Simulation of dielectric properties and stability of clusters (H2O)i, CO2(H2O)i, and CH4(H2O)i

Molecular dynamics method is used for studying complex permittivity ɛ and the stability of individual water clusters as a function of the number of involved molecules (7 ≤ i ≤ 20) and also the corresponding characteristics of water aggregates with a captured CO₂ or CH₄ molecule. Absorption of the latter molecules leads to considerable changes in dielectric properties and stability of clusters. In particular, upon the addition of a CO₂ molecule to a water cluster, the oscillation parameters of the real and imaginary parts of the permittivity change. Capture of a CH₄ molecule by a water aggregate changes the ɛ(ω) dependence from the relaxation to resonance type. For i ≥ 15, the thermal stability of individual water clusters can be lower than that of aggregates CO₂(H₂O) _i and CH₄(H₂O) _i . The mechanical stability of (H₂O) _{i ≥ 13} clusters can exceed that of heteroclusters under consideration. Clusters (H₂O) _i and CO₂(H₂O) _i have approximately the same dielectric stability, whereas aggregates CH₄(H₂O) _i exhibit lower stability with respect to electric perturbations. Original Russian Text ? A.E. Galashev, V.N. Chukanov, A.N. Novruzov, O.A. Novruzova, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 2, pp. 143–153.     

Computer simulation of oxygen and nitrate ion absorption by water clusters

Using the molecular dynamics method, the joint absorption of oxygen and nitrate ions by water clusters is studied in terms of the polarizable model of flexible molecules. Significant fluctuations are observed in the number of hydrogen bonds in the clusters during the addition of NO₃⁻ ions to water-oxygen aggregates. Dielectric permittivity noticeably changes upon the addition of O₂ molecules to water clusters and nitrate ions to oxygen-containing water clusters. After the absorption of oxygen molecules and nitrate ions, water clusters markedly lose the ability to IR absorption. The Raman spectrum of a medium formed from disperse aqueous system, oxygen, and nitrate ions displays a greater number of bands than the spectrum of a system of pure water clusters.     

Spectral characteristics of water clusters in the presence of nitrogen dioxide

The absorption of NO₂ molecules by a water cluster containing 25 molecules was studied by molecular dynamics. The calculated dielectric characteristics of a system of (NO₂) _i (H₂O)₂₅ clusters (1 ≤ i ≤ 6) were compared with similar data for a cluster system of pure water. The ability of the disperse water system that trapped NO₂ molecules to absorb IR radiation increased, and the rate of the absorbed energy emission decreased. The Raman spectrum of the disperse system that absorbed NO₂ molecules changed most significantly in the low-frequency range. The emission time of cluster-generated radiation was much smaller than the lifetime of the clusters.     

Computer study of the absorption of ethane by aqueous ultradispersed medium: IR spectra

Absorption of ethane molecules by water clusters containing 10–20 molecules is studied by the molecular dynamics method. The (H₂O) _n (I), C₂H₆(H₂O) _n (II), and (C₂H₆)₂(H₂O) _n (III) cluster systems are composed on the basis of specific statistical weights. Spectral characteristics of system and single clusters are determined in the frequency range of 0 ≤ ω ≤ 1000 cm^?1. In this frequency range, both real and imaginary parts of dielectric permittivity decrease monotonically after the absorption of C₂H₆ molecules by an aqueous ultradispersed system. Integral coefficient of IR absorption increases, while average (over frequency) reflection coefficient decreases after the absorption of ethane molecules. The intensity of IR scattering by the systems of clusters containing C₂H₆ molecules lowers. Maximal values of radiation power for water clusters with various sizes are balanced with the capture of ethane molecules by the clusters, whereas oscillations in the size dependence of the density of electrons that are active with respect to IR radiation decrease.     

Molecular dynamics study of spectral characteristics of water-carbon oxide disperse systems

Absorption and reflection IR spectra of aqueous disperse systems that absorbed carbon oxide molecules are calculated. Systems of small and large clusters containing 2 ≤ n ≤ 10 and 11 ≤ n ≤ 20 water molecules, respectively, are studied. Each cluster can absorb one or two carbon oxide molecules. Both real and imaginary parts of dielectric permittivity of disperse systems depend on the number of adsorbed CO molecules to a greater extent than that of water molecules in clusters. The integral intensity of the absorption of IR radiation by cluster systems increases after the absorption of carbon oxide molecules by clusters. However, the ability to absorb and reflect IR radiation decreases with an increase in the concentration of absorbed CO molecules. Upon the growth of heteroclusters due to addition of water molecules, integral intensity of the absorption of thermal radiation can be enhanced or damped. In general, the clusterization and capture of CO molecules by clusters result in an anti-greenhouse effect.     

