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Direct Spectroscopic evidence for hydrogen bonded clusters of like-charged ions is reported for ionic liquids. The measured infrared O-H vibrational bands of the hydroxyethyl groups in the cations can be assigned to the dispersion-corrected DFT calculated frequencies of linear and cyclic clusters. Compensating the like-charge Coulomb repulsion, the...
Citations
... Such clusters involve many attractive non-covalent interactions, including hydrogen bonds, which can overcome the repulsive interactions between cations and anions. Several studies have shown that these clusters facilitate the passage of ionic liquids through cell membranes, unlike classical salts [45][46][47]. Therefore, ionicity is often associated with phenomena such as toxicity, pharmacological activity, and biological availability resulting from ionic liquids' interaction with cell membranes [48][49][50][51]. ...
The importance of developing effective antimicrobial agents has become more evident recently, and quaternary phosphonium-based ionic liquids with fatty acid anions have shown great potential in this regard. Tetrabutylphosphonium-hexanoate, -octanoate, -decanoate, and -dodecanoate were synthesized as potential antimicrobial ionic liquids. Confirmation of the structure of the synthesized ionic liquids was determined by IR and NMR spectroscopy. Also, the thermal stability was examined by thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis. Physicochemical characterization was carried out, which included the measurement of density, viscosity, and electrical conductivity in a wide temperature range to understand the interactions that occur in synthesized ionic liquids. The antimicrobial activity against various microorganisms was determined, including three Gram-negative and Gram-positive bacteria, two yeast, and four filamentous fungal strains. Based on the results, ILs were more efficient against Gram-positive than Gram-negative bacteria, whereby the longest-chained anion contributes to the best antibacterial activity. Yeast Candida guillermondii and all tested filamentous fungi were most sensitive to tetrabutylphosphonium-decanoate.
... 11,12 The formation of these clusters is enthalpically favored and caused by cooperative effects within the cyclic geometry. 2,3 With increasing temperature, the (c-c) clusters disappear to the benefit of (c-a) ion pair formation due to entropic reasons. 4,13 This temperature behavior can be clearly traced from infrared (IR) spectra, wherein the (c-c) and (c-a) cluster species are represented by two distinguished vibrational bands in the ν OH stretching region. ...
... The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.3c00463. 2 H NMR spectra for all ILs and theoretical aspects of the 2 H NMR T 1 and T 2 relaxation times for anisotropic media (PDF) Transparent Peer Review report available (PDF) ...
Attractive interactions between ions of like charge remain an elusive concept. Observing and quantifying this type of interaction in liquids and solutions is still a major challenge. Recently, we have shown that cation-cation interactions are present in hydroxyl-functionalized ionic liquids and that they can be controlled by the shape, charge distribution and functionality of the ions. In the present study, we demonstrate that cationic cluster formation does not only change the local structures of the ionic liquids but also influences the dynamics of the cations in a characteristic way. We show that solid-state 2H NMR spectroscopy is well suited for the study of molecular motion, even if the hydrogen bonded species of interest are indistinguishable due to fast deuteron exchange. We also provide valuable information about the applicability of well-accepted relaxation models.
... Instead, we report a "concave" curvature for the (c-c) H-bonds with a slight decrease at low and a stronger decrease at high DMSO concentrations. This observation is in line with the results of earlier studies on other ILs indicating larger clusters showing cooperative effects are replaced by smaller clusters, in particular dimers, characterized by less cooperativity.23,24 This interpretation is supported by the distributions of the (c-c) clusters as present in the mixtures. ...
The concept of hydrogen bonding is celebrating its 100th birthday. Hydrogen bonds (H-bonds) play a key role in the structure and function of biological molecules, the strength of materials, and molecular binding. Herein, we study H-bonding in mixtures of a hydroxyl-functionalized ionic liquid with the neutral, H-bond-accepting molecular liquid dimethylsulfoxide (DMSO) using neutron diffraction experiments and molecular dynamics simulations. We report the geometry, strength, and distribution of three different types of H-bond OH···O, formed between the hydroxyl group of the cation and either the oxygen atom of another cation, the counteranion, or the neutral molecule. Such a variety of different strengths and distributions of H-bonds in one single mixture could hold the promise of providing solvents with potential applications in H-bond-related chemistry, for example, to alter the natural selectivity patterns of catalytic reactions or the conformation of catalysts.
... Despite this expectation, however, structural motifs involving H-bonded cationic clusters were observed in the bulk liquid phase of hydroxy-functionalized ILs. [19][20][21][22][23][24][25][26][27][28][29][30][31][32] In particular, FTIR measurements in the pure ILs clearly showed two distinct vibrational bands that are assigned to (c + À a À ) and (c + À c + ) hydrogen bonded species. It is worth noting that the (c + À c + ) hydrogen bonds are evidently stronger than those in the (c + À a À ) case as indicated by the magnitudes of the redshifts of their corresponding vibrational bands. ...
