Towards large-scale, fully ab initio calculations of ionic liquids

Charge transfer and polarisability in ionic liquids: a case study

The practical use of ionic liquids (ILs) is benefiting from a growing understanding of the underpinning structural and dynamic properties, facilitated through classical molecular dynamics (MD) simulations. The predictive and explanatory power of a classical MD simulation is inextricably linked to the underlying force field. A key aspect of the forcefield for ILs is the ability to recover charge based interactions. Our focus in this paper is on the description and recovery of charge transfer and polarisability effects, demonstrated through MD simulations of the widely used 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C₄C₁im][NTf₂] IL. We study the charge distributions generated by a range of ab initio methods, and present an interpolation method for determining atom-wise scaled partial charges. Two novel methods for determining the mean field (total) charge transfer from anion to cation are presented. The impact of using different charge models and different partial charge scaling (unscaled, uniformly scaled, atom-wise scaled) are compared to fully polarisable simulations. We study a range of Drude particle explicitly polarisable potentials and shed light on the performance of current approaches to counter known problems. It is demonstrated that small changes in the charge description and MD methodology can have a significant impact; biasing some properties, while leaving others unaffected within the structural and dynamic domains.

A review on machine learning algorithms for the ionic liquid chemical space

There are thousands of papers published every year investigating the properties and possible applications of ionic liquids. Industrial use of these exceptional fluids requires adequate understanding of their physical properties, in order to create the ionic liquid that will optimally suit the application. Computational property prediction arose from the urgent need to minimise the time and cost that would be required to experimentally test different combinations of ions. This review discusses the use of machine learning algorithms as property prediction tools for ionic liquids (either as standalone methods or in conjunction with molecular dynamics simulations), presents common problems of training datasets and proposes ways that could lead to more accurate and efficient models.

Targeted modifications in ionic liquids - from understanding to design

Ionic liquids are extremely versatile and continue to find new applications in academia as well as industry. This versatility is rooted in the manifold of possible ion types, ion combinations, and ion variations. However, to fully exploit this versatility, it is imperative to understand how the properties of ionic liquids arise from their constituents. In this work, we discuss targeted modifications as a powerful tool to provide understanding and to enable design. A 'targeted modification' is a deliberate change in the structure of an ionic liquid. This includes chemical changes in an experiment as well as changes to the parameterisation in a computer simulation. In any case, such a change must be purposeful to isolate what is of interest, studying, as far as is possible, only one concept at a time. The concepts can then be used as design elements. However, it is often found that several design elements interact with each other - sometimes synergistically, and other times antagonistically. Targeted modifications are a systematic way of navigating these overlaps. We hope this paper shows that understanding ionic liquids requires experimentalists and theoreticians to join forces and provides a tool to tackle the difficult transition from understanding to design.

Ether functionalisation, ion conformation and the optimisation of macroscopic properties in ionic liquids

Ionic liquids are an attractive material class due to their wide liquid range, intrinsic ionic conductivity, and high chemical as well as electrochemical stability. However, the widespread use of ionic liquids is hindered by significantly higher viscosities compared to conventional molecular solvents. In this work, we show how the transport properties of ionic liquids can be altered significantly, even for isostructural ions that have the same backbone. To this end, structure-property relationships have been determined for a set of 16 systematically varied representative ionic liquids. Variations in molecular structure include ammonium vs. phosphonium, ether vs. alkyl side chains, and rigid vs. flexible anions. Ab initio calculations are used to relate molecular structures to the thermal, structural and transport properties of the ionic liquids. We find that the differences in properties of ether and alkyl functionalised ionic liquids are primarily dependent on minimum energy geometries, with the conformational flexibility of ether side chains appearing to be of secondary importance. We also show unprecedented correlations between anion conformational flexibility and transport properties. Critically, increasing fluidity upon consecutive introduction of ether side chains and phosphonium centres into the cation is found to be dependent on whether the anion is flexible or rigid. We demonstrate that targeted design of functional groups based on structure-property relationships can yield ionic liquids of exceptionally high fluidity.

Ionic liquids from a fragmented perspective

The efficacy of using fragmentation methods, such as the effective fragment potential, the fragment molecular orbital and the effective fragment molecular orbital methods is discussed. The advantages and current limitations of these methods are considered, potential improvements are suggested, and a prognosis for the future is provided.

