Showing papers on "Water cluster published in 2017"

Ab initio molecular dynamics simulations of liquid water using high quality meta-GGA functionals

Abstract: We have used ab initio molecular dynamics (AIMD) to characterize water properties using two meta-generalized gradient approximation (meta-GGA) functionals, M06-L-D3 and B97M-rV, and compared their performance against a standard GGA corrected for dispersion, revPBE-D3, at ambient conditions (298 K, and 1 g cm−3 or 1 atm). Simulations of the equilibrium density, radial distribution functions, self-diffusivity, the infrared spectrum, liquid dipole moments, and characterizations of the hydrogen bond network show that all three functionals have overcome the problem of the early AIMD simulations that erroneously found ambient water to be highly structured, but they differ substantially among themselves in agreement with experiment on this range of water properties. We show directly using water cluster data up through the pentamer that revPBE-D3 benefits from a cancellation of its intrinsic functional error by running classical trajectories, whereas the meta-GGA functionals are demonstrably more accurate and would require the simulation of nuclear quantum effects to realize better agreement with all cluster and condensed phase properties.

Operando Properties of Gas Diffusion Layers: Saturation and Liquid Permeability

Abstract: Polymer electrolyte fuel cells (PEFC) require a sophisticated water management to operate efficiently, especially at high current densities which are needed to reach system cost targets. The description of the complicated two-phase water transport remains a challenge in PEFC models and requires experimental validation on various length scales. In this work, operando X-ray tomographic microscopy (XTM) with scan times of 10 s was used to depict the liquid water at defined conditions at a technically relevant cell temperature of 80°C. Cells with Toray TGP-H-060 gas diffusion layer (GDL) with microporous layer (MPL) and different rib width were operated with different feed gas humidifications (under- and oversaturated) and current densities between 0.75 to 3.0 A/cm2. Based on the quantification of the local and average saturation, the distribution of water cluster size is analyzed. Different categories of the water cluster connectivity are defined and quantified. The analysis is complemented with numerical simulations of the permeability in the liquid phase of the GDL that is correlated to saturation for the different GDL domains. The numerical simulations of the pressure drop of liquid water flow from the catalyst layer toward the gas channels in channel-rib repetition units allows for conclusions on cluster growth mechanisms.

Mechanistic insights into aqueous phase propanol dehydration in H-ZSM-5 zeolite

Abstract: Aqueous phase dehydration of 1-propanol over H-ZSM-5 zeolite was investigated using density functional theory (DFT) calculations. The water molecules in the zeolite pores prefer to aggregate via the hydrogen bonding network and be protonated at the Bronsted acidic sites (BAS). Two typical configurations, i.e., dispersed and clustered, of water molecules were identified by ab initio molecular dynamics simulations of the mimicking aqueous phase H-ZSM-5 unit cell with 20 water molecules per unit cell. DFT calculated Gibbs free energies suggest that the dimeric propanol-propanol, the propanol-water, and the trimeric propanol-propanol-water complexes are formed at high propanol concentrations in aqueous phase, which provide a kinetically feasible dehydration reaction channel of 1-propanol to propene. The calculation results indicate that the propanol dehydration via the unimolecular mechanism becomes kinetically discouraged due to the enhanced stability of the protonated dimeric propanol and the protonated water cluster acting as the BAS site for alcohol dehydration. This article is protected by copyright. All rights reserved.

A Combined Experimental and Theoretical Study on the Formation of a Cyclic Tetrameric Water Cluster and a Similar Type of Cyclic Cluster in Copper(II) Schiff Base Complexes

Abstract: Two copper(II) complexes, [CuL1]⋅2H2O (1) and [CuL2]⋅H2O (2) [where H2L1 {= N,N′-bis(3-methoxysalicylidene)propane-1,3-diamine} and H2L2 {=N,N′-bis(3-ethoxysalicylidene)propane-1,3-diamine} are potential hexadentate N2O4 donor compartmental Schiff bases], were synthesized and characterized. In each complex, copper(II) was placed in the inner N2O2 environment, with a four-coordinate distorted square planar environment keeping the outer O4 compartment pendant. Lattice water molecules were hydrogen bonded with the pendant O4 compartments of the Schiff base ligands. Both complexes formed self-assembled dimers via chelate ring⋅⋅⋅chelate ring and C–H⋅⋅⋅π interactions. A cyclic tetrameric water cluster was stabilized by the oxygen rich region of the Schiff-base ligand in complex 1. A similar type of cyclic cluster involving two water molecules was also observed in complex 2. These supramolecular assemblies were analysed via a DFT study at the M06-2X/def2-TZVP level of theory.