... It is worth noting that the (c + À c + ) hydrogen bonds are evidently stronger than those in the (c + À a À ) case as indicated by the magnitudes of the redshifts of their corresponding vibrational bands. [19][20][21][22] We systematically showed how these three molecular ion parameters support like-charge attraction. The use of polarizable cations, weakly interacting anions, and long alkyl chains results in (c + À c + ) clustering already at room temperature. ...
We show that the carboxyl‐functionalized ionic liquid 1‐(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC‐CH2‐py][NTf2] exhibits three types of hydrogen bonding: the expected single hydrogen bonds between cation and anion, and, surprisingly, single and double hydrogen bonds between the cations, despite the repulsive Coulomb forces between the ions of like charge. Combining X‐ray crystallography, differential scanning calorimetry, IR spectroscopy, thermodynamic methods and DFT calculations allows the analysis and characterization of all types of hydrogen bonding present in the solid, liquid and gaseous states of the ionic liquid (IL). We find doubly hydrogen bonded cationic dimers (c⁺=c⁺) in the crystalline phase. With increasing temperature, this binding motif opens in the liquid and is replaced by (c⁺−c⁺−a⁻ species, with a remaining single cationic hydrogen bond and an additional hydrogen bond between cation and anion. We provide clear evidence that the IL evaporates as hydrogen‐bonded ion pairs (c⁺−a⁻) into the gas phase. The measured transition enthalpies allow the noncovalent interactions to be dissected and the hydrogen bond strength between ions of like charge to be determined.
... The cationic dimers demonstrate that the short-range donor-acceptor covalency forces successfully compete with the powerful long-range electrostatic repulsions [22][23][24][25][26][27][28][29][30][31][32][33][34][35]. ...
... The cationic dimers demonstrate that the short-range donor-acceptor covalency forces successfully compete with the powerful long-range electrostatic repulsions [22][23][24][25][26][27][28][29][30][31][32][33][34][35]. The characteristic covalency features of the double (H . . . ...
We show that carboxyl-functionalized ionic liquids (ILs) form doubly hydrogen-bonded cationic dimers (c+=c+) despite the repulsive forces between ions of like charge and competing hydrogen bonds between cation and anion (c+–a−). This structural motif as known for formic acid, the archetype of double hydrogen bridges, is present in the solid state of the IL 1−(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC−CH2−py][NTf2]. By means of quantum chemical calculations, we explored different hydrogen-bonded isomers of neutral (HOOC–(CH2)n–py+)2(NTf2−)2, single-charged (HOOC–(CH2)n–py+)2(NTf2−), and double-charged (HOOC– (CH2)n−py+)2 complexes for demonstrating the paradoxical case of “anti-electrostatic” hydrogen bonding (AEHB) between ions of like charge. For the pure doubly hydrogen-bonded cationic dimers (HOOC– (CH2)n−py+)2, we report robust kinetic stability for n = 1–4. At n = 5, hydrogen bonding and dispersion fully compensate for the repulsive Coulomb forces between the cations, allowing for the quantification of the two equivalent hydrogen bonds and dispersion interaction in the order of 58.5 and 11 kJmol−1, respectively. For n = 6–8, we calculated negative free energies for temperatures below 47, 80, and 114 K, respectively. Quantum cluster equilibrium (QCE) theory predicts the equilibria between cationic monomers and dimers by considering the intermolecular interaction between the species, leading to thermodynamic stability at even higher temperatures. We rationalize the H-bond characteristics of the cationic dimers by the natural bond orbital (NBO) approach, emphasizing the strong correlation between NBO-based and spectroscopic descriptors, such as NMR chemical shifts and vibrational frequencies.
... 68 The next step, probably a much more difficult one, is to find HIs that go against their nature in anti-electrostatic interactions. Beyond the above cited triel case of TrCl 4 À Á Á ÁNH 3 by Scheiner et al. 81 and other hydrogen/halogen bonding systems, 68,158,[172][173][174][175] we would like to highlight the work of Huber and others in the LA À Á Á ÁLB À HI pattern shown in Fig. 16. 82,156 If we have anti-electrostatic HI adducts with anionic Lewis acids, can we have examples of the opposite case, with cationic Lewis bases? ...