The Ionic Liquid Property Explorer: An Extensive Library of Task-Specific Solvents

Ionic liquids have a broad spectrum of applications ranging from gas separation to sensors and pharmaceuticals. Rational selection of the constituent ions is key to achieving tailor-made materials with functional properties. To facilitate the discovery of new ionic liquids for sustainable applications, we have created a virtual library of over 8 million synthetically feasible ionic liquids. Each structure has been evaluated for their-task suitability using data-driven statistical models calculated for 12 highly relevant properties: melting point, thermal decomposition, glass transition, heat capacity, viscosity, density, cytotoxicity, CO 2 solubility, surface tension, and electrical and thermal conductivity. For comparison, values of six properties computed using quantum chemistry based equilibrium thermodynamics COSMO-RS methods are also provided. We believe the data set will be useful for future efforts directed towards targeted synthesis and optimization.

Insight into conformationally-dependent binding of 1-n-alkyl-3-methylimidazolium cations to porphyrin molecules using quantum mechanical calculations

The first step in the biodegradation of imidazolium-based ionic liquids involves the insertion of the –OH group into the alkyl side chain, and it is believed to be triggered by cytochrome P450. In this work, we investigate the effect of conformations on binding energies of ionic liquid cations to the catalytic center of P450.

Atomistic reverse nonequilibrium molecular dynamics simulation of the viscosity of ionic liquid 1-n-butyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf2N]

The shear viscosity of room-temperature ionic liquid (IL) 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf2N] is calculated over a temperature range 298-353 K, using the reverse nonequilibrium molecular dynamics simulation technique. The results of this work show that while the use of equilibrium molecular dynamics simulation techniques might be inefficient for viscosity calculations of ILs, the reverse nonequilibrium molecular dynamics technique is an efficient tool for this purpose. Our findings indicate that the shear rate for crossover from the Newtonian plateau to the shear thinning regime, corresponds to the relaxation time for the slowest microscopic scale motions, i.e., exchange of counterions in an ion's solvation shell (ion-pair relaxation time). The closeness of the time scales and activation energies for zero-shear-rate viscosities to the relaxation times and the corresponding activation energies for ion-pair formation/rupture, connects macroscopic dynamic properties with local atomic-level motions of the IL. The calculated viscosity coefficients and relaxation times for reorientations of the cation and anion as well as their corresponding activation energies are in very good agreement with experimental data.

Prediction of 1H NMR chemical shifts for ionic liquids: strategy and application of a relative reference standard

The computational predictions of ¹H NMR chemical shifts for ionic liquids were investigated. To calculate the chemical shifts more accurately, the approach of relative reference standard (RRS) was proposed. This straightforward computational technique uses organic compounds and ionic liquids that are similar to the studied ionic liquids as standards. The calculated chemical shifts of single ion pairs were strongly influenced by the anion type and the local environment. Using the RRS methodology, the calculated results agreed well with the experimental chemical shifts due to the cancellation of errors caused by the anion. Ionic clusters consisting of 4 ion pairs were also employed to model the ionic liquids with strongly coordinating anions. Large clusters slightly improve the accuracy by reducing systematic errors. Although the experimental ¹H NMR data of the reference ionic liquid should be used, the RRS methodology has been shown to predict ¹H NMR chemical shifts efficiently at different levels of theory.

Using an RRS method to calculate the ¹H NMR chemical shifts of ionic liquid agreed well with the experimental value.

Accurate prediction of the structure and vibrational spectra of ionic liquid clusters with the generalized energy-based fragmentation approach: critical role of ion-pair-based fragmentation

The GEBF method with the ion-pair-based fragmentation has been developed to facilitate ab initio calculations of general ionic liquid clusters.

Valence electronic structure of [EMIM][BF4] ionic liquid: photoemission and DFT+D study

The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF₄] ionic liquid was recorded and analyzed. Together with the gas-phase UPS spectrum of the [EMIM][BF₄] vapor and ab initio calculation methods, detailed insight into the electronic structure of this simple ionic liquid is possible. The low binding energy tail in the UPS spectrum is about 7.4 eV, in agreement with previous estimations of the HOMO–LUMO gap of the [EMIM][BF₄] ion-pair. The bulk ab initio calculations are able to explain most of the features in the spectrum. However, DFT consistently lacks accuracy in the description of the top of the valence band. The dispersion corrected PBE calculation (PBE-D3) did offer very good agreement with the experimental structure, but the recently-developed vdW-DF functionals C09, optPBE, optB88 and CX were found to offer the best agreement in terms of the electronic structure.

The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF₄] ionic liquid was recorded and compared to previously measured vapor phase UPS spectrum.

Structure and lifetimes in ionic liquids and their mixtures

With the aid of molecular dynamics simulations, we study the structure and dynamics of different ionic liquid systems.

A coarse-grained polarizable force field for the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate

We present a coarse-grained polarizable molecular dynamics force field for the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF₆]). For the treatment of electronic polarizability, we employ the Drude model. Our results show that the new explicitly polarizable force field reproduces important static and dynamic properties such as mass density, enthalpy of vaporization, diffusion coefficients, or electrical conductivity in the relevant temperature range. In situations where an explicit treatment of electronic polarizability might be crucial, we expect the force field to be an improvement over non-polarizable models, while still profiting from the reduction of computational cost due to the coarse-grained representation.