DFT study of water adsorption on lignite molecule surface

Abstract: High moisture content is a main characteristic of low-rank coal, such as lignite. Numerous oxygen containing functional groups in lignite make it represent some special properties, and these functional groups affect the adsorption mechanisms of water molecules on lignite surface. This study reports some typical water · · · lignite conformations, along with a detailed analysis of the geometry, electrostatic potential distribution, reduced density gradient of interaction, and interaction energy decomposition. The results show that water molecules tend to aggregate around functional groups, and hydrogen bonds play a dominant role in the interaction. The adsorption energy of water cluster on lignite surface is larger than that of isolated water molecule, a good linear relationship between the interaction distance and adsorption energy of layers has been found. Since water is a polar molecule, the local minima and maxima of electrostatic potential in conformations increase along with more water adsorbing on lignite surface. Reduced density gradient analysis shows that H-bonds, van der Waals interaction, and a little steric make up the interaction between water cluster and lignite molecule. In these studied conformations which mainly are H-bond complexes, electrostatic and exchange repulsion play a dominant role, whereas polarization and dispersion make relatively small contribution to the interaction. Attractive and repulsive interaction both affect the stability of water · · · lignite conformations.

Which Density Functional Should Be Used to Describe Protonated Water Clusters

Abstract: Protonated water cluster is one of the most important hydrogen-bond network systems. Finding an appropriate DFT method to study the properties of protonated water clusters can substantially improve the economy in computational resources without sacrificing the accuracy compared to high-level methods. Using high-level MP2 and CCSD(T) methods as well as experimental results as benchmark, we systematically examined the effect of seven exchange-correlation GGA functionals (with BLYP, B3LYP, X3LYP, PBE0, PBE1W, M05-2X, and B97-D parametrizations) in describing the geometric parameters, interaction energies, dipole moments, and vibrational properties of protonated water clusters H+(H2O)2–9,12. The overall performance of all these functionals is acceptable, and each of them has its advantage in certain aspects. X3LYP is the best to describe the interaction energies, and PBE0 and M05-2X are also recommended to investigate interaction energies. PBE0 gives the best anharmonic frequencies, followed by PBE1W, B97-D a...

Deciphering the aqueous chemistry of glyoxal oxidation with hydrogen peroxide using molecular imaging

Abstract: Aqueous surfaces after photochemical and dark reactions of glyoxal and hydrogen peroxide (H2O2) have been studied using a microfluidic reactor coupled with an in situ liquid time-of-flight secondary ion mass spectrometry (ToF-SIMS) for the first time Spectral principal component analysis was used to determine similarities and differences among various photochemical aging and dark reaction samples and controls Compared with previous results using bulk solutions, our unique liquid surface molecular imaging approach provided observations of glyoxal hydration (ie, first and secondary products), oxidation products (ie, glyoxylic acid, oxalic acid, formic acid, tartaric acid), oligomers, cluster ions, and water clusters with sub-micrometer spatial resolution Observations of oxidation products give the physical foundation to deduce new reaction pathways at the aqueous surface The first chemical mapping of water cluster changes between dark and photochemical aging suggests that glyoxal oxidation affects the hydrophobicity and water microenvironment at the surface, influencing the particle's ability of reactive uptake and subsequent cloud condensation nucleation and/or ice nucleation activation Moreover, SIMS three-dimensional chemical mapping has made it possible to visualize the surface mixing state for the first time We potentially provide a new method to investigate complex surface chemistry as an important source of aqueous secondary organic aerosol (aqSOA) formation in atmospheric chemistry

