Showing papers on "Water cluster published in 2016"

Concerted hydrogen-bond breaking by quantum tunneling in the water hexamer prism

Abstract: The nature of the intermolecular forces between water molecules is the same in small hydrogen-bonded clusters as in the bulk. The rotational spectra of the clusters therefore give insight into the intermolecular forces present in liquid water and ice. The water hexamer is the smallest water cluster to support low-energy structures with branched three-dimensional hydrogen-bond networks, rather than cyclic two-dimensional topologies. Here we report measurements of splitting patterns in rotational transitions of the water hexamer prism, and we used quantum simulations to show that they result from geared and antigeared rotations of a pair of water molecules. Unlike previously reported tunneling motions in water clusters, the geared motion involves the concerted breaking of two hydrogen bonds. Similar types of motion may be feasible in interfacial and confined water.

Quantum dynamics in the smallest water droplet

Abstract: Water plays a central role in scientific disciplines ranging from geology to astronomy to biology. Yet it is an extraordinarily dif cult liquid to understand because of its complex, ever-changing patterns of hydrogen bonds. Studies of small water clusters have provided important insights into the concerted hydrogen-bond motions that can occur in water. These studies are also crucial for developing an accurate potential function for simulating the properties of liquid water and ice ( 1 ). On page 1310 of this issue, Richardson et al. ( 2 ) provide evidence for a concerted type of motion in which two hydrogen bonds in a water cluster are broken simultaneously (see the figure). The results have implications for many areas of scientific study, including the chemistry of polar solvents, the conformations of proteins, and the dissolution of ions in minerals.

A Protonated Water Cluster as a Transient Proton-Loading Site in Cytochrome c Oxidase.

Abstract: Cytochrome c oxidase (CcO) is a redox-driven proton pump that powers aerobic respiratory chains. We show here by multi-scale molecular simulations that a protonated water cluster near the active site is likely to serve as the transient proton-loading site (PLS) that stores a proton during the pumping process. The pKa of this water cluster is sensitive to the redox states of the enzyme, showing distinct similarities to other energy converting proton pumps.

Water-Induced Structural Changes in Crown Ethers from Broadband Rotational Spectroscopy.

Abstract: The complexes of 12-crown-4 ether (12C4) with water, generated in a supersonic jet, have been studied using broadband Fourier transform microwave spectroscopy. Three 1:1 and one 1:2 clusters have been observed and their structures unambiguously identified through the observation of isotopologue spectra. The structures of the clusters are based on networks of O–H···O and C–H···O hydrogen bonds. The most abundant 1:1 cluster is formed from the most stable S4 symmetry conformer of 12C4, even though it is not the energetically favored water complex. Interestingly, the structures of the most stable water cluster and the other remaining observed 1:1 and 1:2 complexes are formed from the second or the fifth most abundant conformers of 12C4. This shows the existence of a mechanism that changes the conformation of 12C4 so that the host–guest interactions can be maximized, even for a “soft” ligand like water.

Recognition of self-assembled water-nitrate cluster in a Co(III)-2,2 ′ -bipyridine host: Synthesis, X-ray structure, DNA cleavage, molecular docking and anticancer activity

Abstract: A mononuclear cobalt(III) complex [Co(bpy)2Cl2]NO3⋅2H2O (1) (bpy = 2,2′-bipyridine) has been synthesized and crystallographically characterized. Self-assembly of the lattice water molecules from rectangular tetrameric water cluster interacts with nitrate anion along the c-axis forming a six membered hexagonal water-nitrate cluster. It presents a new mode of association of water molecules with nitrate molecules which is not predicted theoretically or found experimentally. The molecule effectively cleaves bacterial genomic DNA and shows important cytotoxicity against human hepatocarcinoma cell (HepG2).

