Ion-pair Clusters Research Articles

Micro- and Mesoporous Carbons and Composites: Nanopores through Salt Templating

The development of more efficient and sustainable synthesis approaches for nanoporous materials is highly demanding. Specifically, porous functional carbons and their composites are very attractive. Yet, besides the quest for convenient precursors, hard templating or activation processes still have to be applied for the introduction of an adequate porosity. Major drawbacks are the use of hazardous chemicals for the template removal and multiple, rather energy consuming synthesis steps.[2, 3] Herein, we present a new tool called “salt templating”, allowing both the synthesis of functional carbons/composites and the introduction of tunable, high porosity.[4] The general concept is based on crosslinking of a carbon precursor in the presence of a molten salt phase which simultaneously acts as solvent and template. This unique synthesis gives access to a wide variety of compositions, morphologies and porosities where the appropriate choice of salts controls the pore size and architecture on the nanoscale. The pore size can correspond to ion pairs, ion pair clusters and their geometric percolation structures. Contrary to current methods, it is possible to proceed in a single-step synthesis, while the porogen can easily be removed with water and in principal recovered afterwards for further use. Exemplarily shown for (but not restricted to) ionic liquids (IL) as precursors, not only heteroatom-doped carbons but also composite materials can be synthesized. The IL constitutes the nitrogen-doped carbon matrix and also act as nitrogen source, e.g. for metal nitride formation. After aqueous removal of the salt phase, e.g. VN nanoparticles embedded in a nitrogen-doped carbon matrix with surface areas of up to 2400 m2g-1, are obtained. Adding to the simplicity of the salt templating approach, examples are chosen to demonstrate the potential of the method as a platform toward the rational design of nanoporous materials in a more sustainable fashion.

The effect of solvent polarity on the rate of the Mitsunobu esterification reaction

The rate of the Mitsunobu esterification reaction of ethanol or isopropanol with benzoic acid was found to be much faster in non-polar solvents. The logarithm of the rate constant was inversely proportional to the solvent polarity, as defined by ET values. Typically, the rate constant for ethyl benzoate formation in THF was 100 times greater than that in MeCN. The presence of either sodium benzoate or excess benzoic acid resulted in a decrease in rate. Each of the main species involved in the Mitsunobu esterification reaction, the alcohol starting material, dialkoxyphosphorane, alkoxyphosphonium salt and ester product, was detected by proton NMR analysis. The possible role of ion pair aggregates or clusters, prior to rate-determining SN2 attack of carboxylate on the alkoxyphosphonium ion, is discussed. An explanation is provided as to why the yield in the Mitsunobu reaction is often higher in non-polar solvents.

Magnetic separation of Dy(III) ions from homogeneous aqueous solutions

The possibility to enrich paramagnetic dysprosium(III) ions in a magnetic field gradient is proved by means of interferometry, which may open the route for a magnetic separation of rare earth ions from aqueous solutions. The separation dynamics are studied for three different concentrations of DyCl3 and compared with those found recently in a sulphate solution of the 3d ion Mn(II). In view of the similar-sized hydration spheres for Dy(III) and Mn(II), the slower separation dynamics in DyCl3 is attributed to both a higher densification coefficient and the strong impact of Brownian motion due to the absence of ion-pair clusters.

Molecular Cluster Distribution in Sea Water Near the Wall: Antifouling Aspect

It was investigated how the “wall effect” influences the cluster distribution in water and 3.5 wt.% NaCl solution being in a glass cylinder. The cluster distribution in a mass assemble up to 2.2 million Dalton was determined in dependence on the distance to the wall at normal atmospheric pressure. The average cluster mass, the rate of collapsed and skeletal clusters and the number of cluster kinds depended on the distance to the wall and on stirring methods. In different liquid’s layers and at different distances to the wall, there were registered newly appearing clusters as well as disappearing ones especially, solvated clusters of ion pairs (SCIPs). Under non-stirring conditions, the base water cluster (H2O)12 and (NaCl·40H2O)2 are the most sensitive to the wall distance. Other SCIPs like dimers, tetramers and hexamers remain stable at stirring in the vessel center (12 rad/s) and don’t react to wall approaching however at cylinder rotation (8.9 rad/s) these SCIPs are sensitive to the wall. The wall nature, the distance to the wall and the constant magnetic field were concluded to influence the cluster distribution differently. Cluster properties were assumed to be applied by sea water living organisms for their navigation.

