Martin Brehm

Unexpected Hydrogen Bond Dynamics in Imidazolium-Based Ionic Liquids

Employing first-principles molecular dynamics simulations, we characterize the structural and dynamical hydrogen bonding in the ionic liquid [C(2)C(1)im][SCN]. The geometric picture indicates a superior role for the most acidic hydrogen bond (at H2) as compared to the two other hydrogen atoms at the rear. Despite the structural picture, the hydrogen bond dynamics at H2 is found to decay faster than the according dynamics at the H4 and H5 proton. Neglecting the directionality provides a dynamics which reflects the geometrical analysis. Two movements are identified. First, a fast (<0.3 ps) hopping of the anion above and below the imidazolium ring and second translational motion of the anion away from the cation in-plane of the imidazolium ring (5-10 ps).

Performance of Quantum Chemically Derived Charges and Persistence of Ion Cages in Ionic Liquids. A Molecular Dynamics Simulations Study of 1-n-Butyl-3-methylimidazolium Bromide

We carried out classical molecular dynamics simulations with a standard and two quantum chemistry based charge sets to study the ionic liquid 1-n-butyl-3-methylimidazolium bromide, [C(4)C(1)im][Br]. We split the cation up into different charge groups and found that the total charge and the charge distribution in the imidazolium ring are completely different in the three systems while the total charge of the butyl chain is much better conserved between the methods. For comparison, the spatial distribution functions and the radial distribution functions as well as different time correlation functions were calculated. For the structural properties we obtained a good agreement between the standard and one of the two quantum chemistry based sets, while the results from the second quantum chemistry based set led to a completely different picture. The opposite was observed for the dynamic properties, which agree well between the standard set and the second quantum chemistry based set, whereas the dynamics in the first charge set obtained by quantum chemistry calculations proceeded much too slow, which is not obvious from the total charge. We observed, that the structure of the butyl chain is mostly unaffected by the choice of the charge set. This is an indirect proof for separation into ionic parts and nonpolar domains. A second focus of the article is the investigation of dynamical heterogeneity and the ion cages. Therefore, we analyzed the reorientational dynamics in the three systems and at five different temperatures in system with the standard charge set. Generally speaking, we detected four different time domains. The fastest movement can be found for the continuous hydrogen bond and the nearest neighbor ion pair dynamics. In the second time domain the movement of the butyl chain took place. The third time domain consisted in the increasing movement of the imidazolium ring as well as in the continuous distortion of an ion cage, i.e., the departure of one of the several counterions from the central ions first shell, and the intermittent hydrogen bond dynamics. The remaining domain involves the translational displacement of the ions.

How hydrogen bonds influence the mobility of imidazolium-based ionic liquids. A combined theoretical and experimental study of 1-n-butyl-3-methylimidazolium bromide.

The virtual laboratory allows for computer experiments that are not accessible via real experiments. In this work, three previously obtained charge sets were employed to study the influence of hydrogen bonding on imidazolium-based ionic liquids in molecular dynamics simulations. One set provides diffusion coefficients in agreement with the experiment and is therefore a good model for real-world systems. Comparison with the other sets indicates hydrogen bonding to influence structure and dynamics differently. Furthermore, in one case the total charge was increased and in another decreased by 0.1 e. Both the most acidic proton as well as the corresponding carbon atom were artificially set to zero, sequentially and simultaneously. In the final setup a negative charge was placed on the proton in order to introduce a barrier for the anion to contact the cation via this most acidic hydrogen atom. The following observations were made: changing the hydrogen bonding ability strongly influences the structure while the dynamic properties, such as diffusion and viscosity, are only weakly changed. However, the introduction of larger alterations (stronger hydrogen bonding and antihydrogen bonding) also strongly influences the diffusion coefficients. The dynamics of the hydrogen bond, ion pairing, and the ion cage are all affected by the level of hydrogen bonding. A change in total charges predominantly influences transport properties rather than structure. For ion cage dynamics with respect to transport porperties, we find a good correlation and a weak or no correlation for the ion pair or the hydrogen bond dynamics, respectively. Nevertheless, the hydrogen bond does influence ion cage dynamics. Therefore, we confirm that ionic liquids rather consist of loosely interacting counterions than of discrete ion pairs. Hydrogen bonding affects the properties only in a secondary or indirect manner.

Effect of Dispersion on the Structure and Dynamics of the Ionic Liquid 1‐Ethyl‐3‐methylimidazolium Thiocyanate

We present a comprehensive density functional study, using the Perdew-Burke-Ernzerhof (PBE) functional, to elucidate the effect of including or neglecting the dispersion correction on the structure and dynamics of the ionic liquid 1-ethyl-3-methylimidazolium thiocyanate. We have investigated the structure of the liquid phase and observed that specific interactions between the anions and cations of the ionic liquid were not accurately represented if the dispersion was neglected. The dynamics of the system is more accurately described if the dispersion correction is taken into account and its omission also leads to an incorrect representation of the hydrogen-bonding dynamics. Finally, the power spectrum is predicted and in good agreement with experimental results. Thus, we conclude that it is possible to represent the structure and dynamics of systems containing ionic liquids accurately using ab initio molecular dynamics and a correction for dispersion.

Short Time Dynamics of Ionic Liquids in AIMD-Based Power Spectra

Power spectra of several imidazolium-based ionic liquids, 1,3-dimethylimidazolium chloride, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium dicyanamide 5, 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium thiocyanate, and 1-butyl-3-methylimidazolium dicyanamide, are presented based on ab initio molecular dynamics simulations. They provide an alternative tool of analysis of several electronic structure-based properties, in particular, those related to the strength of hydrogen bonding in liquids. Moreover, they can be employed to interpret experimental IR or Raman spectra, avoiding the additional calculations required for theoretical IR or Raman spectra. The obtained power spectra are shown to be in good agreement with experimental spectra, and electronic structure properties related to them are analyzed. Further, there are indications for a locality of the power spectra on a relatively short time scale of ≈10 ps or system size of about 8 ion pairs as already speculated in previous work.