Sensitive detection of n -alkanes using a mixed ionization mode proton-transfer-reaction mass spectrometer

Abstract: . Proton-transfer-reaction mass spectrometry (PTR-MS) is a technique that is widely used to detect volatile organic compounds (VOCs) with proton affinities higher than water. However, n-alkanes generally have a lower proton affinity than water and therefore proton transfer (PT) by reaction with H3O+ is not an effective mechanism for their detection. In this study, we developed a method using a conventional PTR-MS to detect n-alkanes by optimizing ion source and drift tube conditions to vary the relative amounts of different primary ions (H3O+, O2+, NO+) in the reaction chamber (drift tube). There are very few studies on O2+ detection of alkanes and the mixed mode has never been proposed before. We determined the optimum conditions and the resulting reaction mechanisms, allowing detection of n-alkanes from n-pentane to n-tridecane. These compounds are mostly emitted by evaporative/combustion process from fossil fuel use. The charge transfer (CT) mechanism observed with O2+ was the main reaction channel for n-heptane and longer n-alkanes, while for n-pentane and n-hexane the main reaction channel was hydride abstraction (HA). Maximum sensitivities were obtained at low E ∕ N ratios (83 Td), low water flow (2 sccm) and high O2+ ∕ NO+ ratios (Uso = 180 V). Isotopic 13C contribution was taken into account by subtracting fractions of the preceding 12C ion signal based on the number of carbon atoms and the natural abundance of 13C (i.e., 5.6 % for n-pentane and 14.5 % for n-tridecane). After accounting for isotopic distributions, we found that PT cannot be observed for n-alkanes smaller than n-decane. Instead, protonated water clusters of n-alkanes (M ⋅ H3O+) species were observed with higher abundance using lower O2+ and higher water cluster fractions. M ⋅ H3O+ species are probably the source for the M + H+ species observed from n-decane to n-tridecane. Normalized sensitivities to O2+ or to the sum of O2++ NO+ were determined to be a good metric with which to compare sensitivities for n-alkane detection between experiments. Double hydride abstraction was observed from the reaction with O2+. Sensitivity to CT increased with carbon chain length from n-pentane to n-dodecane, sensitivity to HA increased from n-heptane to n-dodecane and sensitivity to PT increased from n-decane to n-tridecane. Sensitivity to CT exponentially decreased with molecular ionization energy, which is inversely related to the carbon chain length. We introduce a calibrated fragmentation algorithm as a method to determine the concentrations of n-alkanes and demonstrate its effectiveness using a custom n-alkane mixture and a much more complex oil example representing perhaps the most difficult mixture available for application of the method. We define optimum conditions for using the mixed ionization mode to measure n-alkanes in conventional PTR-MS instruments regardless of whether they are equipped with switchable reagent ion (SRI) capabilities.

Characterizing the local solvation environment of OH(-) in water clusters with AIMD.

Abstract: In this work, we use ab initio molecular dynamics coupled with metadynamics to explore and characterize the glassy potential energy landscape of the OH− in a 20 and 48 water cluster. The structural, energetic, and topological properties of OH− are characterized for both clusters and the molecular origins of the IR signatures are examined. We find that in both the small and large clusters, the OH− can donate or accept a varying number of hydrogen bonds confirming that the amphiphilic character does not depend on cluster size. However, we highlight some important differences found between the energetic and topological properties of both families of clusters which may have implications on understanding the changes in the solvation structure of OH− between bulk and interfacial environments. By studying the IR spectra of smaller subsets of molecules within the 20 water molecule cluster, we find that the IR spectrum of the bare OH− as well as the water molecule donating a strong hydrogen bond to it exhibits cha...

Low Temperature Kinetics of the First Steps of Water Cluster Formation.

Abstract: We present a combined experimental and theoretical low temperature kinetic study of water cluster formation. Water cluster growth takes place in low temperature (23-69 K) supersonic flows. The observed kinetics of formation of water clusters are reproduced with a kinetic model based on theoretical predictions for the first steps of clusterization. The temperature- and pressure-dependent association and dissociation rate coefficients are predicted with an ab initio transition state theory based master equation approach over a wide range of temperatures (20-100 K) and pressures (10^{-6}-10 bar).