Ionic Liquids for Supercapacitor Applications

Supercapacitors are electrochemical energy storage devices in which the charge is accumulated through the adsorption of ions from an electrolyte on the surface of the electrode. Because of their large ionic concentrations, ionic liquids have widely been investigated for such applications. The main properties that have to be optimized are the electrochemical window, the electrical conductivity, and the interfacial capacitances. Ionic liquids allow a significant improvement of the former, but they suffer from their high viscosity. In this review, I will discuss the advantages and the inconvenience of using ionic liquids in supercapacitors. Some innovative approaches using mixtures of ionic liquids or redox-active ions will also be critically addressed.

Quantum Chemical Methods for the Prediction of Energetic, Physical, and Spectroscopic Properties of Ionic Liquids

The accurate prediction of physicochemical properties of condensed systems is a longstanding goal of theoretical (quantum) chemistry. Ionic liquids comprising entirely of ions provide a unique challenge in this respect due to the diverse chemical nature of available ions and the complex interplay of intermolecular interactions among them, thus resulting in the wide variability of physicochemical properties, such as thermodynamic, transport, and spectroscopic properties. It is well understood that intermolecular forces are directly linked to physicochemical properties of condensed systems, and therefore, an understanding of this relationship would greatly aid in the design and synthesis of functionalized materials with tailored properties for an application at hand. This review aims to give an overview of how electronic structure properties obtained from quantum chemical methods such as interaction/binding energy and its fundamental components, dipole moment, polarizability, and orbital energies, can help shed light on the energetic, physical, and spectroscopic properties of semi-Coulomb systems such as ionic liquids. Particular emphasis is given to the prediction of their thermodynamic, transport, spectroscopic, and solubilizing properties.

Trends in Two- and Three-Body Effects in Multiscale Clusters of Ionic Liquids

Applications of higher correlated levels of ab initio theory to condensed systems require a significant amount of computational resources. The recent development of the fragment molecular orbital (FMO) approach alleviates this issue by splitting the system into individual fragments and achieves the accuracy of the method by accounting for all possible two-body and three-body interactions. In this work a comprehensive application of the FMO approach in combination with a second order of Møller-Plesset perturbation theory method, MP2, is presented for multiscale clusters of ionic liquids such as [C₁mim]X, [C₁mpyr]X, [C₂py]X, and [NMe₄]X, where X = chloride and tetrafluoroborates, BF₄^-, with the clusters varying in size from 4, 8, 16, to 32 ion pairs. Reliable cutoff criteria for the inclusion of two-body and three-body interactions are identified for both HF energy and MP2 correlation energy to achieve the desired accuracy of 1 kJ mol^-1. The importance of two-body and three-body interactions in ionic liquids is also discussed.

Difference in chemical bonding between lithium and sodium salts: influence of covalency on their solubility

The current theoretical study explains the difference in solubility between lithium and sodium salts in ionic liquids due to increased covalency in lithium salts.

Influence of electric potential on the apparent viscosity of an ionic liquid: facts and artifacts

According to recent findings, the steady shear viscosity of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf2N]) decreases significantly under the influence of electric potential. This implies a causal connection between nanoscale ordering at the electrified interface and a macroscopic change of transport properties. To study this phenomenon in more detail, we reproduced the above-mentioned measurements; however, we find no evidence that the viscosity of [Emim][Tf2N] is a function of electric potential. Additionally, our results show that steady shear measurements can lead to artifacts that, at first glance, may appear to be potential-induced changes in viscosity. We demonstrate that the artifacts result from a sliding electrical contact at the working electrode of the electrochemical cell and we suggest to consider our findings for future viscosity measurements of ionic liquids.

Self-interaction error in DFT-based modelling of ionic liquids

The modern computer simulations of potential green solvents of the future, involving the room temperature ionic liquids, heavily rely on density functional theory (DFT). In order to verify the appropriateness of the common DFT methods, we have investigated the effect of the self-interaction error (SIE) on the results of DFT calculations for 24 ionic pairs and 48 ionic associates. The magnitude of the SIE is up to 40 kJ mol(-1) depending on the anion choice. Most strongly the SIE influences the calculation results of ionic associates that contain halide anions. For these associates, the range-separated density functionals suppress the SIE; for other cases, the revPBE density functional with dispersion correction and triple-ζ Slater-type basis is suitable for computationally inexpensive and reasonably accurate DFT calculations.

Predictive thermodynamics for ionic solids and liquids

Thermodynamic properties of ionic solids and liquids may reliably be predicted using volume-based thermodynamics (VBT) and thermodynamic difference rules (TDR).

