Precise dielectric spectra have been determined at 25 °C over the exceptionally broad frequency range of 0.1 ≤ ν/GHz ≤ 3000 for the imidazolium-based room-temperature ionic liquids (RTILs) [bmim][BF 4 ], [bmim][PF 6 ], [bmim][DCA], and [hmim][BF 4 ]. The spectra are dominated by a low-frequency process at ∼1 GHz with a broad relaxation time distribution of the Cole−Davidson or Cole−Cole type, which is thought to correspond to the rotational diffusion of the dipolar cations. In addition, these RTILs possess two Debye relaxations at ∼5 GHz and ∼0.6 THz and a damped harmonic oscillation at ∼2.5 THz. The two higher-frequency modes are almost certainly due to cation librations, but the origin of the ∼5 GHz mode remains obscure.
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Stoppa et al. (2008) studied this question.
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