Molecular investigation of oil–water separation using PVDF polymer by molecular dynamic simulation

Abstract: In this study, adsorption of water nanodroplets, oil nanodroplets and oil–water mixtures on a poly vinylidene fluoride (PVDF) surface is investigated computationally by a molecular dynamics (MD) approach and a mechanism for adsorption of the droplets is proposed. MD simulation results revealed that the PVDF surface is hydrophobic and oleophilic and can be used to separate oil from water. The acquired results are in good agreement with experimental results. As to investigate the origins for hydrophobicity of the PVDF surface, mobility of water molecules on the PVDF surface has been meticulously investigated. Investigations have proven that once the water molecules are organized singly or in small groups with two or three molecules, they exhibit very low levels of mobility, whereas water molecules in groups of four or more have high mobility. Some descriptive studies were performed based on MD simulation results to investigate the behavior observed in the systems including: (i) distribution of partial charges on the surface of PVDF. (ii) Dependence of the orientation of water molecules near the PVDF surface on the size of water cluster. (iii) Dependence of the distance of water molecules to the PVDF surface on the water cluster size. Manner of charge distribution on the PVDF demonstrated a formation of nano-domains with positive and negative charges on the surface. Dependence of the behavior of water molecules on the nano-domains on the water cluster size was investigated.

Exploring the hydrated microstructure and molecular mobility in blend polyelectrolyte membranes by quantum mechanics and molecular dynamics simulations

Abstract: Quantum mechanics and molecular dynamics simulations were employed to examine several structural and dynamical characteristics in blend SPEEK–SPPO based membranes at varied water content and temperature values. QM results showed that water molecules were localized around the SPEEK and SPPO sulfonate groups due to the hydrogen bonding interactions, which caused proton dissociation at the increased hydrations. By increasing the hydration level, more water molecules occupied the sulfonate fragments because of the improved sulfonate–water interactions, whereas the hydrogen bond interaction of sulfonate–hydronium ion was weakened, enabling the hydronium ions to be away from the sulfonate groups. Based on water cluster size distribution and structure factor evaluations, it seemed that by improving the water content, isolated smaller aqueous clusters appeared under lower hydration levels, which merged together to form larger clusters comprising almost all molecules. Diffusivities for water and hydronium ion were observed to be enhanced by an increase in water uptake, which were attributed to the fact that larger hydrophilic clusters across the swollen blend SPEEK–SPPO PEMs promoted molecular mobility. Similarly, enhancing operational temperature gave rise to an enhancement in the membrane transport dynamics. Finally, predicted water and hydronium ion diffusion coefficients were noted to be smaller in hydrated SPEEK–SPPO membrane as compared to Nafion under identical conditions, which was in agreement with the experimental results.

Cooperative protein structural dynamics of homodimeric hemoglobin linked to water cluster at subunit interface revealed by time-resolved X-ray solution scattering.

Abstract: Homodimeric hemoglobin (HbI) consisting of two subunits is a good model system for investigating the allosteric structural transition as it exhibits cooperativity in ligand binding. In this work, as an effort to extend our previous study on wild-type and F97Y mutant HbI, we investigate structural dynamics of a mutant HbI in solution to examine the role of well-organized interfacial water cluster, which has been known to mediate intersubunit communication in HbI. In the T72V mutant of HbI, the interfacial water cluster in the T state is perturbed due to the lack of Thr72, resulting in two less interfacial water molecules than in wild-type HbI. By performing picosecond time-resolved X-ray solution scattering experiment and kinetic analysis on the T72V mutant, we identify three structurally distinct intermediates (I1, I2, and I3) and show that the kinetics of the T72V mutant are well described by the same kinetic model used for wild-type and F97Y HbI, which involves biphasic kinetics, geminate recombination, and bimolecular CO recombination. The optimized kinetic model shows that the R-T transition and bimolecular CO recombination are faster in the T72V mutant than in the wild type. From structural analysis using species-associated difference scattering curves for the intermediates, we find that the T-like deoxy I3 intermediate in solution has a different structure from deoxy HbI in crystal. In addition, we extract detailed structural parameters of the intermediates such as E-F distance, intersubunit rotation angle, and heme-heme distance. By comparing the structures of protein intermediates in wild-type HbI and the T72V mutant, we reveal how the perturbation in the interfacial water cluster affects the kinetics and structures of reaction intermediates of HbI.