Computer-assisted study of characteristics of water clusters in the presence of nitrogen dioxide

The interaction of IR radiation with water clusters that have absorbed NO₂ molecules is studied by the molecular dynamics method in terms of the polarizable model. Induced dipole moments of H₂O and NO₂ molecules diminish during the addition of one to six NO₂ molecules to (H₂O)₅₀ cluster. The integral intensity of IR absorption by a system consisting of (NO₂) _i (H₂O)₅₀ heteroclusters with 1 ≤ i ≤ 6 decreases, whereas the power of heat emission rises as compared with an (H₂O) _n system. The decrease in the IR absorption and the increase in the IR emission by water clusters with the capture of NO₂ molecules are nonmonotonic. The absorption of NO₂ molecules by water clusters causes a noticeable reduction in the intensity of the first peak and the confluence of the fourth and fifth peaks in the Raman spectrum.     

Cluster mechanism of the atmosphere ozone destruction by bromine ions

Interaction of bromine ions absorbed by water cluster with adsorbed oxygen and ozone molecules has been investigated by the molecular dynamics method. It was shown that the part of O₂ molecules was removed from the system by evaporating Br⁻ ions, while all O₃ molecules and Br ions were kept in the system during 25 ps. The increase the concentration of the Br⁻ ions in the clusters resulted in a reduction of the absorption intensity and emission in IR spectra at the presence of oxygen, whereas the absorption intensity in the appropriate IR spectra of ozone-containing systems increased with the growth of a number of the Br⁻ ions. Raman spectra of oxygen-containing systems were poorly sensitive to the concentration of the Br⁻ ions but the absorption intensity of Raman spectra for systems with ozone considerably decreased with the growth of a number of bromine ions.     

Molecular dynamics calculation of spectral characteristics of water clusters in the presence of ozone molecules and chlorine ions

Simultaneous interaction of the (H₂O)₅₀ cluster with O₃ molecules and Cl⁻ ions was studied by the molecular dynamics method. Six O₃ molecules located near the cluster were absorbed by the aqueous aggregate, and Cl⁻ ions in turn left the zone of the interaction with the cluster. Some of Cl⁻ ions penetrated inside the formed (O₃)₆(H₂O)₅₀ cluster and come into collision with O₃ molecules that split the ozone molecule into atoms. When Cl⁻ ions were removed sufficiently far away from the cluster, the water cluster with absorbed O₃ molecules and O atoms was observed for 15.6 ps. The interaction of water molecules with Cl⁻ ions gives rise to an increase in the integral intensity of absorption and emission IR spectra, and also to an essential decrease in the analogous characteristics of the Raman spectrum in the frequency range of 0 ≤ ω ≤ 1000 cm⁻¹. The presence of Cl⁻ ions did not affect essentially the location of the main band in the IR spectra, but considerably changed the shape of the bands in the Raman spectrum.     

Modeling the infrared and raman spectra of silicon dioxide clusters absorbing water

The effect of absorbed water on the dielectric properties of silicon dioxide nanoparticles is studied by the molecular dynamic method. It is demonstrated using the model of flexible molecules that increasing the number of water molecules in the (SiO₂)₅₀ cluster to 40 results in an enhancement of absorption of infrared radiation over the frequency range 0 cm⁻¹ ≤ ω ≤ 1000 cm⁻¹. It is ascertained that the absorption of water molecules by the (SiO₂)₅₀ cluster considerably alters the shape of Raman spectra of light, smoothing all the peaks after the first one, and that water molecules are concentrated near the cluster surface formed by SiO₂ structural units.     

Ab initio DFT studies of oxygen K edge NEXAFS spectra for the V2O3(0001) surface

Theoretical near edge X-ray absorption fine structure (NEXAFS) spectra describing oxygen 1s core excitation have been evaluated for the differently coordinated oxygen species appearing near the V₂O₃(0001) surface with half metal layer V^′OV termination. Adsorption of oxygen above vanadium centers of the V^′OV terminated surface (O_tV^′O termination) results in very strongly bound vanadyl oxygen, which has also been considered for core excitation in this study. The angle-resolved spectra are based on electronic structure calculations using ab initio density functional theory (DFT) together with model clusters. Experimental NEXAFS spectra for V₂O₃(0001) show a rather strong dependence of peak positions and relative intensities on the photon polarization direction. This dependence is well described by the present theoretical spectra and allows us to assign spectral details in the experiment to specific O 1s core excitations where final state orbitals are determined by the local binding environments of the differently coordinated oxygen centers. As a result, a combination of the present theoretical spectra with experimental NEXAFS data enables an identification of differently coordinated surface oxygen species at the V₂O₃(0001) surface.     

