Hydrogen-bonded Network Structure Research Articles

New Bis-Pyrazole-Bis-Acetate Based Coordination Complexes: Influence of Counter-Anions and Metal Ions on the Supramolecular Structures

A new flexible bis-pyrazol-bis-acetate ligand, diethyl 2,2’-(pyridine-2,6-diylbis (5-methyl-1H-pyrazole-3,1-diyl))diacetate (L), has been synthesised, and three coordination complexes, namely, [Zn(L)2](BF4)2 (1), [MnLCl2] (2) and [CdLCl2] (3) have been obtained. All ligands and complexes were characterised by IR, mass spectroscopy, thermogravimetric analysis and single-crystal X-ray diffraction. Single crystal X-ray diffraction experiment revealed that the primary supramolecular building block of 1 is a hexagonal chair shaped 0D hydrogen bonded synthon (stabilised by C–H∙∙∙O hydrogen bonding and C=O∙∙∙π interactions), which further built into a 2D corrugated sheet-like architecture having a 3-c net honeycomb topology, and finally extended to a 3D hydrogen bonded network structure having a five nodal 1,3,3,3,7-c net, through C–H∙∙∙F interactions. On the other hand, the two crystallographically independent molecules of 2 exhibited two distinct supramolecular structures such as 2D hydrogen bonded sheet structure and 1D zigzag hydrogen bonded chain, sustained by C–H∙O and C–H∙∙∙Cl interactions, which are further self-assembled into a 3,4-c network structure, and 3 showed a 2D hydrogen bonded sheet structure. The supramolecular structural diversity in these complexes is due to the different conformations adopted by the ligands, which are mainly induced by different metal ions with coordination environments controlled by different anions. Hirshfeld surface analysis was explored for the qualitative and quantitative analysis of the supramolecular interactions.

Wettability of Graphene and Interfacial Water Structure

Understanding the graphene-water interaction, referred to as ‘wettability,’ is important for various applications, such as water desalination, filtration, energy storage, and catalysis. However, most studies on graphene's wettability have been performed with either macroscopic water contact angle measurements or molecular dynamics simulations. The detailed hydrogen-bonding network structure of water molecules at the graphene-water interface has not been fully understood at the molecular level. Here, using vibrational sum frequency generation (VSFG) spectroscopy, we elucidate the interfacial water structure and graphene hydrophobicity at a multilayer graphene-water interface. As the number of graphene layers increases, water molecules with dangling OH group become more populated. We compare the contact angles of water on the multilayer graphene surfaces with VSFG results. An excellent correlation between water adhesion energy of graphene and fraction of dangling OH groups estimated from the water OH stretch VSFG spectrum is established. This observation suggests that the VSFG could be an incisive technique for measuring water's adhesion energy on any spatially confined or blocked surface where the water contact angle cannot be measured. We further anticipate that the VSFG result on the transition from wetting transparency to translucency upon increasing the number of graphene layers will be used to understand the wettability of low-dimensional materials and the role of water structure on electric double layers of graphene-based electrodes.

Preparation and molecular simulation of grafted polybenzimidazoles containing benzimidazole type side pendant as high-temperature proton exchange membranes

Phosphoric acid-doped polybenzimidazole membranes have exhibited great potential in high-temperature proton exchange membrane applications, but proton conductivity and stability should be further improved for high performance fuel cells. Herein, a series of grafted polybenzimidazoles containing benzimidazole type side pendant were successfully prepared by facile N-substitution reaction without any catalyst. These grafted polybenzimidazole membranes with side chains of benzimidazole group exhibit excellent properties in the doping level of phosphoric acid, retention stability of phosphoric acid and proton conductivity under certain mechanical properties, oxidation resistance and thermal stability. Importantly, the phosphoric acid (PA) retention stability and proton conductivity can be preciously controlled by adjusting the length of side chain and the number of benzimidazole groups. With the acid doping level of 19.3, the grafted membrane exhibits a high conductivity of 101 mS cm−1 and a great power density of 305 mW cm−2 at 160 °C without humidification. The molecular dynamics simulation show that the improved performance of the grafted membrane is mainly due to the increase in the free volume of the polymer membrane, the enhancement of the binding energy and the densification of the hydrogen bond network structure between the polymer membrane and phosphoric acid.

