Ionic liquids posses efficiency as solvents, co-solvents or agents for applications involving biomolecules. Due to the increasing interest in systems containing proteins and ionic liquids, we hereby present results from molecular dynamics simulations on solutions containing the polypeptide melittin in pure water, in the neat ionic liquid 1-butyl-3-methyl-imidazolium acetate ([BMI][OAc]) and in the equimolar [BMI][OAc]/H2O mixture. When compared to the solutions containing the ionic liquid, melittin displays higher mobility and flexibility, lower stability and poorer secondary structure preservation in water. The intramolecular hydrogen bonds in melittin do not play a major role in the structural preservation, but intermolecular hydrogen bonds between melittin and the solvent are important. The micro-solvation of melittin demonstrates that anions and water molecules are in closer contact to melittin, whereas the cations maintain larger distances to the polypeptide. The presence of [BMI][OAc] reduces fluctuation in melittin's structure. Only small differences have been found in the structural arrangement of melittin in the neat ionic liquid and the ionic liquid/water mixture.

We performed classical Molecular Dynamics computer simulations to analyze solutions of the gases CO2, N-2, and CH4 in four 1-n-butyl-3-methylimidazolium-based ionic liquids (1-n-butyl-3-methylimidazolium acetate, 1-n-butyl-3-methylimidazolium prolinate, 1-n-butyl-3-methylimidazolium bromide, and 1-n-butyl-3-methylimidazolium tetrafluoroborate). Typical experimental conditions (10 bar gas pressure and room temperature) have been chosen to study mixtures of the ionic liquids with the gases at a single gas molar fraction of 0.25. Structural aspects are discussed to judge the absorption capacities of the ionic liquids. We observed that CO2 coordinates preferentially within the polar domain of the ionic liquids with the bromide and tetrafluoroborate anions presenting the best performances. The other gases, N2 and CH4, remain in the less polar domains of the ionic liquids. Cluster size analysis indicates phase separation for these two gases. Considering both, the absorption tendency and gas separation capacity of the ionic liquids, the anion is desired to be small and possessing multiple coordination sites. In this aspect, the tetrafluoroborate anion accomplished the best results.

The interaction and solvent influence on two different imidazolium based ion pairs have been investigated by molecular dynamics simulations and Nuclear Magnetic Resonance. The cations 1,2,3,4,5-pentamethyl imidazolium and 1,3,4,5-tetramethyl imidazolium were considered with the imidazolate anion to evaluate the influence of the acidic hydrogen at the position 2 of the imidazolium ring on the ion pair formation in different solvents. The selected solvents are chloroform, dichloromethane, acetone, dimethylsulfoxide, and water, covering a broad range of polarity and permittivity. The binding free energy of the ion pair was computed by umbrella sampling. We observed that, with the increase of the dielectric constant, the ion pairs become more transient being separated in water. The free energies of binding corroborate ion pair stabilization by the hydrogen bond at carbon 2 of the imidazolium cations. In dichloromethane, we obtained weaker bound ion pairs than in acetone due to intercalation of dichloromethane into the ion pair. Thus, the ion pair stability is not only a consequence of the solvent's dielectric constant, but also due to local structural details.

A series of functionalized N-alkylimidazolium based ionic liquids (ImILs) were designed, through anion (carboxylates and halogenated) and cation (N-alkyl side chains) structural modifications, and studied as potential sorbents for CO2 . The sorption capacities of as prepared bare ImILs could be enhanced from 0.20 to 0.60 molar fraction by variation of cation-anion-CO2 and IL-CO2 -water interaction. By combining NMR spectroscopy with molecular dynamics simulations, a good description of interactions between ImIL and CO2 can be obtained. Three types of CO2 sorption modes have been evidenced depending on the structure of the ImIL ion pair: Physisorption, formation of bicarbonate, and covalent interaction through the nucleophilic addition of CO2 to the cation or anion. The highest CO2 sorption capacity was observed with the ImIL containing the 1-n-butyl-3-methylimidazolium cation associated with the carboxylate anions (succinate and malonate). This study provides helpful clues for better understanding the structure-activity relationship of this class of materials and the ion pair influence on CO2 capture.

