Computer-aided study of oxygen absorption by water clusters. IR spectra of heteroclusters

Spectral characteristics of (H₂O)_n, (O₂)_m(H₂O)_n, and (O)_i(H₂O)_n cluster systems, where m≤2, i≤4, and 10 ≤ n ≤ 50, are studied with the molecular dynamics method using a flexible molecule model. The IR absorption spectra are changed substantially as a result of O₂ molecule dissociation, and in the presence of atomic oxygen in the clusters, the spectra are characterized by a deep minimum at 520 cm^?1. The absorption of oxygen causes a marked reduction in reflection coefficient R of monochromatic IR radiation. The number of peaks in the R(ω) spectra decreases to two in the case of molecular oxygen absorption and is no larger than four in the case of atomic oxygen absorption. The absorption of atomic oxygen by the clusters is also accompanied by a significant increase in the dissipation of energy accumulated by the clusters. This effect weakens when molecular oxygen is absorbed. An increase in atomic oxygen concentration in the clusters renders their radiation harder.     

Computer simulation of the structure of water clusters containing absorbed ethane molecules

The structure of water clusters that have absorbed ethane molecules is studied by the molecular dynamics method. Structural analysis is performed by the construction of Voronoi polyheda for oxygen atoms and hybrid polyheda whose centers coincide with the centers of oxygen atoms and the faces are formed according to the positions of hydrogen atoms. The (H₂O)₂₀ cluster can retain no more than four ethane molecules remaining at the same time stable. When a water cluster adds more than four ethane molecules, the volumes of Voronoi polyheda acquire values close to the volume per molecule in the bulk liquid water. As the number of ethane molecules in a water cluster increases, the number of hydrogen atoms adjacent to oxygen, as well as the average number of units in cyclic formations composed of hydrogen atoms, also increases. In this case, the number of H-O-H angles formed by the nearest geometric neighbors close to 89° becomes dominant. The coefficient of nonsphericity reflecting the local arrangement of hydrogen atoms around the oxygen atoms decreases as the C₂H₆ molecules are added to water cluster and approaches to the value of this coefficient for the rhombic dodecahedron in the case of adsorption of six ethane molecules.     

IR spectra of water clusters with captured ethane molecules: Computer simulation

Uptake of ethane molecules by a monodisperse aqueous system was simulated by molecular dynamics. The cluster (H₂O)₂₀ characterizing the system remains stable until the number of the captured C₂H₆ molecules becomes larger than four. Addition of ethane molecules to the disperse aqueous system decreases both the real and imaginary parts of the dielectric permittivity in the frequency range 0 ≤ ω ≤ 1000 cm^?1. The integral IR absorption coefficient of the disperse system containing C₂H₆ molecules increases, and the frequency-average reflection coefficient decreases. The continuous reflection spectra transform into band spectra. The heat-radiating power of the clusters decreases upon absorption of ethane molecules. The cluster that took up two ethane molecules exhibits the highest radiating power. This cluster has the largest number of active electrons interacting with the arriving wave.     

Far-infrared absorption of water clusters by first-principles molecular dynamics

Based on first-principle molecular dynamic simulations, we calculate the far-infrared spectra of small water clusters (H(2)O)(n) (n = 2, 4, 6) at frequencies below 1000 cm(-1) and at 80 K and at atmospheric temperature (T>200 K). We find that cluster size and temperature affect the spectra significantly. The effect of the cluster size is similar to the one reported for confined water. Temperature changes not only the shape of the spectra but also the total strength of the absorption, a consequence of the complete anharmonic nature of the classical dynamics at high temperature. In particular, we find that in the frequency region up to 320 cm(-1), the absorption strength per molecule of the water dimer at 220 K is significantly larger than that of bulk liquid water, while tetramer and hexamer show bulklike strengths. However, the absorption strength of the dimer throughout the far-infrared region is too small to explain the measured vapor absorption continuum, which must therefore be dominated by other mechanisms.     

Adsorption of ammonia by water clusters. Computer experiment

The method of molecular dynamics is used to study the adsorption of from one to six ammonia molecules by water clusters composed of 50 molecules. The adsorption of NH₃ molecules markedly increases the IR absorption spectrum intensity, substantially decreases emission power in the frequency range of 0 ≤ ω ≤ 3500 cm^?1, and transforms a continuous reflectance spectrum into a banded one. A rough surface formed by adsorbed ammonia molecules reduces the absorption coefficient and refractive index of the system of water-ammonia clusters in the entire frequency range. Adsorption of ammonia molecules by water clusters greatly diminishes the number of electrons that are active with respect to electromagnetic radiation.     

Molecular simulation of oxygen on supported platinum clusters

Molecular dynamics simulations are performed to study oxygen adsorption on platinum clusters supported on a graphite surface. The Sutton–Chen many-body potential is used for the Pt–Pt interaction, whereas a Steele potential was used to represent the carbon surface. The oxygen–oxygen intramolecular force is modeled by a harmonic oscillator model and other interactions are described by the Lennard–Jones potential. The results indicate an optimum loading of platinum for maximum specific adsorption of oxygen. Adsorption isotherms are constructed and the energies and orientation of adsorbed oxygen are reported. The relevance of this study to electrode processes is discussed.     