Local structure of NH4Cl solution and its correlation with osmotic coefficient by molecular dynamics simulation and Raman spectroscopy

The NH4Cl aqueous solution ranging from 1.00 wt.% to 22.00 wt.% have been studied by molecular dynamic simulation, Raman spectroscopy and polarized Raman spectroscopy. Radial distribution function (RDF), diffusion property, structure of hydrogen bond network, excess Raman spectroscopy and non-coincidence effect (NCE) were obtained. The contact ion pairs of cation and anion appeared in NH4Cl solution of 15.00 wt.%, leading to fundamental changes in hydrogen bonds and the hydrated shell. The evolution of hydrogen bond with mass fraction in solution was described. The osmotic coefficient of NH4Cl aqueous solution was obtained by thermodynamic calculation, and its linear relationship with double donor single acceptor hydrogen bond (DDA-OH) was preliminarily summarized. Moreover, the experimental results are in good agreement with the simulation results.

Copper(II) Complex of a Urea‐functionalized Pyridyl Ligand: Synthesis, Crystal Structure, and Acetate Binding Properties

A copper(II) acetate complex with a urea‐functionalized pyridyl ligand, [CuL(OAc)2]2·2AcOH (1) [L = N‐(3‐chlorophenyl)‐N'‐(3‐pyridyl) urea], was synthesized by the reaction of L with Cu(OAc)2 in methanol. A zigzag‐shaped hydrogen bond chain of L is obtained via urea N–H···Npyridyl interactions, and a two‐dimensional hydrogen bond network structure is further formed through the C–H···O interaction. In the complex 1, a paddle‐wheel structure is generated by Cu···Oacetate interactions and Cu···Npyridyl interactions. Furthermore, hydrogen bonding chain structure is extended through weak C–H···O hydrogen bond interactions. Through ultraviolet‐visible (UV/Vis) spectroscopy, the acetate binding properties of L in solution were also evaluated. Variable temperature magnetic susceptibility measurement indicates that the metal complex 1 displays antiferromagnetic coupling property.

Spectroscopic evidence for characteristic hydrogen bonding pattern in a system with pseudo-Jahn-Teller effect: the chromium(II) saccharinate hexahydrate

A detailed Fourier-transform infrared spectroscopic study of protiated and series of deuterated analogues of chromium(II) saccharinate hexahydrate has been carried out. The spectral appearances in the regions of O–H and O–D stretching modes (particularly those corresponding to the isotopically isolated, i.e. uncoupled oscillators) strongly imply that the hydrogen bonding network in the studied compound is similar to that in the case of copper(II) saccharinate hexahydrate, which is somewhat different from the other members of the isomorphous series of metal(II) saccharinates hexahydrates. Therefore, a reinvestigation of the hydrogen bonding network structure in the title system by neutron diffraction technique is proposed. The similarity of the hydrogen bonding network in the mentioned two compounds is due to the presence of a Jahn–Teller active ion in both of them, which, in a non-ideal octahedral immediate surrounding leads to a second-order (or pseudo-) Jahn–Teller effect. This is clearly seen e.g. from the bond length distortion indices which are an order of magnitude higher in the chromium and copper compounds as compared to the other members of the isomorphous series, that do not contain a Jahn-Teller active ion. A very good correlation between the crystallographically determined proton donor – proton acceptor O···O distances and the experimentally measured O–D stretching frequencies of the corresponding uncoupled oscillators has been found in the studied compound. The derived correlation equation has an excellent predictive value within a rather wide range of distances/frequencies.

