A DLPNO-CCSD(T) benchmarking study of intermolecular interactions of ionic liquids.

The accuracy of correlation energy recovered by coupled cluster single-, double-, and perturbative triple-excitations, CCSD(T), has led to the method being considered the gold standard of computational chemistry. The application of CCSD(T) has been limited to medium-sized molecular systems due to its steep scaling with molecular size. The recent development of alternative domain-based local pair natural orbital coupled-cluster method, DLPNO-CCSD(T), has significantly broadened the range of chemical systems to which CCSD(T) level calculations can be applied. Condensed systems such as ionic liquids (ILs) have a large contribution from London dispersion forces of up to 150 kJ mol-1 in large-scale clusters. Ionic liquids show appreciable charge transfer effects that result in the increased valence orbital delocalization over the entire ionic network, raising the question whether the application of methods based on localized orbitals is reliable for these semi-Coulombic materials. Here the performance of DLPNO-CCSD(T) is validated for the prediction of correlation interaction energies of two data sets incorporating single-ion pairs of protic and aprotic ILs. DLPNO-CCSD(T) produced results within chemical accuracy with tight parameter settings and a non-iterative treatment of triple excitations. To achieve spectroscopic accuracy of 1 kJ mol-1 , especially for hydrogen-bonded ILs and those containing halides, the DLPNO settings had to be increased by two orders of magnitude and include the iterative treatment of triple excitations, resulting in a 2.5-fold increase in computational cost. Two new sets of parameters are put forward to produce the performance of DLPNO-CCSD(T) within chemical and spectroscopic accuracy.

Predicting Entropic Effects of Water Mixing with Ionic Liquids Containing Anions of Strong Hydrogen Bonding Ability: Role of the Cation.

Ionic liquids (ILs) such as choline dihydrogen phosphate exhibit an extraordinary solubilizing ability for proteins such as cytochrome C when mixed with 20 wt % water. Most widely used imidazolium-based ionic liquids coupled with dihydrogen phosphate do not exhibit the same solubilizing properties, suggesting that a multifunctional cation such as choline might play a key role in enhancing these properties of ionic liquid mixtures with water. In this theoretical work, we compare intermolecular interactions between the water molecule and ionic liquid ions in two ion-paired clusters of choline- and 1-butyl-3-methyl-imidazolium-based ionic liquids coupled with acetate, dihydrogen phosphate, and mesylate. Gibbs free energy (GFE) of solvation of water in these ionic liquids was calculated. Incorporation of a water molecule into ionic liquid clusters was accompanied by negative GFEs of solvation in both types of cations. These results were in good agreement with previously reported experimental GFEs of solvation of water in ILs. Compared to imidazolium-based clusters, strong interionic interactions of choline ionic liquids resulted in more negative GFEs due to their smaller deformation upon the addition of a water molecule, with dihydrogen phosphate and mesylate predicting the lowest GFEs of -30.1 and -43.5 kJ/mol-1, respectively. Lower GFEs of solvation of water in choline-based clusters were also accompanied with smaller entropic penalties, suggesting that water easily incorporates itself into the existing ionic network. Analysis of the intramolecular bonds within the water molecule showed that the choline hydroxyl group donates electron density to the neighboring water molecule, leading to additional polarization. The predicted infrared spectra of clusters of ionic liquids with water showed a pronounced red shift due to strongly polarized O-H bonds, in excellent agreement with the experimentally measured infrared spectra of ionic liquid mixtures with water. Increased polarization of water in choline-based ionic liquids undoubtedly creates more effective solvents for stabilizing biological molecules such as proteins.

A Systematic Study of DFT Performance for Geometry Optimizations of Ionic Liquid Clusters.