Local Structure in Ionic Liquids Investigated by Hyper-Rayleigh Scattering

A series of ionic liquids involving imidazolium, pyrrolidinium, and alkyl ammonium cations with different anions (namely, [BF(4)], [PF(6)], [TFSI], [SCN], and [(CN)(2)N]) have been studied by hyper-Rayleigh scattering (HRS), which is a powerful technique to probe the local structure of liquids in a multipolar description. The interpretation of the HRS measurements in terms of an elementary structural (ES) entity has revealed a dominating octopolar nature of ES scatterers. By combining the HRS analysis with density functional theory calculations for different-sized ion pair clusters, we show that the octopolar nature to ES hyperpolarizability in ionic liquids (ILs) originates from a complex local structure due to the formation of 'transient' ion clusters within the time of observation of HRS (~10(-12) to 10(-14) s). We emphasize that such a structural organization puts clearly into evidence the influence of nonadditive interaction processes within first shell of neighbors (<1 nm) leading to a coherent HRS structure factor in ILs.

Role of Cationic Residues in Fine Tuning the Flexibility of Charged Single α-helices

A few highly charged natural peptide sequences have recently been suggested to form stable α-helical structures in aqueous solution. Here we show that these sequences represent a more widespread structural motif called 'charged single α-helix' (CSAH). We have developed two conceptually different computational methods capable of scanning large databases: SCAN4CSAH is based on sequence features characteristic for salt bridge stabilized single α-helices, while FT_CHARGE applies Fourier transformation to charges along sequences. Using the consensus of the two approaches, a remarkable number of proteins were found to contain putative CSAH domains. Recombinant fragments of 50-60 residues in length corresponding to selected hits (from myosin 6, Golgi complex associated protein-60, and mitogen-activated protein kinase M4K4) were found to adopt a highly stable α-helical structure in water. Molecular dynamic simulations of five CSAH peptides (the above three plus peptides from caldesmon and myosin 10) showed that the formation of dynamic ion-pair clusters significantly contribute to the stability of the helices. Furthermore, cationic residues were found to play a differential role in tuning the local flexibility of the CSAH domains. We conclude that sequence specific tuning of flexibility of CSAH peptides could have important role in the mechanical performance of CSAH-containing myosin motors, such as myosin 6 and 10, or in other protein functions.This work was supported by the Hungarian National Science Fund (OTKA).

Cluster Structure of Imidazolium Salts in Methanol Controlled by the Balance of Interactions: Cation-Anion, Cation-Solvent, and Anion-Solvent

We have studied the cluster structure of 1-butyl-3-methylimidazolium halide, bmimX (X = Cl, Br, I), in methanol solution by means of an electrospray mass spectrometer, which is specially designed for analysis of clusters isolated from solution. In positive ion mode experiments, the ratio of solvated bmim(+), bmim(+)(MeOH)(n) and ion-pair clusters, bmim(bmim(+)X(-))(m) was dependent on the counter anion. As for bmimCl solutions, few solvated bmim(+) clusters were observed, and the ion-pair clusters were clearly observed. On the other hand, bmimBr and bmimI with large anions, the solvated bmim(+) clusters increased obviously, and the ion-pair clusters were in turn remarkably decreased. In negative ion-mode experiments, the solvation for Br(-) by the methanol is found to be the most prominent among those for Cl(-), Br(-), and I(-). These results were reasonably explained in consideration of the balance between ion-solvent and ion-counterion interactions.

Undissociated versus Dissociated Structures for Water Clusters and Ammonia−Water Clusters: (H2O)n and NH3(H2O)n−1 (n = 5, 8, 9, 21). Theoretical Study

To understand the autoionization of pure water and the solvation of ammonia in water, we investigated the undissociated and dissociated (ion-pair) structures of (H2O) n and NH3(H2O)n-1 (n = 5, 8, 9, 21) using density functional theory (DFT) and second order Moller-Plesset perturbation theory (MP2). The stability, thermodynamic properties, and infrared spectra were also studied. The dissociated (ion-pair) form of the clusters tends to favor the solvent-separated ion-pair of H3O+/NH4+ and OH-. As for the NH3(H2O)20 cluster, the undissociated structure has the internal conformation, in contrast to the surface conformation for the (H2O)21 cluster, whereas the dissociated structure of NH3(H2O)20 has the surface conformation. As the cluster size of (H2O)n/NH3(H2O)n-1 increases, the difference in standard free energy between undissociated and dissociated (ion-pair) clusters is asymptotically well corroborated with the experimental free energy change at infinite dilution of H3O+/NH4+ and OH-. The predicted NH and OH stretching frequencies of the undissociated and dissociated (ion-pair) clusters are discussed.