Hydrogen Motion of CsH2PO4 Electrolyte Using 1H and 31P High-Resolution Nuclear Magnetic Resonance (NMR) Spectroscopy

This study investigated hydrogen-bonded CsH2PO4 (CDP) solid acid using 1H and 31P high-resolution nuclear magnetic resonance (NMR). Below the superprotonic phase transition temperature, the temperature dependence of the 1H NMR spectra was observed with two different hydrogen-bonded networks in the CDP structure. The systematic evolution of the lineshape with temperature dependence indicates hydrogen hopping in the chemical structure of hydrogen-bonded networks in a solid acid lattice. The proton conduction under two types of hydrogen-bonds—interchain and intrachain—in CsH2PO4 in the chemical environments of PO4 tetrahedra is discussed.

A Constant Potential Molecular Dynamics Simulation Study of the Atomic‐Scale Structure of Water Surfaces Near Electrodes

Summary of main observation and conclusionNovel and technologically important processes and phenomena arise at water surfaces in the presence of electric fields. However, experimental measurements on water surfaces are challenging, and the results are scarce and inconclusive. In this work, the constant potential molecular dynamics method, in which the electrode charges are allowed to fluctuate to keep the electric potential fixed, was implemented in the study of a near‐electrode water surface systems. This simulation system was set up with a vapor/liquid‐water/vapor slab and two electrodes under different sets of applied electrostatic potential, yielding i) a detailed characterization of the external E‐field dependent electrostatic potential/density/dipole moment density profiles, and ii) the relationship between the water surface width and the applied electrode voltage differences which has been rarely reported. The adjustments in the number density profiles in the vicinity of water surfaces due to external E‐fields were observed, while the capillary interfacial widths for the surfaces near both cathode and anode were found with different increment rates under increasing E‐fields. By examining dipole density profiles across the water surfaces, we found that external E‐field induced polarization occurs in both bulk and surface regimes, yet the surface polarization densities vary asymmetrically with respect to the increasing E‐fields. Detailed discussions were carried out to explain the correlation between water surface tension and surface widths, as well as the interplay between the surface polarization densities and the hydrogen bond network structure. We conclude that the mechanical and structural properties of the water surfaces could be tuned by both magnitude and direction of the strong external E‐fields. We also recognize that more surface properties with application value, such as dielectric permittivity tensor or surface potential, could also be regulated by the external E‐fields.

Supramolecular assembly of two copper(II) coordination compounds of topiroxostat with dialkylformamide ligands

Abstract A well-known selective xanthine oxidase inhibitor topiroxostat, 4-[3-(4-pyridinyl)-1H-1,2,4-triazol-5-yl]-2-pyridinecarbonitrile (ptpc), was selected to react with Cu(NO3)2 · 3H2O in dialkylformamide solvents, affording two novel supramolecular coordination compounds [Cu(ptpc)2(NO3)(DMF)2] · NO3 (1) and Cu(ptpc)2(NO3)2(DEF)2 (2). Single-crystal X-ray diffraction revealed that compound 1 crystallizes in the monoclinic space group C2/c. The components are assembled to a 3D supramolecular framework through hydrogen-bonding interactions between the large [Cu(ptpc)2(NO3)(DMF)2]+ cationic moieties and the nitrate anions. Compound 2 crystallizes in the triclinic space group P1̅ and shows a 2D hydrogen-bonded network structure. The results clearly indicate that dialkylformamide solvents with different sizes have an important influence on the structures of the complexes. The luminescence properties of 1 and 2 in the solid state and their thermal stabilities were also investigated.

