Clusters of the Ionic Liquid 1-Hydroxyethyl-3-methylimidazolium Picrate: From Theoretical Prediction in the Gas Phase to Experimental Evidence in the Solid State

Effect of isopropyl side chain branching and different anions on electronic structure, vibrational spectra, and hydrogen bonding of isopropyl-imidazolium-based ionic liquids: Experimental and theoretical investigations

In the present work, four branched methylated, 1,2-dimethyl-3-isopropyl-imidazolium (i-[C₃Dmim⁺]) and protonated,1-methyl-3-isopropyl-imidazolium (i-[C₃mim⁺])-based ionic liquids (ILs) with varying anion (Br^-, BF₄^-, PF₆^-, and NTf₂^-) were synthesized and investigated by NMR, infrared (IR) and Raman spectroscopy. Based on infrared and Raman spectroscopy, complete vibrational assignments have been performed. The IR and Raman analysis revealed that the vibrational spectra are virtually unaffected upon methylation, while significant frequency changes were observed by changing anion. Furthermore, to determine the electronic structure, energetic stability, and vibrational properties of these i-[C₃Dmim]Y, i-[C₃mim]Y (Y = Br, BF₄, PF₆, and NTf₂) ion pairs, quantum chemical calculations including the dispersion correction method are performed both on single ions and on ionic couples. The calculated electron density was analyzed to expose non-covalent intra- and interionic interactions by the quantum theory of atoms in molecules (AIM) and interpreted in terms of both anion dependence and type of interaction. Computational results suggest that for all ionic couples the most energetically stable configuration is obtained with the anions located close to the C2 position of the imidazolium cation. However, in the case of i-[C₃mim]NTf₂ and i-[C₃Dmim]BF₄, similar energies were obtained in configurations 2 and 3 where the anion is located above the imidazolium ring. For i-[C₃mim]Br a stronger hydrogen bond is predicted than for other studied ILs. Calculations indicate that a red shift of the CH stretching bands should occur due to hydrogen bonding; indeed, such displacement of bands is experimentally observed.

Hydrogen Bonds between Ions of Opposite and Like Charge in Hydroxyl-Functionalized Ionic Liquids: an Exhaustive Examination of the Interplay between Global and Local Motions and Intermolecular Hydrogen Bond Lifetimes and Kinetics

Hydroxyl-functionalized ionic liquids (ILs) represent a new interesting class of ILs where hydrogen bonds (HBs) play an important role: here, "typical" HBs between cations and anions (ca) are competing with "atypical" HBs connecting pairs of cations (cc). We study the equilibrium and kinetics of (cc) and (ca) HBs in 1-(n-hydroxyalkyl)-pyridinium bis(trifluoromethlysulfonyl)imide [HOC_nPy][NTf₂] ILs by means of molecular dynamics simulations. (cc) HBs are found to be between 0.96 and 3.76 kJ mol^-1 stronger than their (ca) counterparts, depending on the alkyl chain length. HB lifetimes and kinetics are analyzed by means of HB population and reactive flux correlation functions. Essentially, four different HB lifetimes have to be considered, spanning about 3 orders of magnitude, each valid in its own right and each associated with different aspects of HB breaking and HB reformation. The long-time limiting behavior of the HB population correlation function is controlled by diffusion of the ions and can be quantitatively described by analytical expressions. The short-time HB behavior is tied to the localized dynamics of the hydroxyl group exploring its local solvation environment. A minimalist kinetic two-domain model is introduced to realistically describe the time evolution of the HB population correlation function for both (ca) and (cc) HBs over 5 orders of magnitude. By employing the reactive flux method, we determine the kinetics of HB breaking, unaffected by diffusion processes. We determine both, the ultrafast upper boundary and the average rate of HB breaking, allowing recrossing-events during the transient relaxation time period. For sufficiently long alkyl chains, all those computed HB lifetimes indicate a higher kinetic stability of (cc) HBs over (ca) HBs; for short chains, it is vice-versa.

Kinetics of Hydrogen Bonding between Ions with Opposite and Like Charges in Hydroxyl-Functionalized Ionic Liquids

Hydrogen-bonded structures and their lifetimes in ionic liquids (ILs) are governed by the subtle balance between Coulomb interactions, hydrogen bonding, and dispersion forces. Despite the dominant Coulomb interaction, local and directional hydrogen bonds (HBs) can play an important role in the behavior of ILs. Compared to water, the archetype of hydrogen-bonded liquids, ILs have larger constituents and higher viscosities but are typically lacking a three-dimensional HB network. Hydroxyl-functionalized ionic liquids are even more special: regular HBs between cations and anions (ca) are accompanied by HBs between pairs of cations (cc). Recently, infrared (IR) measurements have suggested that the (cc) HBs are even stronger than their (ca) counterparts and their strength can be controlled via the hydroxyalkyl chain length. In this paper, we show by means of molecular dynamics (MD) simulations that the presence of HBs has a profound effect on the molecular mobility of the ions. We investigate the kinetic mechanism of hydrogen bonding in ILs and show that the lifetimes and hence the stability of (cc) HBs increase with the chain length, making them more stable than the respective (ca) HBs. The observed HB equilibrium can explain the peculiar chain length dependence of the relative molecular mobilities of the ions by a direct comparison between hydroxyl-functionalized ILs with their nonfunctionalized counterparts.