Mechanism of sodium chloride diossolving in water and the process of the solution aging

By the method of Flicker Noise Spectroscopy (FNS) we studied the process of sodium chloride dissolving in water (3.5 wt %) at 294 K on the level of long-range order formation in liquid. It is established that the process of dissolving the salt includes the following steps: formation of instable colloid particles, instable solvate clusters of the salt ion pairs (iSCIP), instable dissolved polymers of globular type, and 12 h later beginning of a new process of formation stable solvate clusters of the salt ion pairs (sSCIP) and stable polymer globules on their basis. Stroke diagrams of the cluster nano-structure and a model of aging the salt solution are presented. Influence of atmospheric pressure on the SCIP average weight is demonstrated. It is concluded that on the SCIP surface there is a porous layer of solvate water with membrane properties. The cluster structure of the 3.5 wt % solution prepared by dissolving the salt is shown to differ from that of the solution obtained by dilution. The solutions long-range order is formed by the nano dispersion of the water solvated salt crystal nuclei. The mechanism of the solution aging is discussed.

Clusters of Hydrated Methane Sulfonic Acid CH3SO3H·(H2O)n (n = 1−5): A Theoretical Study

Ab initio and density functional methods have been used to examine the structures and energetics of the hydrated clusters of methane sulfonic acid (MSA), CH3SO3H.(H2O)n (n = 1-5). For small clusters with one or two water molecules, the most stable clusters have strong cyclic hydrogen bonds between the proton of OH group in MSA and the water molecules. With three or more water molecules, the proton transfer from MSA to water becomes possible, forming ion-pair structures between CH3SO3- and H3O+ moieties. For MSA.(H2O)3, the energy difference between the most stable ion pair and neutral structures are less than 1 kJ/mol, thus coexistence of neutral and ion-pair isomers are expected. For larger clusters with four and five water molecules, the ion-pair isomers are more stable (>10 kJ/mol) than the neutral ones; thus, proton transfer takes place. The ion-pair clusters can have direct hydrogen bond between CH3SO3- and H3O+ or indirect one through water molecule. For MSA.(H2O)5, the energy difference between ion pairs with direct and indirect hydrogen bonds are less than 1 kJ/mol; namely, the charge separation and acid ionization is energetically possible. The calculated IR spectra of stable isomers of MSA.(H2O)n clusters clearly demonstrate the significant red shift of OH stretching of MSA and hydrogen-bonded OH stretching of water molecules as the size of cluster increases.

Photoelectron Spectrum of Isolated Ion-Pairs in Ionic Liquid Vapor

The gas-phase valence binding energy spectrum of isolated ion-pairs of the commonly used [1-ethyl-3-methylimidazolium][bis(trifluoromethylsulfonyl)imide)] room-temperature ionic liquid is obtained by photoionization of a molecular beam of ionic liquid vapor by extreme ultraviolet light. The isolated ion-pair nature of the ionic liquid vapor is corroborated by single photon ionization mass spectroscopy, complemented by computed vaporization energetics of ion-pairs and clusters of ion-pairs. The valence binding energy spectrum of the isolated ion-pairs is discussed in comparison with available liquid-phase data and theoretical density functional theory calculations.

Excited state proton transfer in 2-hydroxypyridine–ammonia clusters: Theoretical investigation

The electronic and structural properties of 2-hydroxypyridine (2HP) and 2HP · (NH 3) n ( n = 1–4) and their tautomers in the excited state are studied using the CIS and TD-B3LYP calculations. The role of ammonia in acting as a bridge between tautomers was also discussed. Specific attention is paid to resolve the threshold size for excited state proton transfer in these clusters. It was found that at least three ammonias are needed to produce a stable excited ion-pair clusters. The ammonia catalyzed enol–keto tautomerization reactions involve concerted proton transfer mechanism. These behaviors are found also in the ground state in our previous study, with DFT calculations performed.