Confirmation of local water structure confined in ionic liquids using H/D exchange

We investigated the local structure of water molecules confined inside ionic liquid (IL) nanodomain structures of 1-butyl-3-methylimidazolium tetrafluoroborate, (abbreviated as [bmim][BF4]) using NMR and Raman spectroscopy to monitor H/D exchange reaction between H2O and D2O water molecules. The results showed that fewer than 5 water molecules were probably present in the water domain of the IL nanospace at less than ~60 water mol%, where the translational motion of water molecules was inhibited and proton exchange rate was highly restricted. These phenomena were attributed to the absence of bulk hydrogen-bonded network structures in the water domain. The H atoms of the HDO molecule tended to interact with BF4− anions, whereas the D atoms of HDO interacted with the C2 atoms of the bmim+ cation. As a result, water molecules became trapped inside the IL framework, which is likely to be the main cause of the reduction in H/D exchange rate among water molecules in the mixture.

Understanding the effect of nanoconfinement on the structure of water hydrogen bond networks.

Using an integrated approach of network theory and atomistic molecular dynamics simulations, we performed a detailed topological analysis on hydrogen bond networks of water confined between either two graphene sheets or two lipid bilayers to explore the structural perturbation of these networks under nanoscale confinement. The hydrogen bond network structure can be perturbed to a considerable extent when water is confined by such surfaces, yet no small-world behaviour is observed. The presence of ions also reduces the network complexity but its effect may be small depending on the type of confining surfaces. We developed an information flow model to evaluate the fluctuating nature of hydrogen bond networks and to characterise the dynamic, long-distance hydrogen-bonded chains in water. We found that the length and directionality of the hydrogen bond "trails" are highly susceptible to the type of confining surfaces and the degree of confinement. In particular, the endpoints of the hydrogen bond trails are not completely random in confined water, in which inherent distributions are determined by the density of water and the density of hydrogen bonds. This work forms the basis for the study of the pure effect of hydrogen bond network topology on various transport processes, such as proton transfer, that occur along a sequence of hydrogen bonds in a biochemical system. Our results suggest that a combined effect of the structure and lifetime of the hydrogen bond network of interfacial water may contribute to high lateral proton diffusivity at the surface of a lipid membrane.

Why the Gas Uptake Behavior of Dry Salt Water Is Vastly Different above 279 K? A Dynamics-Controlled Process That Can Be Intensified by the Cooling Stimulation Method

The effect of temperature on CO2 uptake behavior of dry salt water was investigated by kinetics measurements and molecular dynamics simulations. The gas uptake capacity was found to be dramatically decreased at 279 K by the direct cooling adsorption procedure. By means of simulations, it was realized that the orientation of water and the hydrogen bond network structure near the silica surface are sensitive to temperature. With the temperature decreased, the water layer becomes local-structured by forming more hydrogen bonds, facilitating the capture of CO2 by forming the hydrate precursor easily. The change of the structure is irreversible once the CO2 uptake process occurred; hence, the gas uptake can carry on continuously even if the temperature rises. A cooling stimulation procedure was found to efficiently trigger the gas uptake process at temperatures higher than 279 K. By using this method, gas adsorption can be induced because the energy barrier of nucleation is reduced. This technology can save a ...

Fracture of the Intermolecular Hydrogen Bond Network Structure of Glycerol Modified by Carbon Nanotubes

An intermolecular hydrogen bond network structure is usually formed in the liquid state and affects physicochemical properties, such as melting point, boiling point, and viscosity. The intermolecul...

Chloridobis(ethane-1,2-diamine-κ2 N,N′)(3-methylpyridine-κN)cobalt(III) dichloride monohydrate

In the title hydrated salt, [CoCl(C6H7N)(C2H8N2)2]Cl2·H2O, the CoIII ion exhibits a distorted octahedral coordination envirnoment defined by four N atoms of two ethane-1,2-diamine ligands, another N atom of the pyridine ligand and a Cl− ligand. The pyridine N atom and the Cl− ligand are in cis positions relative to each other. The crystal packing is dominated by intermolecular N—H...Cl, O—H...Cl and O—H...H hydrogen-bonding interactions involving the amino groups of the complex cation, the lattice water molecule and the non-coordinating Cl− anions. Weak C—H...Cl interactions consolidate the three-dimensional hydrogen-bonded network structure.