Isothermal equation of state and high-pressure phase transitions of synthetic meridianiite (MgSO4·11D2O) determined by neutron powder diffraction and quasielastic neutron spectroscopy

Abstract: We have collected neutron powder diffraction data from MgSO4·11D2O (the deuterated analogue of meridianiite), a highly hydrated sulfate salt that is thought to be a candidate rock-forming mineral in some icy satellites of the outer solar system. Our measurements, made using the PEARL/HiPr and OSIRIS instruments at the ISIS neutron spallation source, covered the range 0.1 < P < 800 MPa and 150 < T < 280 K. The refined unit-cell volumes as a function of P and T are parameterized in the form of a Murnaghan integrated linear equation of state having a zero-pressure volume V0 = 706.23 (8) A3, zero-pressure bulk modulus K0 = 19.9 (4) GPa and its first pressure derivative, K′ = 9 (1). The structure's compressibility is highly anisotropic, as expected, with the three principal directions of the unit-strain tensor having compressibilities of 9.6 × 10−3, 3.4 × 10−2 and 3.4 × 10−3 GPa−1, the most compressible direction being perpendicular to the long axis of a discrete hexadecameric water cluster, (D2O)16. At high pressure we observed two different phase transitions. First, warming of MgSO4·11D2O at 545 MPa resulted in a change in the diffraction pattern at 275 K consistent with partial (peritectic) melting; quasielastic neutron spectra collected simultaneously evince the onset of the reorientational motion of D2O molecules with characteristic time-scales of 20–30 ps, longer than those found in bulk liquid water at the same temperature and commensurate with the lifetime of solvent-separated ion pairs in aqueous MgSO4. Second, at ∼ 0.9 GPa, 240 K, MgSO4·11D2O decomposed into high-pressure water ice phase VI and MgSO4·9D2O, a recently discovered phase that has hitherto only been formed at ambient pressure by quenching small droplets of MgSO4(aq) in liquid nitrogen. The fate of the high-pressure enneahydrate on further compression and warming is not clear from the neutron diffraction data, but its occurrence indicates that it may also be a rock-forming mineral in the deep mantles of large icy satellites.

Formation of Exotic Networks of Water Clusters in Helium Droplets Facilitated by the Presence of Neon Atoms

Abstract: Water clusters are formed in helium droplets via the sequential capture of monomers. One or two neon atoms are added to each droplet prior to the addition of water. The infrared spectrum of the droplet ensemble reveals several signatures of polar, water tetramer clusters having dipole moments between 2D and 3D. Comparison with ab initio computations supports the assignment of the cluster networks to noncyclic “3 + 1” clusters, which are ∼5.3 kcal/mol less stable than the global minimum nonpolar cyclic tetramer. The (H2O)3Ne + H2O ring insertion barrier is sufficiently large, such that evaporative helium cooling is capable of kinetically quenching the nonequilibrium tetramer system prior to its rearrangement to the lower energy cyclic species. To this end, the reported process results in the formation of exotic water cluster networks that are either higher in energy than the most stable gas-phase analogs or not even stable in the gas phase.

Effect of Water Clustering on the Activity of Candida antarctica Lipase B in Organic Medium

Abstract: The effect of initial water activity of MTBE (methyl tert-butyl ether) medium on CALB (Candida antarctica lipase B) catalyzed esterification reaction is investigated using experimental methods and classical molecular dynamics (MD) simulations. The experimental kinetic studies show that the initial reaction rate of CALB-catalyzed esterification reaction between butyric acid and ethanol decreases with increasing initial water activity of the medium. The highest rate of esterification is observed at the lowest water activity studied. MD simulations were performed to gain a molecular insight on the effect of initial water activity on the rate of CALB-catalyzed reaction. Our results show that hydration has an insignificant effect on the structure and flexibility of CALB. Rather, it appears that water molecules bind to certain regions (“hot spots”) on the CALB surface and form clusters. The size of the water clusters at these hot spot regions gradually increase and expand with increasing water activity. Consequently, the surface area of CALB covered by the water molecules also increases. Specifically, our results indicate that a particular water cluster located close to the active site partially cover the binding pocket of substrate at high water activity. As a consequence, the effective concentration of substrate at the catalytic site decreases. Therefore, the reaction rate slows down with increasing water activity, which correlates well with the observed decrease in the experimentally determined initial reaction rate.