Electron density and electron temperature measurements in nanosecond pulse discharges over liquid water surface

Abstract: Time-resolved electron density, electron temperature, and gas temperature in nanosecond pulse discharges in helium and O2–He mixtures near liquid water surface are measured using Thomson/pure rotational Raman scattering, in two different geometries, (a) 'diffuse filament' discharge between a spherical high-voltage electrode and a grounded pin electrode placed in a reservoir filled with distilled water, with the tip exposed, and (b) dielectric barrier discharge between the high-voltage electrode and the liquid water surface. A diffuse plasma filament generated between the electrodes in helium during the primary discharge pulse exhibits noticeable constriction during the secondary discharge pulse several hundred ns later. Adding oxygen to the mixture reduces the plasma filament diameter and enhances constriction during the secondary pulse. In the dielectric barrier discharge, diffuse volumetric plasma occupies nearly the entire space between the high voltage electrode and the liquid surface, and extends radially along the surface. In the filament discharge in helium, adding water to the container results in considerable reduction of plasma lifetime compared to the discharge in dry helium, by about an order of magnitude, indicating rapid electron recombination with water cluster ions. Peak electron density during the pulse is also reduced, by about a factor of two, likely due to dissociative attachment to water vapor during the discharge pulse. These trends become more pronounced as oxygen is added to the mixture, which increases net rate of dissociative attachment. Gas temperature during the primary discharge pulse remains near room temperature, after which it increases up to T ~ 500 K over 5 µs and decays back to near room temperature before the next discharge pulse several tens of ms later. As expected, electron density and electron temperature in diffuse DBD plasmas are considerably lower compared to peak values in the filament discharge. Use of Thomson scattering for measurements of electron density and electron temperature in near-surface plasmas is hindered by strong light scattering off the surface.

Water-Mediated Energy Dynamics in a Homodimeric Hemoglobin.

Abstract: We examine energy dynamics in the unliganded and liganded states of the homodimeric hemoglobin from Scapharca inaequivalvis (HbI), which exhibits cooperativity mediated by the cluster of water molecules at the interface upon ligand binding and dissociation. We construct and analyze a dynamic network in which nodes representing the residues, hemes, and water cluster are connected by edges that represent energy transport times, as well as a nonbonded network (NBN) indicating regions that respond rapidly to local strain within the protein via nonbonded interactions. One of the two largest NBNs includes the Lys30-Asp89 salt bridge critical for stabilizing the dimer. The other includes the hemes and surrounding residues, as well as, in the unliganded state, the cluster of water molecules between the globules. Energy transport in the protein appears to be controlled by the Lys30-Asp89 salt bridge critical for stabilizing the dimer, as well as the interface water cluster in the unliganded state. Possible connections between energy transport dynamics in response to local strain identified here and allosteric transitions in HbI are discussed.

Internal electric fields in small water clusters [(H2O)n; n = 2–6]

Abstract: The electric field experienced by a water molecule within a water cluster depends on its position relative to the rest of the water molecules. The stabilization energies and the red-shifts in the donor O–H stretching vibrations in the water clusters increase with the cluster size concomitant with the increase in the electric field experienced by the donor O–H of a particular water molecule due to the hydrogen bonding network. The red-shifts in O–H stretching frequencies show a spread of about ±100 cm−1 against the corresponding electric fields. Deviations from linearity were marked in the region of 100–160 MV cm−1, which can be attributed to the strain in the hydrogen bonding network, especially for structures with DDAA and DDA motifs. The linear Stark effect holds up to 200 MV cm−1 of internal electric field for the average red-shifts in the O–H stretching frequencies, with a Stark tuning rate of 2.4 cm−1 (MV cm−1)−1, suggesting the validity of the classical model in small water clusters.

On Dipole Moments and Hydrogen Bond Identification in Water Clusters.

Abstract: It is demonstrated that the localized orbitals calculated for a water cluster have small delocalization tails along the hydrogen bonds, that are crucial in determining the resulting dipole moments of the system. (By cutting them, one gets much smaller dipole moments for the individual monomers—close to the values one obtains by using a Bader-type analysis.) This means that the individual water monomers can be delimited only in a quite fuzzy manner, and the electronic charge density in a given point cannot be assigned completely to that or another molecule. Thus, one arrives to the brink of breaking the concept of a water cluster consisting of individual molecules. The analysis of the tails of the localized orbitals can also be used to identify the pairs of water molecules actually forming hydrogen bonds.