Selective and low potential electrocatalytic oxidation of NADH using a 2,2-diphenyl-1-picrylhydrazyl immobilized graphene oxide-modified glassy carbon electrode

DPPH (2,2-diphenyl-1-picrylhydrazil), a free radical-containing organic compound, is used widely to evaluate the antioxidant properties of plant constituents. Here, we report an efficient electroactive DPPH molecular system with excellent electrocatalytic sensor properties, which is clearly distinct from the traditional free radical-based quenching mechanism. This unusual molecular status was achieved by the electrochemical immobilization of graphene oxide (GO)-stabilized DPPH on a glassy carbon electrode (GCE). Potential cycling of the DPPH adsorbed-GCE/GO between − 1 and 1 V (Ag/AgCl) in a pH 7 solution revealed a stable and well-defined pair of redox peaks with a standard electrode potential, E0′ = 0 ± 0.01 V (Ag/AgCl). Several electrochemical characterization studies as well as surface analysis of the GCE/GO@DPPH-modified electrode by transmission electron microscopy, Raman, and infrared spectroscopy collectively identified the imine/amine groups as the redox centers of the electroactive DPPH on GO. The use of different carbon-supports showed that only oxygen-functionalized GO and MWCNTs could provide major electroactivity for DPPH. This highlights the importance of a strong hydrogen-bonded network structure assisted by the concomitant π-π interactions between the organic moiety and oxygen function groups of carbon for the high electroactivity and stability of the GCE/GO@DPPH-NH/NH2-modified electrode. The developed electrode exhibited remarkable performance towards the electrocatalytic oxidation of NADH at 0 V (Ag/AgCl). The amperometric i-t sensing of NADH showed high sensitivity (488 nA μM−1 cm−2) and an extended linear range (50 to 450 μM) with complete freedom from several common biochemical/chemical interferents, such as ascorbic acid, hydrazine, glucose, cysteine, citric acid, nitrate, and uric acid.

Interplay between translational diffusion and large-amplitude angular jumps of water molecules.

Understanding the microscopic mechanism of water molecular translational diffusion is a challenging topic in both physics and chemistry. Here, we report an investigation on the interplay between the translational diffusion and the large-amplitude angular jumps of water molecules in bulk water using molecular dynamics simulations. We found that large-amplitude angular jumps are tightly coupled to the translational diffusions. Particularly, we revealed that concurrent rotational jumps of spatially neighboring water molecules induce inter-basin translational jumps, which contributes to the fast component of the water translational diffusion. Consequently, the translational diffusion shows positional heterogeneity; i.e., the neighbors of the water molecules with inter-basin translational jumps have larger probability to diffuse by inter-basin translational jumps. Our control simulations showed that a model water molecule with moderate hydrogen bond strength can diffuse much faster than a simple Lennard-Jones particle in bulk water due to the capability of disturbing the hydrogen bond network of the surrounding water molecules. Our results added to the understanding of the microscopic picture of the water translational diffusion and demonstrated the unique features of water diffusion arising from their hydrogen bond network structure compared with those of the simple liquids.

Hydrogen-bonded structures from adamantane-based catechols

Adamantane-based bis- and tris-catechols were synthesized to examine the effect of hydrogen bonds on the arrangement and packing of the components in the crystalline state. Single-crystal X-ray crystallographic analysis revealed that hydrogen bonds formed by the hydroxyl groups of catechol groups play essential roles in the production of various types of unique structures. 1,3-Bis(3,4-dihydroxyphenyl)adamantane (1) provided hydrogen-bonded network structures composed of helical chains in crystal from chloroform/methanol, and layer structures in crystal from ethyl acetate/hexane. The complexation of 1 with 1,3,5-trinitrobenzene or 1,2,4,5-tetracyanobenzene resulted in the formation of co-crystals, respectively. One-dimensional hydrogen-bonded structures were constructed from the adamantane-based molecules, which participated in charge-transfer interactions with guests. 1,3,5-Tris(3,4-dihydroxyphenyl)adamantane also afforded crystal, and the components were assembled into infinite polymers.

