Showing papers on "Water cluster published in 1996"

High resolution uv spectroscopy of phenol and the hydrogen bonded phenol/water cluster

Abstract: The S1←S0 000 transitions of phenol and the hydrogen bonded phenol(H2O)1 cluster have been studied by high resolution fluorescence excitation spectroscopy. All lines in the monomer spectrum are split by 56±4 MHz due to the internal rotation of the −OH group about the a axis. The barrier for this internal motion is determined in the ground and excited states; V2″=1215 cm−1, and V2′=4710 cm−1. The rotational constants for the monomer in the ground state are in agreement with those reported in microwave studies. The excited state rotational constants were found to be A′=5313.7 MHz, B′=2620.5 MHz, and C′=1756.08 MHz. The region of the redshifted 000 transition of phenol(H2O)1 shows two distinct bands which are 0.85 cm−1 apart. Their splitting arises from a torsional motion which interchanges the two equivalent H atoms in the H2O moiety of the cluster. This assignment was confirmed by spin statistical considerations. Both bands could be fit to rigid rotor Hamiltonians. Due to the interaction between the overal...

Hydration Energies of Divalent Beryllium and Magnesium Ions: An ab Initio Molecular Orbital Study

Abstract: Ab initio molecular orbital calculations have been used to investigate contributions of water molecules in the first and second coordination shells to the overall hydration energy of divalent beryllium and magnesium cations. Enthalpy and free energy changes at 298 K have been calculated at a variety of computational levels for the reactions M2+ + [H2O]p → M2+·nH2O·mH2O, where M = Be or Mg, [H2O]p (p = 2, 4, 6, 8; p = n + m) are water clusters, and M2+·nH2O·mH2O are ion−water complexes with n and m water molecules in the first and second coordination shells, respectively. These reactions involve the disruption of the water cluster and naturally include the competitive effects of ion−water and water−water interactions inherent in the hydration process. At the MP2(FULL)/6-311++G**//RHF/6-31G* computational level, the values of ΔG298 for the reactions which complete the first hydration shells, Be2+ + [H2O]4 → Be2+·4H2O and Mg2+ + [H2O]6 → Mg2+·6H2O, are −352.0 and −266.7 kcal/mol, accounting for 61.2% and 60....

Identification of the Serratia endonuclease dimer: structural basis and implications for catalysis.

Abstract: The Serratia endonuclease is an extracellularly secreted enzyme capable of cleaving both single- and double-stranded forms of DNA and RNA. It is the first member of a large class of related and usually dimeric endonucleases for which a structure is known. Using X-ray crystallography, the structure of monomer of this enzyme was reported by us previously (Miller MD et al., 1994, Nature Struct Biol 1:461-468). We now confirm the dimeric nature of this enzyme through light-scattering experiments and identify the physiologic dimer interface through crystal packing analysis. This dimerization occurs through an isologous twofold interaction localized to the carboxy-terminal subdomain of the enzyme. The dimer is a prolate ellipsoid with dimensions 30 A x 35 A x 90 A. The dimer interface is flat and contains four salt links, several hydrogen bonds, and nonpolar interactions. Buried water is prominent in this interface and it includes an unusual "cubic" water cluster. The position of the two active sites in the dimer suggests that they can act independently in their cleavage of DNA, but have a geometrical advantage in attacking substrate relative to the monomer.

Quantum statistical mechanical simulation of the ion–water cluster I−(H2O)n: The importance of nuclear quantum effects and anharmonicity

Abstract: Monte Carlo simulations of quantum statistical mechanical properties using the Feynman path integral method were carried out at temperatures of 70, 100, 200, and 300 K to study the structure and energetics of the ion–water cluster I−(H2O)n, with n=1–6. Simulation results at low temperatures (i.e., 70 K) can be directly compared with data from photoelectron spectroscopy (PES). The temperature dependence of binding enthalpies and stabilization energies indicate that at low temperatures the classical description of nuclear motion is qualitatively incorrect. The binding enthalpies are also computed within the harmonic approximation using the energy and frequencies at the minimum energy geometry, a method widely used in conjunction with electronic structure calculations to extend zero temperature information to finite temperatures. It is found that the harmonic approximation works well for the cluster with a single water molecule, but not for clusters with more than one water molecule, where the temperature dependence of binding enthalpies is in qualitative disagreement with the exact enthalpy obtained from simulation.