Water cluster fragmentation probed by pickup experiments

Abstract: Electron ionization is a common tool for the mass spectrometry of atomic and molecular clusters. Any cluster can be ionized efficiently by sufficiently energetic electrons, but concomitant fragmentation can seriously obstruct the goal of size-resolved detection. We present a new general method to assess the original neutral population of the cluster beam. Clusters undergo a sticking collision with a molecule from a crossed beam, and the velocities of neat and doped cluster ion peaks are measured and compared. By making use of longitudinal momentum conservation, one can reconstruct the sizes of the neutral precursors. Here this method is applied to H2O and D2O clusters in the detected ion size range of 3-10. It is found that water clusters do fragment significantly upon electron impact: the deduced neutral precursor size is ∼3-5 times larger than the observed cluster ions. This conclusion agrees with beam size characterization by another experimental technique: photoionization after Na-doping. Abundant post-ionization fragmentation of water clusters must therefore be an important factor in the interpretation of experimental data; interestingly, there is at present no detailed microscopic understanding of the underlying fragmentation dynamics.

Chirality recognition in concerted proton transfer process for prismatic water clusters

Abstract: Proton transfer and chiral conversion via hydrogen bonds (HBs) are important processes in applications such as chiral recognition, enzymatic catalysis, and drug preparation. Herein, we investigate the chiral conversion and interlayer recognition, via concerted intralayer proton transfer (CIPT) processes, of small prismatic water clusters, in the form of bilayer n−membered water rings (BnWRs, n = 4, 5, 6). Density functional theory (DFT) calculations show that despite the small energy variations between the initial and final states of the clusters of less than 0.3 kcal·mol−1, the vibrational circular dichroism (VCD) spectrum provides clear chiral recognition peaks in the range of 3,000 to 3,500 cm−1. The vibrational modes in this region correspond to stretching of intralayer HBs, which produces strong signals in the infrared (IR) and Raman spectra. The electronic circular dichroism (ECD) spectrum also reveals obvious chiroptical characteristics. The molecular orbitals involved in the interlayer interaction are dominated by O 2p atomic orbitals; the energy of these orbitals increased by up to 0.1 eV as a result of the CIPT processes, indicating corresponding recognition between monolayer water clusters. In addition, isotopic substitution by deuterium in the BnWRs results in characteristic peaks in the VCD spectra that can be used as fingerprints in the identification of the chiral structures. Our findings provide new insights into the mechanism of chiral recognition in small prismatic water clusters at the atomic level as well as incentives for future experimental studies.

On the applicability of one- and many-electron quantum chemistry models for hydrated electron clusters

Abstract: We evaluate the applicability of a hierarchy of quantum models in characterizing the binding energy of excess electrons to water clusters. In particular, we calculate the vertical detachment energy of an excess electron from water cluster anions with methods that include one-electron pseudopotential calculations, density functional theory(DFT) based calculations, and ab initio quantum chemistry using MP2 and eom-EA-CCSD levels of theory. The examined clusters range from the smallest cluster size (n = 2) up to nearly nanosize clusters with n = 1000 molecules. The examined cluster configurations are extracted from mixed quantum-classical molecular dynamics trajectories of cluster anions with n = 1000 water molecules using two different one-electron pseudopotenial models. We find that while MP2 calculations with large diffuse basis set provide a reasonable description for the hydrated electron system, DFT methods should be used with precaution and only after careful benchmarking. Strictly tested one-electron psudopotentials can still be considered as reasonable alternatives to DFT methods, especially in large systems. The results of quantum chemistry calculations performed on configurations, that represent possible excess electron binding motifs in the clusters, appear to be consistent with the results using a cavitystructure preferring one-electron pseudopotential for the hydrated electron, while they are in sharp disagreement with the structural predictions of a non-cavity model.

