Halogen bonding effect on electrochemical anion oxidation in ionic liquids

Three Ionic liquids (ILs) based on an imidazolium core have been compared and used as solvents for the oxidation of various anions. Electrochemical experiments as well as NMR titrations and X-ray diffraction analyses unambiguously confirm the crucial role of non-covalent halogen bonding on the oxidation potentials and consequently the electrochemical window of the respective ILs.

How Meaningful Is the Halogen Bonding in 1-Ethyl-3-methyl Imidazolium-Based Ionic Liquids for CO2 Capture?

We report on several parameters that can be used to describe the 1-ethyl-3-methyl-4,5-(X₂)imidazolium cations (where X = H, Br, and I) within the Canongia-Lopez and Padua Force Field (CL&P) framework. Geometrical parameters like intramolecular distances and radial distribution functions are close to the experimental structure. Density values obtained with our force field are within the expected ones from CL&P calculations in related systems. This information is used to simulate through molecular dynamics the solubilization of CO₂ by these ILs. For pure ILs, the addition of halides in position 4 and 5 promotes an enhanced hydrogen bond interaction at position 2 with the oxygen atoms in the anion. It is found that CO₂ should be in the interstices of the anion-cation 3D network with longer distances than those found in other reports at ab initio levels, suggesting that halogen bond, if present, may be not the driving force interaction in these systems. Therefore, it seems that CO₂ interacts linearly via an oxygen atom with the cation and with the anion through a π-stacking or hydrogen-bonded fashions. Solvation enthalpies compare well with the experimental data, thereby suggesting that halogenated ILs dissolve more efficiently in CO₂ than C₂C₁Im⁺ derivatives. This result suggests that halogenated ILs can be considered as reliable candidates for CO₂ capture.

Investigation of the interactions between 1-butyl-3-methylimidazolium-based ionic liquids and isobutylene using density functional theory

To identify ionic liquids (ILs) that could be used as solvents in isobutylene (IB) polymerization, the interactions between IB and eight different ILs based on the 1-butyl-3-methylimidazolium cation ([Bmim]⁺) were investigated using density functional theory (DFT). The anions in the ILs were chloride, hexafluorophosphate, tetrafluoroborate, bis[(trifluoromethyl)sulfonyl]imide, tetrachloroaluminate ([AlCl₄]^-), tetrachloroferrate, acetate, and trifluoroacetate. The interaction geometries were explained by changes in the total energy, intermolecular distances, Hirshfeld charges, and the electrostatic potential surface. The IL solvents were screened by comparing their interaction intensities with IB to the interaction intensities of reference ILs ([AlCl₄]^--based ILs) with IB. The microscopic mechanism for IB dissolution was rationalized by invoking a previously reported microscopic mechanism for the dissolution of gases in ILs. Computation results revealed that hydrogen (H) bonding between C2-H on the imidazolium ring and the anions plays a key role in ion pair (IP) formation. The addition of IB leads to slight changes in the dominant interactions of the IP. IB molecules occupied cavities created by small angular rearrangements of the anions, just as CO₂ does when it is dissolved in an IL. The limited total free space in the ILs and the much larger size of IB than CO₂ were found to be responsible for the poor solubility of IB compared with that of CO₂ in the ILs.

Modeling Halogen Bonds in Ionic Liquids: A Force Field for Imidazolium and Halo-Imidazolium Derivatives

In this work, a force field for molecular dynamics and Monte Carlo simulations of ionic liquids containing imidazolium and halo-imidazolium derivatives is presented. This force field is an extension of the well-known CL&P and OPLS-AA models and was validated by comparing predicted crystalline structures for 22 ionic liquid compounds with the corresponding data deposited at the Cambridge Structural Database. The obtained results indicate that the proposed force field extension allows the reproduction of the crystal data with an absolute average deviation lower than 2.4%. Finally, it was also established that the halogen atoms covalently bound to the studied imidazolium cations are positively charged and do not exhibit a so-called σ-hole feature. For this reason, the formation of halogen bonds in the proposed force field appears naturally from the parametrized atomic point-charge distribution, without the necessity of any extra interaction sites.