Ionic Liquids with Weakly Coordinating [M(III)(OR(F))4](-) Anions

Ionic liquids (ILs) are defined as salts with melting points below 100 °C. They attracted much attention in the last two decades due to their unique set of properties, including high conductivities, low viscosities, negligible vapor pressure, and high electrochemical resistance. ILs are seen as tunable systems, of which (also in mixtures) up to 10(19) combinations may exist. These properties make ILs interesting candidates for a variety of fundamental to industrial applications. Our addition to this field was weakly coordinating, little interacting anions, the highly fluorinated aluminates [Al(OR(F))4](-) (R(F) = C(CF3)3, C(CH3), (CF3)2, and CH(CF3)2 and later also CH2(CF3)). We have used these anions in a broad spectrum of applications, including the stabilization of reactive cations, (polymerization) catalysis, and conducting salts for cyclic voltammetry or in electrochemical cells. Especially the [Al(Ohfip)4](-) (hfip = CH(CF3)2) anions in combination with asymmetric organic cations turned out to be very well suited for the synthesis of ILs with very low melting points, some even far below 0 °C. Also the analogous borates, [B(OR(F))4](-), were shown to yield ILs, and currently a plethora of such aluminate and borate ILs have been synthesized and thoroughly investigated. In many aspects, at least the [Al(Ohfip)4](-) ILs present almost ideally noninteracting prototype ILs with (nearly) isotropic but weak and flat Coulomb potential. Consequently, their overall interionic interactions are significantly reduced compared with other classes of ILs, resulting in an extraordinarily low degree, or (for short cation chain lengths below six) even complete absence of ion pairing. From thorough analysis of the principles governing the physical properties of this highly fluorinated IL class with minimized interactions, we were able to learn basic principles that could be extended, for example, to the prediction of the principal properties of a wide variety of typical ILs. In this Account, we give a comprehensive review of their syntheses, thermal and toxicological behavior, physical as well as dynamic properties, and use in electrochemical applications. We delineate advantages and limitations of the [M(III)(OR(F))4](-) ILs developed in our lab and give an outlook on those fields, in which there is still a lack of knowledge.

Ionic liquids as solvents of polar and non-polar solutes: affinity and coordination

Evolution of H₂O and CO₂ interactions with an ionic liquid (IL) from gas phase to IL phase is described. Affinity is lost and coordination patterns vary in the process, favouring H₂O–anion and CO₂–cation interactions.

Molecular features contributing to the lower viscosity of phosphonium ionic liquids compared to their ammonium analogues

Classical molecular dynamics simulations identify the size of the central atom and the angle flexibility as important molecular features contributing to the lower viscosity of phosphonium based ionic liquids compared to their ammonium analogues.

Simulations of room temperature ionic liquids: from polarizable to coarse-grained force fields

This perspective article summarizes the recent advances in the classical molecular modelling of room temperature ionic liquids.

Importance of dispersion forces for prediction of thermodynamic and transport properties of some common ionic liquids

An extensive study of interaction energies in ion pairs of pyrrolidinium and imidazolium ionic liquids is presented. The Cnmpyr and Cnmim cations with varying alkyl chains from Methyl, Ethyl, n-Propyl to n-Butyl were combined with a wide range of routinely used IL anions such as chloride, bromide, mesylate (CH3SO3 or Mes), tosylate (CH3PhSO3 or Tos), bis(trifluoromethanesulfonyl)amide (NTf2), dicyanamide (N(CN)2 or dca), tetrafluoroborate (BF4) and hexafluorophosphate (PF6). A number of energetically favourable conformations were studied for each cation-anion combination. The interaction energy and its dispersion component of the single ion pairs were calculated using a sophisticated state-of-the-art approach: a second-order of Symmetry Adapted Perturbation Theory (SAPT). A comparison of energetics depending on the cation-anion type, as well as the mode of interaction was performed. Dispersion forces were confirmed to be of importance for the overall stabilisation of ionic liquids contributing from 28 kJ mol(-1) in pyrrolidinium ion pairs to 59 kJ mol(-1) in imidazolium ion pairs. The previously proposed ratio of total interaction energy to dispersion components and melting points was assessed for this set of ionic liquids and was found to correlate with their melting points for the anionic series, producing separate trends for the Cnmim and Cmpyr series of cations. Chlorides, bromides and tetrafluoroborates formed close-to-ideal correlations when both types of cations, Cnmim and Cnmpyr, were combined in the same trend. Correlation of the dispersion component of the interaction energy with transport properties such as conductivity and viscosity was also considered. For imidazolium-based ionic liquids strong linear correlations were obtained, whereas pyrrolidinium ionic liquids appeared to be insensitive to this correlation.