Clusters of two ion pairs of imidazolium-based ionic liquids were optimized with 43 different levels of theory, including DFT functionals and MP2-based methods combined with varying Dunning's basis sets, and added dispersion corrections. Better preforming DFT functionals were then applied to clusters consisting of four ion pairs. Excellent performance of some DFT functionals for the two ion pair clusters did not always match that of the four ion-paired clusters despite interionic distances remaining constant between the optimized two and four ion-paired clusters of the same ionic liquid. Combinations of DFT functional and basis set such as ωB97X-D/cc-pVDZ, M06-2X/aug-cc-pVDZ, B3LYP-D3/cc-pVTZ, and TPSS-D3/cc-pVTZ gave excellent results for geometry optimization of two ion-paired clusters of imidazolium ionic liquids but gave larger deviations when applied to the four ion-paired clusters of varying ionic liquids. Empirical dispersion corrections were seen to be crucial in correctly capturing correlation effects in the studied ionic liquid clusters, becoming more important in larger clusters. Dunning's double-ζ basis set, cc-pVDZ, is associated with the smallest root mean squared deviations for geometries; however, it also produces the largest deviations in total electronic energies. ωB97X-D and M06-2X produced the best performance with the augmented version of this basis set. The triple-ζ basis set, cc-pVTZ, leads to the best performance of most of the DFT functionals (especially the dispersion-corrected ones) used, whereas its augmented version, aug-cc-pVTZ, was not seen to improve results. The combinations of functional and basis set that gave the best geometry and energetics in both two and four ion-paired clusters were PBE-D3/cc-pVTZ, ωB97X-D/aug-cc-pVDZ, and BLYP-D3/cc-pVTZ. All three combinations are recommended for geometry optimizations of larger clusters of ionic liquids. PBE-D3/cc-pVTZ performed the best with an average deviation of 2.3 kJ mol-1 and a standard deviation of 3.4 kJ mol-1 for total electronic energy when applied to four ion-paired clusters. Geometries optimized with FMO2-SRS-MP2/cc-pVTZ produced total energy within 2.0 kJ mol-1 off the benchmark in two ion-paired clusters, with the cc-pVDZ basis set performing unsurprisingly poorly with the same method. The error increased to 4.8 kJ mol-1 on average in four ion-paired clusters, with the smallest RMSD deviations in geometries when compared to the benchmark ones. This study is the first report that investigated the performance of DFT functionals for two and four ion-paired clusters of a wide range of ionic liquids consisting of commonly used cations such as pyrrolidinium, imidazolium, pyridinium, and ammonium. It also identified the importance of assessing the performance of quantum chemical methods for ionic liquids on a variety of cation-anion combinations.

Influence of DFT Functionals and Solvation Models on the Prediction of Far-Infrared Spectra of Pt-Based Anticancer Drugs: Why Do Different Complexes Require Different Levels of Theory?

Computational modeling was applied to far-infrared (FIR) spectra of Pt-based anticancer drugs to study the hydrolysis of these important molecules. Here, we present a study that investigates the influence of different factors-basis sets on non-Pt atoms, relativistic effective core potentials (RECPs) on the Pt atom, density functional theory (DFT) functionals, and solvation models-on the prediction of FIR spectra of two Pt-based anticancer drugs, cisplatin and carboplatin. Geometry optimizations and frequency calculations were performed with a range of functionals (PBE, PBE0, M06-L, and M06-2X), Dunning's correlation-consisted basis sets (VDZ, VTZ, aVDZ, and aVTZ), RECPs (VDZ-pp, VTZ-pp, aVDZ-pp, and aVTZ-pp), and solvation models (IEFPCM, CPCM, and SMD). The best combination of the basis set/DFT functional/solvation model was identified for each anticancer drug by comparing with experimentally available FIR spectra. Different combinations were established for cisplatin and carboplatin, which was rationalized by means of the partial atomic charge scheme, ChelpG, that was utilized to study the charge transfer between the Pt ion and ligands in both cisplatin and carboplatin.

Increased stability of nitroxide radicals in ionic liquids: more than a viscosity effect.