The motion of protons in water–ammonia clusters

Abstract: The dynamics of clusters (H2O)nH+ (n=1,2,3,4) interacting with an NH3 molecule has been studied by first‐principles Born–Oppenheimer molecular dynamics (BOMD) simulations. These small clusters are chosen as prototype systems for studying the mechanisms of proton transfer at atomistic level. We focus on the fundamental steps of proton motion in molecular clusters, the dynamical consequences of proton affinities, and the interplay between proton motion and proton affinity in these systems. A characteristic feature of the motion, the forming and breaking of O–H bonds in H3O+ is analyzed in detail. The transfer process is found to be consecutive along a quasi‐one‐dimensional channel. The umbrella mode in NH3 can easily be excited to direct the lone pair of the ammonia molecule to the water clusters. The hydronium ion, however, reorients mainly via rotation. When NH3 reaches one terminal water molecule of a protonated water cluster, the system undergoes a series of intermediate states in which the mobile protons travel within the water clusters, H3O+ transients are formed as protons approach individual water molecules. The lifetime of the H3O+ transient is 8–20 fs, or 1–3 vibrational periods of the O–H stretch mode. Proton transfer is observed for n=1, 2, 3, although for n=3 NH+4(H2O)3 is in existence with NH3(H2O)2H+. For n=4, NH3(H2O)4H+ is the dominant statistical configuration. Vibrational spectrum of NH3(H2O)4H+ is analyzed in detail. The features of the spectrum can be used, in principle, to probe the proton motion in the transition state region reactions. In these calculations, the electronic charge distribution is calculated concurrently with the nuclear dynamics. An analysis of isocharge density surfaces gives qualitative and quantitative descriptions of the dynamics of electronic redistribution. The BOMD is performed in the framework of density functional theory with local spin density and generalized gradient approximations.

Dynamics of proton attachment to water cluster: Proton transfer, evaporation, and relaxation

Abstract: A proton attachment dynamics to a water cluster is investigated by using a classical molecular dynamics calculation. It is found that three dynamical stages are involved: (1) ultrafast (∼10−14 s) proton attachment to a water molecule of the cluster which followed by (2) the fast (∼10−13 s) sequential proton transfer over several water molecules on the cluster surface and then, (3) the gradual (∼10−11 s) proton penetration to the cluster core. In the first two stages, the large kinetic energy of the order of hundreds kcal/mol is released to the system, which results in the evaporation of a few water molecules from the cluster. The water molecules evaporating in these early stages have large vibrational and translational energies. The mechanism of the energy relaxation and the proton transfer in each process are investigated. The large amplitude vibrational motion promotes sequential concerted proton exchange transfers in the earlier stages (1) and (2). The precise configurational matching of the hydrogen b...

Modification of photochemical reactivity by nafion. photocyclization and photochemical cis-trans isomerization of azobenzene

Abstract: The photophysical and photochemical behavior of azobenzene (AB) incorporated into solvent-swollen acid form Nafion (Nafion-H+) membranes were examined. In water-swollen Nafion-H+ membrane AB exhibits strong fluorescence at room temperature, which has never been observed in isotropic solvents. Photolysis of AB adsorbed in water-swollen Nafion-H+ results in its cyclization to give benzo[c]cinnoline (BC) and benzidine (BZ) in quantitative yield. The product distribution is very dependent upon the number of AB molecules in each water cluster of the Nafion membrane (occupancy number). In the case of the occupancy number greater than 2, BC and BZ are formed equimolarly, while when only one AB molecule exists in each water cluster, BC is exclusively produced. The AB molecules incorporated into methanol-swollen Nafion-H+ membrane do not emit fluorescence and only undergo cis−trans isomerization when they are photoirradiated. These observations suggest that in water-swollen Nafion-H+ AB molecules are solublized in...

Water clusters and uses therefor

Abstract: The present invention provides water cluster compositions characterized by high oxygen reactivity due to protruding, delocalized pπ orbitals. The invention also provides methods of producing the structures. The invention further provides methods of using the water clusters, for example in combustion, and compositions associated therewith.

An investigation of hydrogen transfer in water clusters

Abstract: The potential energy surface of the (H2O)5 water cluster is examined using Kohn–Sham density functional theory, Hartree–Fock theory and second‐order Mo/ller–Plesset theory. Two distinct minima on the energy surface may be interconverted through the transfer of two hydrogen atoms, representing a possible mechanism for ionic dissociation in water clusters. Our calculations suggest a concerted mechanism where the two hydrogen atoms move simultaneously through a late transition state.