Effect of Hydrogen Bonding between Ions of Like Charge on the Boundary Layer Friction of Hydroxy-Functionalized Ionic Liquids

Atomic force microscopy has been used to measure the lubricity of a series of ionic liquids (ILs) at mica surfaces in the boundary friction regime. A previously unreported cation bilayer structure is detected at the IL-mica interface due to the formation of H-bonds between the hydroxy-functionalized cations [(c-c) H-bonds], which enhances the ordering of the ions in the boundary layer and improves the lubrication. The strength of the cation bilayer structure is controlled by altering the strength of (c-c) H-bonding via changes in the hydroxyalkyl chain length, the cation charge polarizability, and the coordination strength of the anions. This reveals a new means of controlling IL boundary nanostructure via H-bonding between ions of the same charge, which can impact diverse applications, including surface catalysis, particle stability, electrochemistry, etc.

Controlling "like-likes-like" charge attraction in hydroxy-functionalized ionic liquids by polarizability of the cations, interaction strength of the anions and varying alkyl chain length

We provide comprehensive understanding of "like-likes-like" charge attraction in hydroxy-functionalized ionic liquids (ILs) by means of infrared spectroscopy (IR), quantum chemistry and differential scanning calorimetry (DSC). We show that hydrogen bonding between cation and cation (c-c) is possible despite the repulsive forces between ions of like charge. Already at room temperature, the (c-c) hydrogen bonds can compete with the regular Coulomb-enhanced hydrogen bonds between cation and anion (c-a). For a large set of well-selected ILs, we show that "like-charge attraction" between the OH-functionalized cations is controllable by the polarizability of the cation, the interaction strength of the anion and the length of the hydroxyalkyl chain. In particular, we clarify whether tethering the OH group away from the positive charge center of the cationic ring with longer hydroxyalkyl chains compensates for unfavourable cation/anion combinations with respect to (c-c) cluster formation. For that purpose, we synthesized and characterized twelve ionic liquids including the differently polarizable cations, 1-(n-hydroxyalkyl)-1-methylpiperidinium [HOC_nMPip]⁺ and 1-(n-hydroxyalkyl)-pyridinium [HOC_nPy]⁺, as well as the weakly and strongly interacting anions, bis(trifluoromethanesulfonyl)imide [NTf₂]^- and methanesulfonate [OMs]^-, respectively. On top, we varied the hydroxyalkyl chain length (HOC_n) (n = 2-5). We systematically show how these three molecular ion parameters affect like-charge attraction. The use of polarizable cations, weakly interacting anions, and long alkyl chain tethers results in (c-c) clustering already at room temperature. Kinetic trapping is not a prerequisite for the existence of (c-c) cluster species in ILs. Moreover, we demonstrate that micro structuring affects macroscopic behavior of this type of ILs. We observed that substantial (c-c) interaction prevents ILs from crystallizing. Instead, these ILs supercool and finally form a glass.

Counting cations involved in cationic clusters of hydroxy-functionalized ionic liquids by means of infrared and solid-state NMR spectroscopy

Sensitive probe of like-charge attraction: analyzing infrared spectra allows counting the number of cations involved in clusters of opposite (c–a) and like-charged (c–c) ions in ionic liquids. This approach is also applicable to molecular liquids.

Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling Constants-Comparison with Salt Bridges and Molecular Systems

We present deuteron quadrupole coupling constants (DQCC) for hydroxyl‐functionalized ionic liquids (ILs) in the crystalline or glassy states characterizing two types of hydrogen bonding: The regular Coulomb‐enhanced hydrogen bonds between cation and anion (c–a), and the unusual hydrogen bonds between cation and cation (c–c), which are present despite repulsive Coulomb forces. We measure these sensitive probes of hydrogen bonding by means of solid‐state NMR spectroscopy. The DQCCs of (c–a) ion pairs and (c–c) H‐bonds are compared to those of salt bridges in supramolecular complexes and those present in molecular liquids. At low temperatures, the (c–c) species successfully compete with the (c–a) ion pairs and dominate the cluster populations. Equilibrium constants obtained from molecular‐dynamics (MD) simulations show van't Hoff behavior with small transition enthalpies between the differently H‐bonded species. We show that cationic‐cluster formation prevents these ILs from crystallizing. With cooling, the (c–c) hydrogen bonds persist, resulting in supercooling and glass formation.

Cluster Formation through Hydrogen Bond Bridges across Chloride Anions in a Hydroxyl-Functionalized Ionic Liquid

Several recent studies of hydroxyl-functionalized ionic liquids (ILs) have shown that cation-cation interactions can be dominating these materials at the molecular level when the anion involved is weakly interacting. The hydrogen bonds between the like ions led to the formation of interesting chain-like, ring-like, or distinct dimeric (i. e. two ion pairs) supermolecular clusters. In the present work, vibrational spectroscopy (ATR-IR and Raman) and density functional theory (DFT) calculations of the hydroxyl-functionalized imidazolium ionic liquid C₂ OHmimCl indicate that anion-cation hydrogen bonding interactions are dominating, leading to the formation of distinct dimeric ion pair clusters. In this arrangement, the Cl^- anions function as a bridge between the cations by establishing bifurcated hydrogen bonds with the OH group of one cation and the C(2)-H of another cation. Cation-cation interactions, on the other hand, do not play a significant role in the observed clusters.

Isolating the role of hydrogen bonding in hydroxyl-functionalized ionic liquids by means of vaporization enthalpies, infrared spectroscopy and molecular dynamics simulations

Hydrogen bonding in hydroxyl-functionalized ionic liquids (right) prevents favourable dispersion interaction between cation and anion (left). We analyze this subtle balance of interactions by combining calorimetry, IR spectroscopy and MD simulations.

Cooperatively enhanced hydrogen bonds in ionic liquids: closing the loop with molecular mimics of hydroxy-functionalized cations

The combined experimental and theoretical approach for the gas and the liquid phases provides a quantitative understanding of the competition between differently H-bonded and charged constituents in liquids.