The kinetics of water-assisted tautomeric 1,2-proton transfer in azoles: a computational approach

Abstract: A complete set of azoles undergoing 1,2-proton transfer, consisting of pyrazole, 1,2,3-triazole, 1,2,4-triazole, tetrazole and pentazole, was computationally investigated regarding proton transfer mechanism in gas phase and water solution. Complexes of one azole molecule with 1–4 water molecules were employed to facilitate the proton transfer by lowering the activation energy, which for the isolated azole molecule is prohibitively large. The calculations were performed at the MP2/aug-cc-pVDZ and B3LYP/6-311++G(d,p) levels of theory which showed very good concordance. It follows that in the most probable transition state one azole molecule is accompanied by two water molecules. The activation barrier in water solution modelled by PCM method was lowered to 18.8 kcal/mol in the case of pyrazole and more for azoles containing more nitrogen atoms in the ring, reaching 6.8 kcal/mol in the case of pentazole. The analysis of the IRC reaction paths showed that proton transfer in the gas phase has more synchronous character and in the water solution is rather stepwise. It also follows from the analysis of bond lengths in the transition state that in the case of pyrazole the transition state is more cation-like and for other azoles, especially pentazole, is more anion-like. In the water environment, the initial step of proton transfer is moving the proton from azole to the water cluster in all cases except pyrazole, where the proton moves first from the water cluster to the azole molecule.

Stepwise Hydration of 2-Aminooxazole: Theoretical Insight into the Structure, Finite Temperature Behavior and Proton-Induced Charge Transfer

Abstract: It was recently suggested that 2-aminooxazole (AO) could contribute to the formation of RNA nucleotides on primitive earth. In this article we have considered by means of computational modeling the influence of microhydration on the structural and spectral properties of this potential prebiotic molecule. The stable structures of AO(H2O)n were obtained first by sampling the potential energy landscapes of clusters containing up to n = 20 water molecules, using a simple but reasonably accurate force field and replica-exchange molecular dynamics simulations. Through reoptimization using an explicit description of electronic structure at the level of density functional theory with the M06-2X functional, the formation energies, ionization energies and electron affinities were determined in the vertical and adiabatic treatments, as well as vibrational and optical spectra covering the far-IR, mid-IR, and lower part of the UV ranges. The results generally show a clear segregation between the aminooxazole solute and the water molecules, a water cluster being formed near the nitrogen and amino group side leaving the hydrocarbon side dry even at temperatures corresponding to the liquid state. The spectral signatures generally concur and show distinct contributions of the solute and solvent, spectral shifts to lower energies being in agreement with earlier calculations in bulk solvent. We have also investigated the importance of microhydration on the charge transfer cross section upon collision with a proton, thereby extending an earlier investigation on the bare AO molecule. The presence of water molecules generally reduces the propensity for charge transfer at small sizes, but the influence of the solvent steadily decreases in larger droplets.

Ab initio investigation of structure, stability, thermal behavior, bonding, and infrared spectra of ionized water cluster (H2O)6.

Abstract: The low-lying isomers of cationic water cluster (H2O)6+ have been globally explored by using particle swarm optimization algorithm in conjunction with quantum chemical calculations. Compared with previous results, our searching method covers a wide range of structural isomers of (H2O)6+ and therefore turns out to be more effective. With these local minima, geometry optimization and vibrational analysis are performed for the most interesting clusters at second-order Moller-Plesset (MP2)/aug-cc-pVDZ level, and their energies are further refined at MP2/aug-cc-pVTZ and coupled-cluster theory with single, double, and perturbative triple excitations/aug-cc-pVDZ level. The interaction energies using the complete basis set limits at MP2 level are also reported. The relationships between their structure arrangement and their energies are discussed. Based on the results of thermal simulation, structural change from a four-numbered ring to a tree-like structure occurs at T ≈ 45 K, and the relative population of six lowest-free-energy isomers is found to exceed 4% at some point within the studied temperature range. Studies reveal that, among these six isomers, two new-found isomers constitute 10% of isomer population at 180 K, and the experimental spectra can be better explained with inclusions of the two isomers. The molecular orbitals for six representative cationic water clusters are also studied. Through topological and reduced density gradient analysis, we investigated the structural characteristics and the bonding strengths of these water cluster radical cations.

In Situ Characterization of Hydrated Proteins in Water by SALVI and ToF-SIMS.