Cooperativity and ground-state proton transfer in 7-hydroxyimidazo[1,2-a]pyridine·ammonia clusters: DFT study

DFT/B3LYP (6-311+G**) has been used to study twelve different geometries of 7-hydroxyimidazo[1,2-a]pyridine·(NH 3) n clusters with n=1–6. Ion-pair (Zwitterionic) clusters resulting from proton transfer from 7-hydroxyimidazo[1,2-a]pyridine (7HIP) to the (NH 3) n cluster have also been investigated. The structure, energetics and relative stabilities of the geometries were compared and analyzed in gaseous phase. The principle of maximum hardness has been tested by calculating chemical hardness and chemical potential to confirm the more stable of different conformers and predict their order of stability. In all cases, the solvent cluster form hydrogen bonds at –O–H or N(2) positions, or at both –O–H and N(2) positions of 7HIP. For n=2–4, ammonia-chain clusters are found, for n=4–6 other topologies appear such as cycle and mixed chain/cycle. The study of neutral forms shows that intermolecular proton transfer from the –O–H group of 7HIP to the (NH 3) n occurs for the critical size n c=5 yielding a locally stable zwitterions 7HIP −· ( NH 3 ) 4 NH 4 + . Some structural parameters such as intra- and intermolecular distances and vibrational frequency of –O–H stretching serving as indicators of proton transfer have been investigated. The natural bond orbital analysis (NBO) of the more stable conformers revealed specifically, that for a given number n of ammonia molecules, the stabilization energy E (2) is largest for LP(1)N A→BD*O 15–H 16 donor–acceptor interaction ( n=2–5), and this value increases with increasing n (22.74 kJ/mol for n=2, 35.38 kJ/mol for n=3, 35.57 kJ/mol for n=4 and 53.54 kJ/mol for n=5), showing that electron density is directly transferred to the –O–H antibonding orbital, which result in weakening and elongating of the –O–H bond and red-shift.

Oscillation spectra of seed NaCl crystals in aqueous solutions

Using the flicker-noise method (FNM), we investigated the oscillations of clusters in aqueous solutions of NaCl in the range of concentrations from 0.1 to 26.0 mass %. It has been established that in the solutions oscillators whose masses are similar to the masses of the models of aggregates of solvate clusters of ion pairs (SCIP) of salt with a different water content are present. In diluted solutions (<10%), the elementary SCIP has the form NaCl· 40H2O. For the entire range of concentrations the SCIPs are given by structures based on the cubic system of the sodium chloride system. The base structure for them is a cube formed from 12 SCIPs of salt. The largest cluster revealed by the FNM method for all investigated concentrations of salt had a mass of ≈1.5 million D. The presence of NaCl in water leads to a collapse of its cluster structure, except for the smallest clusters (H2O)10…11, whose concentration increases with temperature or solution concentration. The distribution of SCIPs changes dramatically at a temperature above 300 K. The possible structures of SCIPs are given and the mechanism of their formation is discussed.

Hydrogen Bond vs Proton Transfer in HZSM5 Zeolite. A Theoretical Study

The interaction of a large set of bases covering a wide range of the basicity scale with HZSM5 medium-size zeolites has been investigated through the use of two model clusters, namely 5T and 7T:63T. The 5T cluster has been treated fully ab initio at the B3LYP level, whereas the 63T cluster has been treated with the ONIOM2 scheme using the B3LYP:MNDO combination for geometry optimizations and B3LYP:HF/3-21G for adsorption energies. The optimized geometries of the different hydrogen bond (HB) and ion pair (IP) complexes obtained with both models are rather similar. However, there are significant dissimilarities as far as the adsorption energies are concerned, in particular when dealing with IP clusters whose intrinsic stability is largely underestimated when the simpler 5T model is used. 5T clusters could be used to obtain reasonable estimates of adsorption energies provided these are scaled by a factor of 1.1 for HB complexes and 1.4 for IP complexes. The zeolite cavity favors the proton transfer process, similarly to that found by third polar partners in gas-phase HB trimers. The intrinsic basicity of the base and its adsorption energy within the zeolite are correlated. From this correlation, is possible to conclude that, in general, bases with proton affinities (PA) larger than 200 kcal mol(-1) should lead to the formation of IPs, whereas bases with PA smaller than this value should form HB complexes.