A kinetic model of water adsorption, clustering and dissociation on the Fe3S4{001} surface.

Abstract: The interaction of water with catalyst surfaces is a common process which requires investigation. Here, we have employed density functional theory calculations to investigate the adsorption of up to ten water molecules on the {001} surface of greigite (Fe3S4), which owing to its redox properties, is of increasing interest as a catalyst, e.g. in electro-catalysis. We have systematically analyzed and characterized the modes of water adsorption on the surface, where we have considered both molecular and dissociative adsorption processes. The calculations show that molecular adsorption is the predominant state on these surfaces, from both a thermodynamic and kinetic point of view. We have explored the molecular dispersion on the surface under different coverages and found that the orientation of the molecule, and therefore the surface dipole, depends on the number of adsorbed molecules. The interactions between the water molecules become stronger with an increasing number of water molecules, following an exponential decay which tends to the interaction energy found in bulk water. We have also shown the evolution of the infra-red signals as a function of water coverage relating to the H-bond networks formed on the surface. Next we have included these results in a classical micro-kinetic model, which introduced the effects of temperature in the simulations, thus helping us to derive the water cluster size on the greigite surface as a function of the initial conditions of pressure, temperature and external potential. The kinetic model concluded that water molecules agglomerate in clusters instead of wetting the surface, which agrees with the low hydrophilicity of Fe3S4. Clusters consisting of four water molecules was shown to be the most stable cluster under a wide range of temperatures and external potential.

Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif

Abstract: An ongoing controversy about water cluster anions concerns the electron-binding motif, whether the charge center is localized at the surface or within the cluster interior. Here, mixed quantum-classical dynamics simulations have been carried out for a wide range of cluster sizes (n ≤ 1000) for (H2O)n– and (D2O)n–, based on a nonequilibrium first-order rate constant approach. The computed data are in good general agreement with time-resolved photoelectron imaging results (n ≤ 200). The analysis reveals that, for surface state electrons, the cluster size dependence of the excited state electronic energy gap and the magnitude of the nonadiabatic couplings have compensating influences on the excited state lifetimes: the excited state lifetime for surface states reaches a minimum for n ∼ 150 and then increases for larger clusters. It is concluded that the electron resides in a surface-localized motif in all of these measured clusters, dominating at least up to n = 200.

On the possibility of chiral structure-density submillimeter inhomogeneities existing in water

Abstract: The existence of stable formations representing submillimeter density inhomogeneities in aqueous solutions was confirmed by using the laser light diffraction (LALLS) method and also the dynamic (DLS) and two-dimensional light scattering methods. The effect of low angle light scattering by giant water clusters depends on the presence of ions or dissolved substances in water solution, and also on the concentration of deuterium (heavy hydrogen isotope). It has been shown that the reduction of deuterium concentration results in the reduction of light scattering ability of protein nanoparticle preparations and water. The formation of long-living discrete water cluster structures possessing chiral properties was substantiated for explaining the manifestation of effects of weak impacts and low concentrations on biological objects.

Hydrogen bond breaking dynamics in the water pentamer: Terahertz VRT spectroscopy of a 20 μm libration.

Abstract: Hydrogen bonds in solid and liquid water are formed and broken via librational vibrations, hence characterizing the details of these motions is vital to understanding these important dynamics. Here we report the measurement and assignment of 875 transitions comprising 6 subbands originating from out-of-plane librational transitions of the water pentamer-d10 near 512 cm-1. The precisely measured (ca. 1 ppm) transitions reveal bifurcation splittings of ∼1884 MHz, a ∼4000× enhancement over ground state splittings and 100× greater than predicted by theory. The pentamer is thus the third water cluster to display greatly enhanced bifurcation tunneling upon single quantum excitation of librational vibrations. From the intensity pattern of the observed transitions, the mechanism of bifurcation is established by comparison with theoretical predictions.