Abstract: This work demonstrates in situ characterization of protein biomolecules in the aqueous solution using the System for Analysis at the Liquid Vacuum Interface (SALVI) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The fibronectin protein film was immobilized on the silicon nitride (SiN) membrane that forms the SALVI detection area. During ToF-SIMS analysis, three modes of analysis were conducted including high spatial resolution mass spectrometry, two-dimensional (2D) imaging, and depth profiling. Mass spectra were acquired in both positive and negative modes. Deionized water was also analyzed as a reference sample. Our results show that the fibronectin film in water has more distinct and stronger water cluster peaks compared to water alone. Characteristic peaks of amino acid fragments are also observable in the hydrated protein ToF-SIMS spectra. These results illustrate that protein molecule adsorption on a surface can be studied dynamically using SALVI and ToF-SIMS in the liquid environment for the first time.

Vibrational Relaxation of the Aqueous Proton in Acetonitrile: Ultrafast Cluster Cooling and Vibrational Predissociation

Abstract: We study the ultrafast O–H stretch vibrational relaxation dynamics of protonated water clusters embedded in a matrix of deuterated acetonitrile, using polarization-resolved mid-IR femtosecond spectroscopy. The clusters are produced by mixing triflic (trifluoromethanesulfonic) acid and H2O in molar ratios of 1:1, 1:2, and 1:3, thus varying the degree of hydration of the proton. At all hydration levels the excited O–H stretch vibration of the hydrated proton shows an ultrafast vibrational relaxation with a time constant T1 < 100 fs, leading to an ultrafast local heating of the protonated water cluster. This excess thermal energy, initially highly localized to the region of the excited proton, first re-distributes over the aqueous cluster and then dissipates into the surrounding acetonitrile matrix. For clusters with a triflic acid to H2O ratio of 1:3 these processes occur with time constants of 320 ± 20 fs and 1.4 ± 0.1 ps, respectively. The cooling of the clusters reveals a long-living, underlying transien...

Water sorption behaviour of two series of PHA/montmorillonite films and determination of the mean water cluster size.

Abstract: Biodegradable polyester-based films constituted of poly(hydroxyalkanoates) (PHA) were successfully extruded with various Cloisite 30B contents. The morphology was highly dependent on the matrix, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate), the polymer crystalline phase fraction, the matrix/nanoclay interfacial regions as well as the nanoclay content. Water vapour resistance was investigated through sorption kinetics, isotherms, modelling aspect, and diffusivity. A typical sigmoid-shaped isotherm was obtained in every case. It emerges that the nanoclay highly contributed to the increase of water solubility of matrices. The dependence of polymer crystallinity on the affinity of the nanocomposite films for water was highlighted. Thermodynamic and kinetic contributions of the sorption process were also correlated with the film morphology. According to the matrix used, water diffusivity in films was differently impacted by the sorbed water amount. The access of sorbed water molecules within films was examined through a mathematical modelling approach and the deduced mean cluster size of water vs. its activity was corroborated by sorption kinetics.

Multifunctional copper dimer: structure, band gap energy, catalysis, magnetism, oxygen reduction reaction and proton conductivity

Abstract: A new dimeric copper complex namely, [Cu2(PDA)2(Ald)2(H2O)2]·8H2O, 1, (where PDA = 2,4-pyridine dicarboxylic acid, Ald = aldrithiol) has been synthesized through a slow diffusion technique. Compound 1 is a molecular structure and assembled through H-bonding forming a supramolecular architecture. The CuO2N3 units bridged through an aldrithiol molecule to form the dimeric structure. The lattice water molecules are linked through H-bonding to form the decameric water cluster. The decameric water clusters are H-bonded to each other to form the 1D chain which resulted in excellent water stability and conduction of protons under humid conditions. Band gap energy and magnetic measurements show that compound 1 is a semiconductor and paramagnetic in nature. Further the compound is shown as a selective heterogeneous catalyst for styrene and cyclohexene epoxidation. This also shows a facile oxygen reduction reaction (ORR) and can be used as a promising Pt-free cathode in alkaline Direct Methanol Fuel Cells (DMFC). The present results suggest that compound 1 is a promising multifunctional material.