Hole interactions are known by different names depending on the key atom of the bond (halogen bond, chalcogen bond, hydrogen bond, etc.), and the geometry of the interaction (σ if in line, π if perpendicular to the Lewis acid plane). However, its origin starts with the creation of a Lewis acid by an underlying covalent bond, which forms an electrostatic depletion and a virtual antibonding orbital, which can create non-covalent interactions with Lewis bases. In this (maybe subjective) perspective, we will claim that hole interactions must be defined via the molecular orbital origin of the molecule. Under this premise we can better explore the richness of such bonding patterns. For that, we will study old, recent and new systems, trying to pinpoint some misinterpretations that are often associated with them. We will use as exemplars the triel bonds, a couple of metal complexes, a discussion on convergent σ-holes, and many cases of anti-electrostatic hole interactions.
... Meanwhile, even for ionic liquids, increasing evidence for attractive interactions between ions of like charge is reported. In hydroxyl-functionalized ILs, we recently observed two types of hydrogen bonds (H-bonds): normal H-bonds between cation and anion, further enhanced by attractive Coulomb forces, and elusive H-bonds between two or more cations leading to cluster formation of like-charged ions that are supposed to be much weaker due to the repulsive Coulomb force [18][19][20][21][22]. Despite this expectation, the hydrogen bonds in cationic clusters are evidently stronger than the ones in ion pairs as shown by stronger redshifted OH vibrational bands in IR spectra [23,24]. ...
... One type of hexamer is built upon these ion pairs solely (see Figure 3b), whereas the other includes only (c-c) bound cationic clusters with additional anions interacting with the positively charged rings of the cations. We observed both types of hydrogen bonds in infrared spectra, whereby (c-c) H-bonds are stronger than (c-a) H-bonds and are strongly favored with decreasing temperature [20][21][22][23]. For cationic and neutral clusters, we employed B3LYP/6-31+G* and B3LYP-D3/6-31+G* calculations performed with the Gaussian 09 program [45]. ...
... For calculating the clusters at the same level of theory, we used the well-balanced 6-31+G* Pople basis set. Including polarization as well as diffuse functions, this basis set is suitable for reasonably calculating hydrogen-bonded clusters of like-charged ions [20][21][22][23]. We used the relatively small 6-31+G* basis set for calculating all clusters at the same level of theory as well as for better comparison with earlier studies of molecular and ionic clusters [46][47][48]. ...
The attractive part of the van der Waals potential is commonly referred to as dispersion forces. Dispersion forces are a ubiquitous phenomenon with significant implications for chemistry, biochemistry, and materials science. Admittedly, the dispersion interactions are considered “weak”. For this reason, the quantification of the dispersion forces is rather challenging. In this paper we used the DFT methodology with def2-TZVPP basis set to evaluate dispersion interactions in molecular and ionic systems. We also quantify the dispersion contributions with help of the complementary to DFT experimental methods based on thermochemical quantities: standard molar vaporization enthalpy and the ideal-gas standard molar enthalpy of formation. We used a bunch of experimental thermodynamic methods (combustion calorimetry, solution calorimetry, differential scanning calorimetry, thermogravimetry, vapour pressure determination with combined Quartz Crystal Microbalance, Knudsen method, transpiration technique, static method) to quantify the dispersion forces in molecular and ionic systems. Experimental studies have been performed on the well-chosen sets of molecular compounds (alkanes, alcohols, amines, aromatics) and ionic liquids (ammonium and phosphonium based ionic liquids). The main idea was based on the predominant view that bulky groups in molecular structures are more likely considered repulsive rather than stabilizing. In view of the fact that a delicate balance between attraction and repulsion can be quantified and rationalized, the approach developed in this work can help establish new design principles based on the conscious inclusion of dispersion as an important stabilizing factor. The ensemble of experimental techniques and quantum chemical methods (DFT with D3 dispersion correction) enabled the development of quantitative scales of the dispersion contributions and their understanding at the molecular level.
... It is noteworthy that for other interactions,agrowing number of experimental evidence for adducts between ions of like charge were reported, for example,for guanidinium ions in water, [13] biomolecules like oligopeptides, [14] metastable colloidal crystallites, [15] or for ionic liquids with weakly coordinating counterions. [16][17][18][19] In the latter example,cationic imidazolium species form clusters which are stabilized by cooperative hydrogen bonds (HB). Thelatter is another type of interaction that is considered to be primarily electrostatic in nature, [20] but for which n!s*orbital interactions are often also non-negligible. ...
