sequence γ-phase < α-phase ≈ liquid < β-phase. On the other hand, in the liquid state, all local motions in the cation as well as its global rotational reorientation are characterized by time scales that are slower compared to that for the PF(6)(-) anion rotation. The time scale τ(c) and the activation energy of PF(6)(-) rotation in this RTIL are found to be comparable with those observed in ordinary alkali and ammonium salts despite the large counterion size and low melting point of the former. The high sphericity of the PF(6)(-) ion is hypothesized to play an important role in the decoupling of its rotational dynamics that appear to be practically independent of the averaged cation-anion interaction.
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