The influence of polarizability on the dielectric spectrum of the ionic liquid 1-ethyl-3-methylimidazolium triflate

Abstract

This work reports for the first time the computational, frequency-dependent dielectric spectrum of the polarizable molecular ionic liquid 1-ethyl-3-methylimidazolium triflate as well as its experimental analogue. In the frequency range from 500 MHz up to 20 GHz the agreement between the computational and the experimental spectrum is quantitative. For higher frequencies up to 10 THz the agreement is still remarkably good. The experimental asymptotic limit e N is 2.3. The difference in the computational value of 1.9 comes solely from the neglect of polarizability of the hydrogen atoms. For reasons of efficiency the simulations are based on the Lagrangian algorithm for the Drude oscillator model which cannot handle polarizable hydrogens. In the computational analysis the complete spectrum of the generalized dielectric constant P Ã 0 ðnÞ is splitted into its translational and non-translational components, called dielectric conductivity W 0 (n) and dielectric permittivity e(n). For 1-ethyl-3-methylimidazolium triflate both components contribute with equal weight and overlap in the complete frequency range. The inclusion of polarization forces, however, is quite different for the two components: the collective non-translational dynamics is accelerated and hence the dielectric permittivity is shifted to higher frequencies. The low frequency region of the dielectric conductivity is also affected while its high frequency part remains almost unchanged. Inductive effects are not only visible at high frequencies but also contribute in the sub-GHz region. The computational peak found in this region correlates with the experimental OKE-spectrum. It may be interpreted as the correlation between the induced dipole moment of the cations and the local electric field exerted by the anionic cage.

Dielectric relaxation and ultrafast transient absorption spectroscopy of [C6mim]+[Tf2N]−/acetonitrile mixtures

“…5 However, the high-frequency mode is just a formal description of the significant contributions from librations and intermolecular vibrations dominating the far-infrared region. 5,8,25 Consistent with the bimodal character of the spectra at intermediate concentrations ( Fig. 2) and similar to IL + dichloromethane mixtures 25 all mixture spectra could be well fitted with the CC+D model (Fig.…”

Dynamics of RTILs: A comparative dielectric and OKE study

“…Simulations suggest that at least for imidazolium RTILs the α relaxation is not a pure rotational motion but also significantly involves translations of cations and anions [47]. This might explain why the effective volumes of rotation derived from a Stoke-Einstein-Debye analysis of τ α yields values incompatible with cation reorientation via rotational diffusion [35].…”

“…This mainly translational mode is associated with the rattling of molecules in their cage of surrounding neighbors superimposed on the formation and decay of these cages. An argument from MD simulations if favor of such an assignment also for RTILs is provided by the strong collective translational contributions (interionic vibrations) to the simulated dielectric spectrum in this frequency range [47,53]. 18.2 ± 1.…”

Do H-bonds explain strong ion aggregation in ethylammonium nitrate + acetonitrile mixtures?

“…Note that this empirical decomposition of the dielectric spectra has to be taken with a grain of salt as computer simulations of Schröder et al have clearly shown that at least for imidazolium-based ionic liquids and their mixtures with water rotational and translational modes of all components stretch over large frequency ranges and strongly overlap. [39][40][41] However, as long as quantitative agreement between experimental and simulated dielectric spectra cannot be routinely achieved empirical decompositions, like the present CC + D or HN i + D models, have to be used.…”