Computer simulation of quantum melting in hydrogen clusters.

We introduce a new criterion, based on multipole dynamical correlations calculated within reptation quantum Monte Carlo, to discriminate between a melting versus freezing behavior in quantum clusters. This criterion is applied to small clusters of para-hydrogen molecules (both pristine and doped with a CO chromophore), for cluster sizes of around twelve molecules. This is a magic size at which para-hydrogen clusters display an icosahedral structure and a large stability. Despite the similar geometric structure of CO@(pH2)12 and (pH2)13, the first system has a rigid, crystalline, behavior; the second behaves more like a superfluid (or, possibly, a supersolid).     

Spectral characteristics of water clusters in the presence of nitrate ions

Nitrate ion adsorption by water clusters is studied using the molecular dynamics method combined with a polarizable model of flexible molecules. It is established that successive addition of one to six NO ₃ ^? ions to an (H₂O)₅₀ cluster decreases the averaged electrical potential related to the center of masses of water molecules. The (H₂O)₅₀ cluster retains its thermodynamic stability, provided that no more than three nitrate ions are added to it. After NO ₃ ^? ions are adsorbed, the real and imaginary components of the dielectric permittivity and the intensity of the Raman spectrum decrease, while the intensity of the IR absorption spectrum increases. Moreover, ion adsorption by the water cluster reduces the IR absorption coefficient and refractive index.     

Indirect determination of oxygen dissolved in water by atomic absorption spectrometry

Summary Indirect Determination of Oxygen Dissolved in Water by Atomic Absorption Spectrometry O₂ dissolved in aqueous medium reacts quantitatively with thallium metal according to O₂+4 Tl+2 H₂O 4 Tl⁺+4 OH^–. The reaction takes place in a glass column filled up with small pieces of sheet metal. The Tl⁺ formed is measured by atomic absorption spectrometry. We study the influence of several factors (pH, temperature, salinity of the water). The results obtained from a statistical sample were compared with those provided by the amperometric method for direct determination of dissolved O₂ and we found a high degree of consistency between them.     

Contribution of atmospheric nitrate to stream‐water nitrate in Japanese coniferous forests revealed by the oxygen isotope ratio of nitrate

Evaluation of the openness of the nitrogen (N) cycle in forest ecosystems is important in efforts to improve forest management because the N supply often limits primary production. The use of the oxygen isotope ratio (δ¹⁸O) of nitrate is a promising approach to determine how effectively atmospheric nitrate can be retained in a forest ecosystem. We investigated the δ¹⁸O of nitrate in stream water in order to estimate the contribution of atmospheric NO in stream‐water NO (f_atm) from 26 watersheds with different stand ages (1–87 years) in Japan. The stream‐water nitrate concentrations were high in young forests whereas, in contrast, old forests discharged low‐nitrate stream water. These results implied a low f_atm and a closed N cycle in older forests. However, the δ¹⁸O values of nitrate in stream water revealed that f_atm values were higher in older forests than in younger forests. These results indicated that even in old forests, where the discharged N loss was small, atmospheric nitrate was not retained effectively. The steep slopes of the studied watersheds (>40°) which hinder the capturing of atmospheric nitrate by plants and microbes might be responsible for the inefficient utilization of atmospheric nitrate. Moreover, the unprocessed fraction of atmospheric nitrate in the stream‐water nitrate in the forest (f_unprocessed) was high in the young forest (78%), although f_unprocessed was stable and low for other forests (5–13%). This high f_unprocessed of the young forest indicated that the young forest retained neither atmospheric NO nor soil NO effectively, engendering high stream‐water NO concentrations. Copyright © 2010 John Wiley & Sons, Ltd.     

Determination of nitrite and nitrate in Venice lagoon water by ion interaction reversed-phase liquid chromatography

Ion interaction reversed-phase liquid chromatography with octylammonium orthophosphate as the interacting reagent and a reversed-phase C₁₈ column was applied to the identification and determination of nitrite and nitrate in Venice lagoon water. Interference by the high chloride concentration was systematically studied and the results obtained with different column packings were compared. With spectrophotometric detection at 230 nm, nitrite at 0.005 mg 1^?1 can be detected and determined even in the presence of 0.70 M chloride. The dependence of the retention time of nitrite on the chloride concentration was studied for two reversed-phase columns with different packings. Concentrations of 0.30 ± 0.05 mg 1^?1 of nitrite and 0.20 ± 0.05 mg 1^?1 of nitrate were found in Venice lagoon water.     