DFT study of ground state proton transfer in 2-pyridone/2-hydroxypyridine–ammonia clusters

Abstract DFT calculations in the ground state have been performed for 2-hydroxypyridine–ammonia clusters 2HP · (NH 3 ) n ( n = 1 to 5) and their tautomers. Structures, relative energies and vibrational frequencies of these species were obtained at the B3LYP/6-31++G** level of theory. It was found that some structural parameters such as intra- and intermolecular distances and vibrational frequencies of O–H and N–H stretching, serve as probes and indicators of proton transfer in these clusters. The study of neutral forms shows that intermolecular proton transfer to the (NH 3 ) n occurs for n = 4 and 5 in enol and keto clusters, respectively. For enol form, ion-pair cluster calculated for n = 4 is less stable than the neutral form by 7.1 kJ/mol. While, for keto form, it is calculated to be 25.4 kJ/mol less stable than the neutral form.

Well-defined Ion-pair Clusters of Alkyl- and Dialkylammonium Salts of a Sterically-hindered Carboxylic Acid. Implication for Hydrogen-bonded Lys Salt Bridges

Abstract Ion-pair clusters of simple alkylammonium carboxylates in solution and in the crystalline state were described as a model of Lys salt bridge. tert-Butylammonium triphenylacetate gives a well-defined tetrameric cluster by a cubic hydrogen bond (H-bond) network, and diisopropylammonium triphenylacetate gives a dimeric cluster by a cyclic network.

A mass spectrometric study of solvated clusters of ions and ion pairs generated from lithium halide solutions in polar solvents: Acetonitrile compared to methanol

Mass spectrometric study on cluster structure of solutions of lithium halides (LiX = LiBr and LiI) in acetonitrile is reported. Solvated ions Li +(CH 3CN) n, X −(CH 3CN) n, and salt clusters Li +(LiX) m(CH 3CN) n and X −(LiX) m(CH 3CN) n, were observed in the spectra. The mechanism of cluster formation is discussed in terms of a competition between solvation interactions and electrostatic interactions. This helps to explain the differences between clustering in methanol and acetonitrile, since the latter one has much larger dipole moment than the former one. Therefore appearance of solvated clusters, including the solvated salt clusters, is more favourable in methanol solutions' spectra.

燃料電池用プロトン伝導膜 高分子荷電膜の輸送現象

Water and ion transport characteristics are discussed with relevance to the microscopic structural properties of perfluorinated ionomer membranes that are used as H+ conducting membranes in polymer electrolyte fuel cells. Some morphological models of ion pair aggregation and cluster formation are briefly introduced, together with reported experimental results like diffraction or spectroscopic studies. Transport models in the ionomer membranes are also discussed that can fit the experimental data. A new concept of ion transport in the polymer membrane in relation to the interaction of ion and water molecules inside the ionic channel is proposed, which is to become of particular use in designing high performance ion conducting polymer membranes.

Proton transfer and tautomerization in 7-hydroxyquinoline⋅(NH3)n clusters: Structure and energetics at the self-consistent field level

The structures and energetics of 23 different 7-hydroxyquinoline⋅(NH3)n clusters with n=1–6 are investigated using ab initio calculations at the SCF/6-31G** level. Clusters based on both the enol and keto tautomers of 7-hydroxyquinoline (7-HQ) are investigated, as well as ion-pair (zwitterionic) clusters resulting from proton transfer from 7-HQ to the (NH3)n cluster. In all cases, the solvent cluster forms hydrogen bonds at both the -O-H and N positions of 7-HQ. For n=2–4, ammonia-chain clusters are found, for n⩾4, additional hydrogen-bond topologies appear such as bifurcated chains, cycles and mixed chain/cycles. Although the bare keto 7-HQ is calculated to be 17 kcal/mol less stable than the enol form, the energy difference decreases continuously to 11 kcal/mol for 7-HQ⋅(NH3)6 with increasing n. For one of the enol clusters involving six NH3 molecules, proton transfer can occur from either the -O-H or N-H group to the cluster, yielding a locally stable zwitterion cluster 7-HQ−⋅(NH3)6H+. For the keto tautomer, locally stable ion-pair clusters were found for n=2, 5, and 6, but these are calculated to be considerably less stable than the neutral enol forms.