Molecular dynamic simulations were carried out to analyze the structure of contact ion pairs containing the imidazolate anion and a variety of methyl substituted 1,3-dimethyl-imidazolium cations in chloroform solution. These ionic liquids were studied under the condition of infinite dilution. The study focused on determining the importance and the effect of different methylations at the cation's imidazolium ring. The structural analysis by radial and spatial distribution functions demonstrated that a stable contact pair is formed in chloroform along the entire simulation period. The cations are localized preferentially above or below the anion's plane ring, reflecting contributions of π+-π− stacking interaction between the two aromatic planar ions. The most acidic hydrogen at position C2 of the imidazolium cation exhibits the strongest structural correlations, at distances within the range of hydrogen bonding, with the imidazolate anion. Methylations at all the cation's ring positions in general weakened the structural correlations. For the methylation at the cation's C2, the difference in the free energy of association has been determined from potential of mean-force calculations in the order of 13kJ/mol favoring the ion pair containing the protonated C2 atom.

Bulk ionic liquids (ILs) have been widely studied by experimental and computational methodologies, due to their enormous variety of potential applications [1, 2, 3]. The imidazolium (Im) group and its derivates have been of large interest, among the large spectrum of organic cations that might compose/form ILs, also for the possibility of tailoring the ILs thermodynamic and physical properties by alkyl substitutions at different locations [2, 4]. The supermolecular structure of neat ImILs, and consequently their properties, are directly linked to the three-dimensional arrangement of cations and anions. The interplay of the three intermolecular forces, electrostatics, dispersive, and hydrogen bonding, controls the organization of the ImIL in the gas and the liquid phases [4, 5, 6, 7]. More recently, it became of interest to understand cation-anion contact ion pairs when the IL is dissolved in other solvents [8, 9] and to characterize the structure and intermolecular interactions in these intimate ion pairs. The structure of contact ion pairs containing the imidazolate anion and a variety of six methyl substituted 1,3dimethyl-imidazolium cations in chloroform solution were studied by molecular dynamic simulations in the present work. These ionic liquids were studied under the condition of infinite dilution, that means, only one ion pair. This study focused on determining the importance and the effect of different methylations at the cation’s imidazolium ring. The novelty of the work also consists in the fact of having an organic molecule, the imidazolate, as counter-ion, instead of the common chloride or hexafluoroborate. The formation of a stable contact pair in chloroform along the entire simulation period was demonstrated by radial and spatial distribution functions (Figure 1). The cations are localized preferentially above or below the anion’s plane ring, reflecting contributions of π+-π− stacking interaction between the two aromatic planar ions. The most acidic hydrogen at position C2 of the imidazolium cation exhibits the strongest structural correlations with the imidazolate anion, at distances within the range of hydrogen bonding. The structural correlations was in general weakened by methylations at all the cation’s ring positions., The difference in the free energy of association for the methylation at the cation’s C2 has been determined in the order of 13 kJ/mol from potential of mean-force calculations, favoring the ion pair containing the protonated C2 atom. 12 a 17/Nov, 2017, Águas de Lindóia/SP, Brasil Figure 1. SDFs of the six different functionalized cations studied around the imidazolate anion (at the center, view from the C4 and C5 side). Upper row: cations with hydrogen at C2 of the imidazolium ring; lower row: the corresponding cations with a methyl group at C2. The SDFs depict the same isodensity surface for all the pairs. Key-words: Imidazolium Ionic Liquid, Contact Pair, Methyl group substitution Click here to enter text. Support: This work has been supported by CNPq and CAPES References: [1] P. Wasserscheid, Nature, 439, 797 (2006). [2] A.A. Pádua, M.F. Costa Gomes, J.N. Canongia Lopes, Acc. Chem. Res. 40, 1087– 1096 (2007). [3] M. Smiglak, J.M. Pringle, X. Lu, L. Han, S. Zhang, H. Gao, D.R. Mac-Farlane, R.D. Rogers, Chem. Commun. 50, 9228–9250 (2014). [4] R.M. Lynden-Bell, M.G. Del Po ́polo, T.G. Youngs, J. Kohanoff, C.G. Hanke, J.B. Harper, C.C. Pinilla, Acc. Chem. Res. 40, 1138–1145 (2007). 
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