Anharmonic effects in ammonium nitrate and hydroxylammonium nitrate clusters

The covalent and ionic clusters of ammonium nitrate and hydroxyl ammonium nitrate are characterized using density functional theory and second-order vibrational perturbation theory. The most stable structures are covalent acid-base pairs for the monomers and ionic acid-base pairs for the dimers. The hydrogen-bonding distances are greater in the ionic dimers than in the covalent monomers, and the stretching frequencies are significantly different in the covalent and ionic clusters. The anharmonicity of the potential energy surfaces is found to influence the geometries, frequencies, and nuclear magnetic shielding constants for these systems. The inclusion of anharmonic effects significantly decreases many of the calculated vibrational frequencies in these clusters and improves the agreement of the calculated frequencies with the experimental data available for the isolated neutral species. The calculations of nuclear magnetic shielding constants for all nuclei in these clusters illustrate that quantitatively accurate predictions of nuclear magnetic shieldings for comparison to experimental data require the inclusion of anharmonic effects. These calculations of geometries, frequencies, and shielding constants provide insight into the significance of anharmonic effects in ionic materials and provide data that will be useful for the parametrization of molecular mechanical force fields for ionic liquids. Anharmonic effects will be particularly important for the study of proton transfer reactions in ionic materials.     

Computer simulation of ion recombination in irradiated nonpolar liquids

A review of the results of computer simulation of the diffusion controlled recombination of ions is presented. The ions generated in clusters of two and three pairs of oppositely charged ions were considered. The recombination kinetics and the ion escape probability at infinite time with and without external electric field have been computed. These results are compared with the calculations based on the single-pair theory.     

Computer evaluation of water sorption on ion exchangers

Water sorption on an ion exchanger can be described by formation of several hydrates. The general least squares program WSLET based on the "pit-mapping" approach has been written for evaluating such equilibria. It is written in FORTRAN 77 for use on personal computers. Besides finding the best values for the equilibrium constants of the various hydrates the program also searches for best values of the stoichiometric indices by using the so-called ESI-method. The program is provided with least-squares procedures and blocks for statistical analysis of the residuals as well as for data simulation. It has been tested with data on the strong base resin Dowex 1 as well as on simulated data. In the present paper an illustrative example is given together with data simulation. It is found that in most cases WSLET helps to find a better fit than earlier models quite efficiently, showing the advantage of the ESI method in combination with least-squares methods.     

Computer simulation of single-ion trajectories in paul-type ion traps

The computer simulation of single-ion trajectories using a number of computer programs is described together with associated theory. The programs permit calculation of ion trajectories while the ion is subjected to collisions with buffer gas of variable pressure, resonance excitation in any of three modes, and static or ramped DC and radiofrequency levels. Initially, the programs were designed for the calculation of ion trajectories in a quadrupole ion trap. The programs now permit such calculations for ions confined in traps having electrodes shaped to include percentages of hexapole and octupole components in the electric field as well as electrode surface geometries for which there is no closed-form expression. The Langevin collision theory is reviewed and a theoretical treatment of the multipole trap is presented.     

Volume change of ion pair formation: Rubidium nitrate and thallium(I) nitrate in water

Density data for dilute aqueous solutions of RbNO ₃ and TlNO ₃ at 25°C lead to the following equations for the apparent molal volumes: RbNO₃: Φ_V = 43.0₇ + 2.46c ^1/2 + 0.15c (0?0.41M) TINO₃: Φ_V = 39.5₀ + 10.83c ^1/2 ? 25.46c + 26.0c ^3/2 (0.03?0.25M) From these expressions and a semiempirical equation for Φ_v of a completely dissociated 1:1 electrolyte, values of ΔΦ_v for ion pair formation at c=0.10 are calculated to be+5 ml-mole^?1 for RbNO ₃ and+12 ml-mole^?1 for TlNO ₃ . Comparison of ΔΦ_v with the electrostriction volume effect suggests that virtually all of the electrostriction is removed in the formation of the TlNO ₃ ion pair but only about half with RbNO ₃ . Values of ΔΦ_v are in poor agreement with those estimated from electrostatic theories of ion pair formation.     

离子色谱法测定米酒中NO-3和NO-2

研究了用带电导检测器的离子色谱对米酒中阴离子NO3^-、NO2^－进行分离测定的方法。NO3^-、NO2^－的线性范围分别为0．5－80mg/L和0.1-20mg/L，检出限分别为0.5mg/L和0.1mg/L。该法适用于SO4^2-、CI^-的测定。     

Tuning ferroelectricity of niobium clusters by oxygen chemisorption

The chemisorption of atomic oxygen on Nb(N) (N=2-16) has been investigated by the scalar relativistic all-electron density-functional calculations with emphasis on its effect on the ferroelectricity of Nb(N). We have shown that the binding of O atom to niobium clusters is site- and size-dependent, for which the bridge-site doping is preferred in the smaller size range from N=2 to 8 and the threefold hollow site one for the larger Nb(N) with 9 < or = N < or = 16. Though the geometrical structures of Nb(N) are modified slightly when doped with an oxygen atom, their ferroelectric properties vary considerably, depending on cluster size and the O adsorption sites, which is mainly caused by the charge transfer between the oxygen atom and niobium clusters. The addition of oxygen can enhance the ferroelectricity of Nb(N) with moderate and essentially zero moments while induce only small dipole change for those Nb(N) with large moments. Thus, the big fluctuation of the Nb(N)'s dipole moments with size is greatly suppressed by the O doping, indicating that one might expect to tune the size-dependent ferroelectricity of Nb(N) by the chemical decoration.