How Can a Carbene be Active in an Ionic Liquid

The solvation of the carbene 1-ethyl-3-methylimidazole-2-ylidene in the ionic liquid 1-ethyl-3-methylimidazolium acetate was investigated by ab initio molecular dynamics simulations in order to reveal the interaction between these two highly important classes of materials: N-heterocyclic carbenes with superb catalytic activity and ionic liquids with advantageous properties as solvents and reaction media. In contrast to previously published data on analogous systems, no hydrogen bond is observed between the hypovalent carbon atom and the most acidic ring hydrogen atoms, as these interaction sites of the imidazolium ring are predominantly occupied by the acetate ions. Keeping the carbene away from the ring hydrogen atoms prevents stabilization of this reactive species, and hence any retarding effect on subsequent reactions, which explains the observed high reactivity of the carbene in acetate-based ionic liquids. Instead, the carbene exhibits a weaker interaction with the methyl group of the imidazolium cation by forming a hitherto unprecedented kind of C⋅⋅⋅H-C hydrogen bond. This unexpected finding not only indicates a novel kind of hydrogen bond for carbenes, but also shows that such interaction sites of the imidazolium cation are not limited to the ring hydrogen atoms. Thus, the results give the solute-solvent interactions within ionic liquids a new perspective, and provide a further, albeit weak, site of interaction to tune in order to achieve the desired environment for any dissolved active ingredient.

Simulating the vibrational spectra of ionic liquid systems: 1-Ethyl-3-methylimidazolium acetate and its mixtures

The vibrational spectra of the ionic liquid 1-ethyl-3-methylimidazolium acetate and its mixtures with water and carbon dioxide are calculated using ab initio molecular dynamics simulations, and the results are compared to experimental data. The new implementation of a normal coordinate analysis in the trajectory analyzer TRAVIS is used to assign the experimentally observed bands to specific molecular vibrations. The applied computational approaches prove to be particularly suitable for the modeling of bulk phase effects on vibrational spectra, which are highly important for the discussion of the microscopic structure in systems with a strong dynamic network of intermolecular interactions, such as ionic liquids.

Triphilic Ionic-Liquid Mixtures: Fluorinated and Non-fluorinated Aprotic Ionic-Liquid Mixtures

We present here the possibility of forming triphilic mixtures from alkyl- and fluoroalkylimidazolium ionic liquids, thus, macroscopically homogeneous mixtures for which instead of the often observed two domains—polar and nonpolar—three stable microphases are present: polar, lipophilic, and fluorous ones. The fluorinated side chains of the cations indeed self-associate and form domains that are segregated from those of the polar and alkyl domains. To enable miscibility, despite the generally preferred macroscopic separation between fluorous and alkyl moieties, the importance of strong hydrogen bonding is shown. As the long-range structure in the alkyl and fluoroalkyl domains is dependent on the composition of the liquid, we propose that the heterogeneous, triphilic structure can be easily tuned by the molar ratio of the components. We believe that further development may allow the design of switchable, smart liquids that change their properties in a predictable way according to their composition or even their environment.

Domain Analysis in Nanostructured Liquids: A Post-Molecular Dynamics Study at the Example of Ionic Liquids.

In the present computational work, we develop a new tool for our trajectory analysis program TRAVIS to analyze the well-known behavior of liquids to separate into microphases. The dissection of the liquid into domains of different subsets, for example, in the case of fluorinated ionic liquids with anionic and cationic head groups (forming together the polar domain), fluorous, and alkyl subsets is followed by radical Voronoi tessellation. This leads to useful average quantities of the subset neighbor count, that is, the domain count that gives the amount of particular domains in the liquid, the domain volume and surface, as well as the isoperimetric quotient, which provides a measure of the deviation of the domains from a spherical shape. Thus, the newly implemented method allows analysis of the domains in terms of their numbers and shapes on a qualitative and also quantitative basis. It is a simple, direct, and automated analysis that does not need evaluation of the structure beforehand in terms of, for example, first solvent shell minima. In the microheterogeneous ionic liquids that we used as examples, the polar subsets always form a single domain in all investigated liquids. Although the fluorous side chains are also more or less connected in one or, maximally, two domains, the alkyl phases are dispersed.

Liquid Structure and Cluster Formation in Ionic Liquid/Water Mixtures – An Extensive ab initio Molecular Dynamics Study on 1-Ethyl-3-Methylimidazolium Acetate/Water Mixtures – Part

Abstract We present an extensive ab initio molecular dynamics study on mixtures of the room temperature ionic liquid 1-ethyl-3-methylimidazolium acetate with water. To show the dependence of the properties on the concentration, we simulated four different systems: Pure IL, pure water, and two binary mixtures with different mixing ratios. We found that the imidazolium rings are stacking on top of each other in the pure ionic liquid, which is a result of the strong dispersion interactions between the cations. With increasing water content, this ordering is disturbed. While in the pure IL the anions are located almost exclusively within the cations ring plane, they occupy both the in-plane and the on-top position in the mixture. For very high water content, the anions are mainly found on top of the imidazolium ring. The ethyl chains of the cations attract each other via dispersion force and form clusters, indicating the existence of microheterogeneity in our simulations. We also analyzed the topology of the hydrogen bond network and found that the cation is forming hydrogen bonds to multiple anions at the same time, and is therefore a bridging agent. Likewise, the anion is bridging between different cations. At moderate water content, two acetate ions are frequently coordinated to the same water molecule, leading to the formation of acetate/water clusters.