Ion-Pair Halogen Bonds in 2-Halo-Functionalized Imidazolium Chloride Receptors: Substituent and Solvent Effects

The interaction of 2-halo-functionalized imidazolium derivatives (n-X⁺ ; X=Cl, Br, I) with a chloride anion through ion-pair halogen bonds (n-X⋅Cl) was studied by means of DFT and ab initio calculations. A method benchmark was performed on 2-bromo-1H-imidazol-3-ium in association with chloride (1-Br⋅Cl); MP2 yielded the best results when compared with CCSD(T) calculations. The interaction energies (ΔE) in the gas phase are high and, although the electrostatic interaction is strong owing to the ion-pair nature of the system, large X⋅⋅⋅Cl^- Wiberg bond orders and contributions from charge transfer (nCl- →σ*C-X) are obtained. These values drop considerably in chloroform and water; this shows that solvent plays a role in modulating the interaction and that gas-phase calculations are particularly unrealistic for experimental applications. The introduction of electron-withdrawing groups in the 4,5-positions of the imidazolium (e.g., -NO₂ , -F) increases the halogen-bond strength in both the gas phase and solvent, including water. The effect of the substituents on the 1,3-positions (N-H groups) also depends on the solvent. The variation of ΔE can be predicted through a two-parameter linear regression that optimizes the weights of charge-transfer and electrostatic interactions, which are different in vacuum and in solvent (chloroform and water). These results could be used in the rational design of efficient chloride receptors based on halogen bonds that work in solution, in particular, in an aqueous environment.

Advantages of Organic Halogen Bonding for Halide Recognition

The study of hydrogen bonding organocatalysis is rapidly expanding. Much research has been directed at making catalysts more active and selective, with less attention on fundamental design strategies. This study systematically increases steric hindrance at the active site of pH switchable urea organocatalysts. Incorporating strong intramolecular hydrogen bonds from protonated pyridines to oxygen stabilizes the active conformation of these ureas thus reducing the entropic penalty that results from substrate binding. The effect of increasing steric hindrance was studied by single crystal X-ray diffraction and by kinetics experiments of a benchmark reaction.

The acidity/basicity of metal-containing ionic liquids: insights from surface analysis and the Fukui function

The acidity/basicity of a series of metal-containing ionic liquids has been studied by surface analysis and the Fukui function.

Halogen bonding interactions in ion pairs versus conventional charge-assisted and neutral halogen bonds: a theoretical study based on imidazolium species

The properties of three different types of halogen bonds in the complexes of imidazolium species are characterized.

Competitive pi interactions and hydrogen bonding within imidazolium ionic liquids

In this paper we have explored the structural and energetic landscape of potential π(+)-π(+) stacked motifs, hydrogen-bonding arrangements and anion-π(+) interactions for gas-phase ion pair (IP) conformers and IP-dimers of 1,3-dimethylimidazolium chloride, [C1C1im]Cl. We classify cation-cation ring stacking as an electron deficient π(+)-π(+) interaction, and a competitive anion on-top IP motif as an anion-donor π(+)-acceptor interaction. 21 stable IP-dimers have been obtained within an energy range of 0-126 kJ mol(-1). The structures have been found to exhibit a complex interplay of structural features. We have found that low energy IP-dimers are not necessarily formed from the lowest energy IP conformers. The sampled range of IP-dimers exhibits new structural forms that cannot be recovered by examining the ion-pairs alone, moreover the IP-dimers are recovering additional key features of the local liquid structure. Including dispersion is shown to impact both the relative energy ordering and the geometry of the IPs and IP-dimers, however the impact is found to be subtle and dependent on the underlying functional.

A knowledge-based halogen bonding scoring function for predicting protein-ligand interactions

Halogen bonding, a non-covalent interaction between the halogen σ-hole and Lewis bases, could not be properly characterized by majority of current scoring functions. In this study, a knowledge-based halogen bonding scoring function, termed XBPMF, was developed by an iterative method for predicting protein-ligand interactions. Three sets of pairwise potentials were derived from two training sets of protein-ligand complexes from the Protein Data Bank. It was found that two-dimensional pairwise potentials could characterize appropriately the distance and angle profiles of halogen bonding, which is superior to one-dimensional pairwise potentials. With comparison to six widely used scoring functions, XBPMF was evaluated to have moderate power for predicting protein-ligand interactions in terms of "docking power", "ranking power" and "scoring power". Especially, it has a rather satisfactory performance for the systems with typical halogen bonds. To the best of our knowledge, XBPMF is the first halogen bonding scoring function that is not dependent on any dummy atom, and is practical for high-throughput virtual screening. Therefore, this scoring function should be useful for the study and application of halogen bonding interactions like molecular docking and lead optimization.