Radical stability has been subject to continuous research due to its importance in polymerization as well as in all-organic batteries. Recently, the SOMO-HOMO conversion was identified as the main factor in controlling the stability of distonic radicals, for which the negative charge resides on the same molecule. Based on this finding, the idea of ionic liquids stabilizing radicals was hypothesized in this study. A series of ionic liquids were tested in EPR measurements of the 3-carboxy-2,2,5,5-tetramethyl-pyrroline-1-oxyl. Unusually high rotational diffusion constants (τR), 4 times larger compared to conventional media such as dichloromethane (DCM), were recorded at room temperature. This finding could only be explained by a strong interaction existing between the radical and ionic liquid ions, which was confirmed with quantum chemical calculations, with interaction energies falling between -17.1 kJ mol-1 for tetramethylphosphonium tetrafluoroborate and -85.6 kJ mol-1 for 1,3-dimethylimidazolium triflate. Elevated temperature measurements performed at 80 °C reduced the viscosity of the ionic liquids to that of DCM, while the τR values remained relatively high, thus further confirming that the rotational hindrance occurred due to radical-ionic liquid interactions. The calculated interaction energies between the radical and ionic liquids ions were also found to correlate well with experimental rotational diffusion constants, thus offering us a valuable tool in tailoring ionic liquids for enhanced stability of nitroxide radicals. The findings of this study showcase the ability of ionic liquids to reduce reactivity of nitroxides without the need for any chemical modification of the radical.

Cluster approach to the prediction of thermodynamic and transport properties of ionic liquids.

The prediction of physicochemical properties of ionic liquids such as conductivity and melting point would substantially aid the targeted design of ionic liquids for specific applications ranging from solvents for extraction of valuable chemicals to biowaste to electrolytes in alternative energy devices. The previously published study connecting the interaction energies of single ion pairs (1 IP) of ionic liquids to their thermodynamic and transport properties has been extended to larger systems consisting of two ion pairs (2 IPs), in which many-body and same-ion interactions are included. Routinely used cations, of the imidazolium and pyrrolidinium families, were selected in the study coupled with chloride, tetrafluoroborate, and dicyanamide. Their two ion pair clusters were subjected to extensive configuration screening to establish most stable structures. Interaction energies of these clusters were calculated at the spin-ratio scaled MP2 (SRS-MP2) level for the correlation interaction energy, and a newly developed scaled Hartree-Fock method for the rest of energetic contributions to interaction energy. A full geometry screening for each cation-anion combination resulted in 192 unique structures, whose stability was assessed using two criteria-widely used interaction energy and total electronic energy. Furthermore, the ratio of interaction energy to its dispersion component was correlated with experimentally observed melting points in 64 energetically favourable structures. These systems were also used to test the correlation of the dispersion contribution to interaction energy with measured conductivity.

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.

Local determination of thin liquid film profiles using colour interferometry.

We explore theoretically the interference of white light between two interfaces as a function of the optical conditions, using separately: a) idealised conditions where the light is composed of three discrete wavelengths; b) a more typically experimentally realisable case where light comprises a sum of three Gaussian wavelength distributions; and c) unfiltered white light from a broadband source comprising a broad distribution of wavelengths. It is demonstrated that the latter case is not only optically simple to arrange, but also provides unambiguous absolute separation information over the range 0-1µm --a useful range in studies of cell adhesion, thin liquid films and lubrication-- when coupled to detection using a typical colour camera. The utility of this technique is verified experimentally by exploring the air film between a cylinder and surface, as well as arbitrary liquid films beneath air bubbles that are interacting with solid surfaces.

Reversible pH- and photocontrollable carbohydrate-based surfactants.

The parallel synthesis and properties of a library of photoswitchable surfactants comprising a hydrophobic butylazobenzene tail-group and a hydrophilic carbohydrate head-group, including the first surfactants to exhibit dual photo- and pH-responsive behavior, is reported. This new generation of surfactants shows varying micelle morphologies, photocontrollable surface tension, and pH-induced aggregation and adsorption.