Structural Origin of Shear Viscosity of Liquid Water.

The relation between the microscopic structure and shear viscosity of liquid water was analyzed by calculating the cross-correlation between the shear stress and the two-body density using the molecular dynamics simulation. The slow viscoelastic relaxation that dominates the steady-state shear viscosity was ascribed to the destruction of the hydrogen-bonding network structure along the compression axis of the shear distortion, which resembles the structural change under isotropic hydrostatic compression. It means that the shear viscosity of liquid water reflects the anisotropic destruction-formation dynamics of the hydrogen-bonding network.

Competition between hydrogen bonds and van der Waals forces in intermolecular structure formation of protonated branched-chain alcohol clusters.

To investigate the influence of bulky alkyl groups on hydrogen-bonded (H-bonded) network structures of alcohols, infrared (IR) spectra of protonated clusters of 2-propanol (2-PrOH) and tert-butyl alcohol (t-BuOH) were observed in the OH and CH stretch regions. In addition, by varying the tag species, the temperature dependence profile of the isomer population of H+(t-BuOH)n was revealed. An extensive search for stable isomers was performed using dispersion-corrected density functional theory methods, and temperature-dependent IR spectral simulations were done on the basis of the harmonic superposition approximation. The computational results qualitatively agreed with the observed size and temperature dependence of the H-bonded network structures of these protonated bulky alcohol clusters. However, the difficulty in the quantitative evaluation of dispersion was also demonstrated. It was shown that H+(2-PrOH)n (n = 4-7) have essentially the same network structures as the protonated normal alcohol clusters studied so far. On the other hand, H+(t-BuOH)n (n = 4-8) showed a clear preference for the smaller-membered ring structures, that is very different from the preference of the protonated normal alcohol clusters. The origin of the different structure preferences was discussed in terms of the steric effect and dispersion.

Hydrogen bond network structures of protonated short-chain alcohol clusters.

Because of the hydrogen bond coordination properties of alcohols, their possible hydrogen bond network structures are categorized into only a few types. Therefore, gas phase clusters of alcohols can be a very simple model system to examine the properties of hydrogen bond networks, such as structural development with cluster size and temperature dependence. In this perspective, we focus on the structural study of protonated short-chain alcohol clusters, whose excess protons (charge) enable size-selective spectroscopy in combination with mass spectrometric techniques. Size-selective infrared spectroscopy and a theoretical multi-scale isomer search were applied to protonated clusters of methanol, which is a prototype of short-chain alcohols, and their hydrogen bond network development is elucidated in detail. Complete isomer population switching with increasing temperature was predicted by the quantum harmonic superposition approximation and this isomer switching was evidenced by the remarkable temperature (internal vibrational energy) dependence of the observed infrared spectra. The characteristics of the temperature dependence of protonated methanol were compared with those of water and neutral methanol. In addition, possible hydrogen bond networks of methanolated ions were discussed on the basis of the results for protonated methanol. Stepwise changes in the internal energy of clusters with inert gas tagging are demonstrated. Convergence of the hydrogen bond network to the bulk-like network in large clusters is also discussed. The hydrogen bond structures of the protonated clusters of longer normal alkyl chain alcohols (ethanol, 1-propanol, 1-butanol, and 1-pentanol) are determined by comparison of their infrared spectra with those of the protonated methanol clusters. It is demonstrated that the normal alkyl chain interferes only slightly with the most stable hydrogen bond structure, although a few exceptional cases were also found. These exception cases serve as good model systems for further theoretical and computational studies.