... [21] This was demonstrated vividly in as tudy by Weinhold and Klein [22] on cation-cation and anion-anion complexes that showed unusual kinetic stability, thereby challenging the seemingly generally accepted electrostatic model of HBs.T his first report of so-called antielectrostatic hydrogen bonds (AEHBs) was followed by numerous theoretical studies [23,24] as well as experimental evidence. [16][17][18][19]25] Forhalogen bonding,the first theoretical studies on antielectrostatic XBs (AEXBs) [26,27] appeared only very recently. Foro rganic compounds,t he only experimental systems that could be considered to contain AEXBs are ahandful of cases in which anionic organohalogen compounds show self-association in solid-state structures (even though in these studies, the contacts were not interpreted as AEXBs and were not analyzed further). ...
Halogen-bonded (XB) complexes between halide anions and a cyclopropenylium-based anionic XB donor were characterized in solution for the first time. Spontaneous formation of such complexes confirms that halogen bonding is sufficiently strong to overcome electrostatic repulsion between two anions. The formation constants of such "anti-electrostatic" associations are comparable to those formed by halides with neutral halogenated electrophiles. However, while the latter usually show charge-transfer absorption bands, the UV-Vis spectra of the anion-anion complexes examined herein are determined by the electronic excitations within the XB donor. The identification of XB anion-anion complexes substantially extends the range of the feasible XB systems, and it provides vital information for the discussion of the nature of this interaction.
... Similar reasoning might be correct for the CuCl 2 -EtOH mixtures. It needs noting that in the linear correlations ( Figure 4B), the calculated wavenumbers, except for complex L, are all intensity-weighted averages of the theoretical O-H absorption positions of the related complexes following the literature method [81]. The quantity variations of the identified species are assessed using the deviation parameter ε d . ...
We report in this article the structural properties, spectral behavior and heterogeneity of ZnCl2-ethanol (EtOH) mixtures in a wide-composition range (1:3 to 1:14 in molar ratios), using ATR-FTIR spectroscopy and quantum chemical calculations. To improve the resolution of the initial IR spectra, excess spectroscopy and two-dimensional correlation spectroscopy were employed. The transformation process was suggested to be from EtOH trimer and EtOH tetramer to EtOH monomer, EtOH dimer and ZnCl2-3EtOH complex upon mixing. The theoretical findings showed that increasing the content of EtOH was accompanied with the flow of negative charge to ZnCl2. This led to reinforcement of the Zn←O coordination bonds, increase of the ionic character of Zn‒Cl bond and weakening and even dissociation of the Zn‒Cl bond. It was found that in some of the ZnCl2-EtOH complexes optimized at the gas phase or under the solvent effect, there existed hydroxyls with a very special interactive array in the form of Cl‒Zn+←O‒H
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Cl-, which incredibly red-shifted to wavenumbers <3000 cm-1. This in-depth study shows the physical insights of the respective electrolyte alcoholic solutions, particularly the solvation process of the salt, help to rationalize the reported experimental results, and may shed light on understanding the properties of the deep eutectic solvents formed from ZnCl2 and an alcohol.
... Several works demonstrated (somehow counterintuitively) that not only the cation-anion-but also the cationcation-clustering induced by H-bonding plays a major role for the structural organization and the crystallization tendency of these liquids. [48][49][50][51][52][53] Striving for a maximum impact of H-bonding, the liquids chosen here are EMIM-TFSI, exhibiting the smallest cationic alkyl chain (hence, the smallest cation size) among the asymmetrical imidazolium-based aprotic ILs, and its structurally modified counterpart OHEMIM. Moreover, the present investigation focuses on the deeply supercooled regimes of these liquids since, at low temperatures, the fraction of H-bonding species is certainly higher than in the highly fluid regime. ...
Combining results from impedance spectroscopy and oscillatory shear rheology, the present work focuses on the relation between the mass and charge flows and on how these are affected by the H-bonding in viscous ionic liquids (ILs). In particular, we compare the relaxational behaviors of the paradigmatic IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) and its OH-functionalized counterpart 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (OHEMIM-TFSI). Our results and their analysis demonstrate that the presence of cationic OH-groups bears a strong impact on the overall dynamics of OHEMIM-TFSI, although no signatures of suprastructural relaxation modes could be identified in their dielectric and mechanical responses. To check whether at the origin of this strong variation is the H-bonding or merely the difference between the corresponding cation sizes (controlling both the hydrodynamic volume and the inter-charge distance), the present study includes 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PMIM-TFSI), mixtures of EMIM-TFSI and PMIM-TFSI with lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and mixtures of OHEMIM-TFSI with PMIM-TFSI. Their investigation clearly reveals that the dynamical changes induced by H-bonding are significantly larger than those that can be attributed to the change in the ion size. Moreover, in the mixtures of OHEMIM-TFSI with PMIM-TFSI, a dilution of the OH-groups leads to strong deviations from ideal mixing behavior, thus highlighting the common phenomenological ground of hydroxy-functionalized ILs and other H-bonded liquids.