PHOTOELECTRON SPECTROSCOPY OF MASS-SELECTED METAL-WATER CLUSTER NEGATIVE IONS: Cu−(H2O)n AND Na−(H2O)n

Abstract: The electronic structures of hydrated metal-atom clusters have been investigated by negative-ion photoelectron spectroscopy. We have obtained the photoelectron spectra of Cu−(H2O)n with n=0−4 and Na−(H2O)n with n=0−12. For the former clusters, we also detected the electron detachment from the CuOH−(H2O)n−1 which coexists with Cu−(H2O)n. The observed bands for both Cu−(H2O)n and Na−(H2O)n were all assigned to the transitions to the states originating in those of the metal atoms, which are shifted as a result of hydration. This result implies that the ground states of the neutral clusters still have a one-center character at the size range examined. In contrast, for the Na–water clusters, increasing character of the Rydberg-type ion-pair state in the negative-ion state is suggested from the vertical-detachment-energy dependence on the solvent number.

Effect of water cluster state influenced by the polarized properties on dielectric constant and loss

Abstract: In order to describe the mechanisms of the effect of water on electrical properties, we study the state of dissolved water which can be analyzed by IR spectroscopy. We investigate the relationship between conduction current in a dielectric liquid and the structure of water clusters formed by hydrogen bonding. The dielectric constant and loss of several kinds of aromatic hydrocarbons are measured and related to electron donor functional groups. Since the hydrogen bonding energy calculated from the IR spectrum is not very different in donor and acceptor groups, the sensitivity of the effect of water on dielectric constant and loss may be caused by the polarization characteristics of the benzene ring.

Effect of molecular hindrance on dc conductivity in dielectric liquids including water

Abstract: The microscopic state of water in dielectric liquids relates to the hydrogen bonding energy between the water molecules and the polarization site of the liquids. It has been reported that the dc conductivity of dielectric liquids including water is estimated by the hydrogen bonding energy. In this report, it is shown that the molecular hindrance relating the length of the side chain does not affect the microscopic state of water, but the conductivity decreases according to the length of the side chain. Our model of the effect of the water cluster on conductivity is the conductive bridge formed by the water cluster. In this model the molecular hindrance disturbs the formation of the water cluster bridge.

Surface Phenomena of Molecular Clusters by Molecular Dynamics Method

Abstract: Liquid droplets of water and argon surrounded by their vapor have been simulated by the milecular dynamics method. To explore the surface phenomena of clusters, each molecule is classified into 'liquid', 'surface', or 'vapor' with respect to the number of neighbor molecules. The contribution of a 'surface' molecule of the water cluster to the far infrared spectrum is almist the same as that of the 'liquid' molecule. Hence, the liquid-vapor interface is viewed as geometrically and temporally varying boundary of 'liquid' molecules with only a single layer of 'surface' molecules that might have different characteristics from the 'liquid' molecules. The time scale of the 'phase change' of each molecule is estimated for the argon cluster by observing the instantancous kinetic and potential energies of each molecule. To compare the feature of clusters with macroscopic droplets, the temperature dependence of the surface tension of the argon cluster is estimated.

Radiation Induced Reactions at “Small” Surfaces and on Small Particles

Abstract: This work describes the formation of hydrated electrons in the supercages of hydrated zeolites X and Y and the chemistry of organic adsorbates on silica surfaces by fast electron beam irradiation of these heterogeneous systems. Electron trapping by water clusters compartmentalized in zeolite supercages was observed to overwhelm the trapping by Na+ clusters in the hydrated forms of zeolites NaX and NaY, while only Na+ cluster trapped electrons were observed in both hydrated and dehydrated zeolites NaA and NaZK-4. The formation of the hydrated electron was examined with respect to the size of the water clusters confined in the supercages of the zeolites, and six “free” water molecules were found to be critical for the hydrated electron formation. Studies on the subsequent reactions of the hydrated electron demonstrate the restricted nature of its reactivity in the zeolite cage structure. Hydrogen atoms and hydrogen gas are produced with significant yields on hydroxylated silica surfaces upon high energy irradiation. Trapping of free charge carriers leads to the observation of surface trapped electrons in 150°C pretreated silicas and trapped holes in 600°C dried silicas. Formation of the 1-hydro-pyrenyl radical (PyH•), the pyrene cation radical and the pyrene anion radical was observed simultaneously with the reduction of hydrogen gas yield on 150°C dried silica sufaces loaded with pyrene. The hydrogen atoms produced on silica surfaces are shown to be the precursor of the molecular hydrogen and the pyrene free radical PyH•. G value measurements suggest that the production of hydrogen atoms is the major radiolytic process occurring on 150°C dried silica surfaces.