The influence of polarizability on the dielectric spectrum of the ionic liquid1-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 ε_∞ 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 is splitted into its translational and non-translational components, called dielectric conductivity ϑ₀(ν) and dielectric permittivity ε(ν). 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.