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WIREs Comput Mol Sci
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Multiresolution calculation of ionic liquids

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Ionic liquids—which are special solvents composed entirely of ions—are difficult albeit interesting to study for several reasons. Owing to the many possible cation and anion combinations that form ionic liquids, common properties are hard to classify for them, which makes the theoretical investigation crucial for ionic liquids. The system size, the amount of possible isomers including cation–anion orientation and coordination, as well as the rotation of the side chain(s) prevent the use of high‐level electronic structure methods, and density functional theory is the method of choice. Dispersion forces—although they are small compared to electrostatics—play a major role in ionic liquids; therefore, methods that describe such kind of interplay are preferred. Between the cation and the anion, there is a sizable charge transfer, which has important consequences for molecular dynamics simulations and force field development. Already based on the first generation of force fields important discoveries were made, namely that ionic liquids are nanostructured. Moreover, it was possible to predict that their distillation is possible. Throughout the construction of these force fields, transferability was taken into account which allowed them to describe homologous series. For studying reactions in ionic liquid (IL) media, continuum models were found to improve the results. Ab initio molecular dynamics (AIMD) and quantum mechanics (QM)/molecular mechanics (MM) approaches are well suited for spontaneous events. In case of very large systems, such as cellulose in ionic liquids, coarse‐grained methods are providing insight and are applied more frequently. This makes ionic liquids real multiscalar systems. WIREs Comput Mol Sci 2015, 5:202–214. doi: 10.1002/wcms.1212

Conflict of interest: The authors have declared no conflicts of interest for this article.

A symbolic representation of the logical building blocks that produce ionic liquids (ILs). In order to arrive at an IL, certain principles have to be considered. The combination of cations, anions, and side chains should incorporate charge delocalization and asymmetry to perturb the optimal crystal lattice of the solid state.
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Isomerization of basic imidazolium‐based ionic liquid ion pairs, dominant in the gas phase, and occurring to a much lower extent in the liquid. The combined distribution functions depicted below show the occurrences of the different combinations of C2–H2 (r 1) and O ⋯ H (r 2) distances in a gas phase and a liquid phase simulation of 1‐ethyl‐3‐methylimidazolium acetate from Refs 65 and 66 The carbene‐like structures with long C2–H2 and short O ⋯ H distances appear in the areas with red frame in the graph. The difference in behavior between the gas and the liquid phases stresses the necessity of applying solvation models when calculating reactions in ILs. It should also be noted here that the experimentally observed dominance of the carbene–acetic acid structure in the gas phase is apparently slightly shifted toward the IL isomer by the applying functionally.
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Specifications of a general force field for ionic liquids: (IC, internal consistency) anions and cations are parameterized with the same functional/rules; special attention is given to the definition of atomic partial point charges, q, ion flexibility (dihedral angles, Φ) and non‐bonded interactions (e.g., LJ ϵ, σ); (T, transferability) parameters are valid within the same homologous family (e.g., ions with different alkyl side chains, Cn); they also allow ion interchange to yield different ionic liquids (a1, a2); (C, compatibility) rules are established to join ions to neutral molecules; the latter are parameterized from well‐established force fields.
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MD simulation snapshots illustrating (a) the nanosegregated structure of a bulk ionic liquid (polar network with red anions and blue cations versus non‐polar domains with gray alkyl side chains in 1‐hexyl‐3‐methylimidazolium butylsulfonate) and (b) the gas‐liquid interface of 1‐octyl‐3‐methylimidazolium tetrafluoroborate (depicting the existence of neutral ion pairs in the gas phase). MD, molecular dynamics.
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Important considerations for the investigation of ionic liquids using the methods of electronic structure theory. Solvation can be treated through continuum models or by clusters in static density functional theory (DFT) or ab initio calculations, but all the possible conformations have to be carefully considered. On the other hand, in the statistical ensemble of ab initio molecular dynamics of periodic simulation boxes solvation is treated explicitly, as well as the conformations. The electronic structure methods have to be chosen carefully, but generally DFT‐D provides reasonable results for low computational costs. *The energy data above correspond to the interaction energy in the two main isomers of 1‐butyl‐3‐methylimidazolium dicyanamide with different methods with TZVP basis set.
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Structure and Mechanism > Computational Materials Science
Electronic Structure Theory > Density Functional Theory
Molecular and Statistical Mechanics > Molecular Dynamics and Monte-Carlo Methods

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