Fast Room Temperature Synthesis of the Thiostannate [Ni(2amp)3]2[Sn2S6]·9.5H2O: Crystal Structure and Properties

Abstract: The new compound [Ni(2amp)3]2[Sn2S6]·9.5H2O (1) was obtained at room temperature using Na4SnS4·14H2O and [Ni(2amp)3][ClO4]2 [2amp = 2-(aminomethyl)pyridine] or [Ni(en)3]Cl2·2H2O as starting materials. The synthesis is fast and crystals grew within 1 d. The two unique anions and cations are embedded in an extended water cluster network and an extensive hydrogen bonding network is formed. The complexes are arranged to form intermolecular π–π interactions between neighboring 2amp ligands. A Hirshfeld surface analysis yields a detailed picture about the intermolecular interactions. Heating the compound up to about 100 °C the crystal water molecules are removed and a new crystalline material is formed. Storing the dehydrated sample in a water atmosphere leads to reformation of the title compound.

Influence of anions on copper(II) complexes of a flexible bis-pyridyltrazole ligand: identification of a discrete water–chloride cluster [(H2O)10(Cl)8]8−

Abstract: Three Cu(II) complexes, [Cu(H2L)(ClO4)2] (1), [Cu(H2L)0.5(µ-SO4)(H2O)]·H2O (2) and [{Cu(H2L)(H2O)}{Cu(H2L)(Cl)]Cl3·4H2O (3), with a flexible ligand 1,2-bis(5-(pyridine-2-yl)-1,2,4-triazole-3-yl)ethane (H2L) were synthesized from various Cu(II) salts. X-ray crystal structure analysis reveals that the H2L ligand demonstrates different coordination modes in each of these complexes. Complex 1 shows a mononuclear structure with ClO4 − anions weakly coordinated to the metal center, which is further extended into a 1-D assembly through hydrogen bonds. Complex 2 is a polymeric species in which the dinuclear units [Cu2(H2L)(H2O)2] are linked through SO4 2− anions to form 1-D chains, which are further associated into a 2-D assembly through a self-assembled decameric water cluster. Complex 3 features an interesting 3-D coordination architecture assembled through extensive hydrogen interactions between chloride anions and water molecules. Notably, a unique discrete water–chloride cluster [(H2O)10(Cl)8]8− built around a chair-like water–chloride octameric core is identified in the crystal matrix of complex 3. The choice of counteranion plays a key role in the diverse structures of these complexes. The spectroscopic properties of the complexes have also been investigated.

The Transferability of Topologically Partitioned Electron Correlation Energies in Water Clusters

Abstract: The electronic effects that govern the cohesion of water clusters are complex, demanding the inclusion of N-body, Coulomb, exchange and correlation effects. Here we present a much needed quantitative study of the effect of correlation (and hence dispersion) energy on the stabilization of water clusters. For this purpose we used a topological energy partitioning method called Interacting Quantum Atoms (IQA) to partition water clusters into topological atoms, based on a MP2/6-31G(d,p) wave function, and modified versions of GAUSSIAN09 and the Quantum Chemical Topology (QCT) program MORFI. Most of the cohesion in the water clusters provided by electron correlation comes from intramolecular energy stabilization. Hydrogen bond-related interactions tend to largely cancel each other. Electron correlation energies are transferable in almost all instances within 1 kcal mol-1 . This observed transferability is very important to the further development of the QCT force field FFLUX, especially to the future modelling of liquid water.

Investigation of Gaseous Elemental Mercury Oxidation by Non-thermal Plasma Injection Method

Abstract: The non-thermal plasma injection method was used to oxidize elemental mercury (Hg0) in this study. A mixture of water vapor and oxygen was selected as the discharge gases. Active species generated by a dielectric barrier discharge plasma reactor were introduced into the flue gas duct, where they reacted with Hg0. Different parameters including active particles, supply voltage, flow rate of injection gas, system temperature, and typical flue gas components were considered. Experimental results indicated that it produced a high yield of oxidative species than using a single discharge gas, and the reason was believed to be the dissociation and excitation of water molecules and oxygen molecules by electron impact. It was assumed that active radicals include O, O3, water cluster ions (O2+·H2O), and ·OH. Increasing the plasma injection volume led to higher content of reactive species in the system, which promoted Hg0 oxidation. Approximately 98.3% of Hg0 was oxidized at 4 kV of voltage with 20 mL/min of plasma ...

