Collective rotational dynamics in ionic liquids: A computational and experimental study of 1-butyl-3-methyl-imidazolium tetrafluoroborate

Abstract

The aim of this study is the analysis of the rotational motion in ionic liquids, in particular, 1-butyl-3-methyl-imidazolium tetrafluoroborate. By comparing single-particle and collective motion it is found that the Madden-Kivelson relation is fairly fulfilled in long-term simulation studies (>100 ns), i.e., the collective reorientation can be predicted by the corresponding single-particle property and the static dipolar correlation factor, GK. Furthermore, simulated reorientation is in accordance with hydrodynamic theories yielding hydrodynamic radii comparable to van der Waals radii. Since viscosity is the central quantity entering hydrodynamic formulas, we calculated and measured the viscosity of our system in order to have two independent cycles of hydrodynamic evaluation, a computational and an experimental one. While the static dielectric constant agrees with dielectric reflectance experiment, the hydrodynamic radii derived from the experiments are much lower as a consequence of enhanced rotational motion. Even more, a considerable dynamic broadening is observed in the experiments.

Communication: Are hydrodynamic models suited for describing the reorientational dynamics of ions in ionic liquids? A case study of methylimidazolium tetra(hexafluoroisopropoxy)aluminates

“…Thus, Ј͑ ͒ and Љ͑ ͒ may still involve frequency-dependent conductance contributions which cannot be separated experimentally from the rotational contributions. MD simulations of ILs 18 and molten NaCl 19 provide, however, no evidence for such contributions in the microwave regime besides the ionic loss. In addition, there lacks evidence for spectral contributions from ion aggregates such as dipolar ion pairs.…”

“…13 As a perturbing feature, MD simulations of dielectric spectra of ILs. 18 do not indicate such anomalies, although the question was not addressed in detail.…”

“…17 There are several scenarios for the origin of the lowfrequency mode which can involve frequency-dependent contributions due to charge transport and orientational dynamics of dipolar species. The gross picture provided by experiments 3,4 and molecular dynamics ͑MD͒ simulations 18,19 for similar ILs attributes the low-frequency mode mainly to collective orientational relaxation of the dipolar cations. Given the high symmetry of ͓Al͑hfip͒ 4 ͔ − , the anion contribution may be neglected in the present case as well.…”

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

“…21 The important point to stress here is that it is the generalized dielectric constant, S*(o), which is measured experimentally by dielectric relaxation spectroscopy 15,44,45 and which can be computed by molecular dynamics simulation. 21,41,[46][47][48] Whilst the nuclear degrees of freedom describing translation and rotation are usually explicitly considered in computer simulations, the electronic degrees are either implicitly contained in the applied force field or may be explicitly modelled by introducing (electronic) polarizability. Since in the former case electrostatic forces are restricted to the interactions between permanent charge distributions, these cannot respond to an external electric field, E 0 (o), of very high frequency, o.…”

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

“…[5][6][7] DRS provides not only static dielectric constants, but also, e.g., accurate frequencies of solvent modes, which can be assigned with the aid of sophisticated molecular dynamics simulations. [8][9][10] Complementary approaches use local, ''microscopic'' molecular probes. This concept is more closely linked to chemical reactivity, because it is the local surrounding or the ''cybotactic'' region of a molecule which crucially influences the outcome of reactions.…”