Molecular Dynamics Study of the Collision-Induced Reaction of H with CO on Small Water Clusters.

Abstract: The successive hydrogenation of CO is supposed to be the main mechanism leading to the formation of complex oxygenated species in the interstellar medium, possibly mediated by ice layers or ice grains. In order to simulate the dynamical influence of a water environment on the first step of the hydrogenation process, we perform molecular dynamics simulations of the reactive collision of H with CO adsorbed on water clusters in the framework of the self-consistent-charge density functional based tight-binding approach (SCC-DFTB) to calculate potential energy surfaces. The reaction probabilities and the reactive cross sections are determined for water cluster sizes up to ten water molecules. The collision results are analyzed in terms of different reaction pathways: reactive or nonreactive, sticking or desorption of the products or reactants. We show that the HCO radical, although potentially formed as an intermediate regardless of the size of the water cluster, is significantly stabilized for cluster sizes l...

A Water Cluster Conduit in Crystal

Abstract: The crystal structure of compound (1), [CuCl(phen)(H2NCH2COO)]∙4H2O, reveals an unusual hydrogen-bond water cluster aggregate T6(2)6(2). Four water molecules in (1) form an isolated water cluster, [(H2O)14]n, resembling a “phenanthro-[1,2]phenanthrene polymer structure shape” along the ac plane. The two face-face parallel [(H2O)14]n planes are bridged by Cl atoms in [CuCl(phen) (H2NCH2COO)] with a strong O-H∙∙∙Cl hydrogen bond to form a water cluster conduit.

Substituent effect on the hydrolysis of chlorosilanes: quantum chemical and QSPR study

Abstract: The substituent effect on the hydrolysis of chlorosilanes was studied computationally. Sixteen practically important compounds with SiCl bond were considered, and the stationary points along their reaction pathway with different-sized water clusters (monomer and tetramer) were investigated using density functional theory. While in the case of a single reactant water molecule the reactions are endothermic for most of the substituents, with the larger reactant water cluster the reactions are mainly exothermic. In the case of the reactant cluster consisting of four water molecules both an inversion and a retention pathway are located. The reaction barrier for both pathways is about 50% that with a single water molecule, and the inversion pathway is somewhat more preferred over retention for most substituents except for cage-like chlorosilanes. Strong correlations were indentified between several factors of substituted chlorosilanes and their activation energy of hydrolysis. Cl-Si-O bond angle in the transition state as an indirect descriptor of steric effect, electrophilicity and partial charge at silicon of the reactant chlorosilane, and the opportunity of extra H-bond formation have influence on the reaction barrier. Quantitative structure-property relationship (QSPR) analysis showed that the hydrolysis activation energy can be well described by using these factors.

Ozone–Water Interaction Revisited Through [(O3)m···(H2O)n] Clusters

Abstract: Ozone–water clusters (O3)m···(H2O)n (n = 2, 4, 5, 8, 10 and m = 1–5) have been theoretically investigated using density functional theory, with appropriate correction for BSSE that allow the accurate calculation of binding energies and change in enthalpies for the formation of clusters. For comparison, water clusters (H2O)n (n = 2, 4, 5, 8, 10) have also been studied at the same level of theory. The results presented herein provide a detailed understanding of the binding of ozone with water clusters of varying sizes. While the earlier reports on ozone–water interaction considered 1:1 complexes, the present study discusses the maximum binding capacity of varied sized water clusters towards ozone molecules. This is an important and interesting observation to decide the nature of interaction between ozone molecules and water clusters in light of the debacle between Van der Waals interactions and H-bonding. The study concludes that the maximum number of ozone molecules complexing with a water cluster is directly proportional to the number of hydrogen atoms available in the cluster for hydrogen bonding (H-bonding). In contrast to the previous studies, the present work emphasizes the binding of water clusters with ozone molecules through H-bonding instead of dipole–dipole interactions.