Oxygen desorption from polycrystalline palladium: Thermal desorption of O<Subscript>2</Subscript> from a chemisorbed layer of O<Subscript>ads</Subscript> in the course of the decomposition of PdO surface oxide and in the release of oxygen from the bulk of palladium

The desorption of oxygen from polycrystalline palladium (Pd(poly)) was studied using temperature-programmed desorption (TPD) at 500–1300 K and the amounts of oxygen absorbed by palladium (n) from 0.05 to 50 monolayers. It was found that the desorption of O₂ from Pd(poly), which occurred from a chemisorbed oxygen layer (O_ads), in the release of oxygen from a near-surface metal layer in the course of the decomposition of PdO surface oxide, and in the release of oxygen from the bulk of palladium (O_abs), was governed by repulsive interactions between O_ads atoms and the formation and decomposition of O_ads-Pd*-O_abs structures (Pd* is a surface palladium atom). At θ ≤ 0.5, the repulsive interactions between O_ads atoms (ɛ_aa = 10 kJ/mol) resulted in the desorption of O₂ from Pd(poly) at 650–950 K. At 0.5 ≤ n ≤ 1.0, the release of inserted oxygen from a near-surface palladium layer occurred during TPD in the course of the migration of O_abs atoms to the surface and the formation-decomposition of O_ads-Pd*-O_abs structures. As a result, the desorption of O₂ occurred in accordance with a first-order reaction with a thermal desorption (TD) peak at T _max ∼ 700 K. At 1.0 ≤ n ≤ 2.0, the decomposition of PdO surface oxide occurred at a constant surface cover-age with oxygen during TPD in the course of the formation-decomposition of O_ads-Pd*-O_abs structures. Because of this, the desorption of O₂ occurred in accordance with a zero-order reaction at low temperatures with a TD peak at T _max ∼ 675 K. At 1.0 ≤ n ≤ 50, oxygen atoms diffused from deep palladium layers in the course of TPD and arrived at the surface at high temperatures. As a result, O₂ was desorbed with a high-temperature TD peak at T > 750 K.     

Crystallization in the SiO2-Al2O3 System from Amorphous Powders

The crystallization of amorphous chemically homogeneous powders in the SiO₂.Al₂O₃ system has been studied by differential scanning calorimetry and X-ray diffraction. Up to 1300°C only one exotherm has been observed. Only mullite crystallizes for compositions ≤69 mol% Al₂O₃ and spinel for those ≤80%. The crystallizations into mullite and spinel are sharp and exothermic, with an enthalpy of 250–300 J/g. The chemical composition of the crystallized mullite regularly increases from 68 to 76 mol% Al₂O₃ with increasing bulk composition from 60 to 75 mol% Al₂O₃.     

Theoretical Study of Structure and Stability of B n O and B n O2 Clusters

The structures and stabilities of B _n O and B _n O₂ clusters with n ≤ 6 are studied systematically by using density-functional theory. The lowest-energy structures of B _n O clusters favor two-dimensional, and can be obtained from B _n or B _n−1O. For B _n O₂, unexpectedly, all of the most stable B _n O₂ clusters but B₆O₂ are linear. Furthermore, in B _n O₂ clusters, the longer the distance between two O atoms, the more stable the structure. To investigate the relative stability of considered clusters, binding energies per atom, reaction energies, and fragmentation energies are also calculated and discussed.     

Study of the Dehydration/Rehydration of Ammonium Tris-Oxalato Aluminate(III) (NH4)3Al(C2O4)3⋅3H2O at Controlled Atmosphere

The study of the dehydration/rehydration of ammonium tris-oxalato aluminate(III) (NH₄)₃Al(C₂O₄)₃⋅3H₂O in flowing dinitrogen saturated with water vapor at room temperature, using thermogravimetric analysis and X-ray diffraction techniques, allowed the determination of the temperature stability domains of (NH₄)₃Al(C₂O₄)₃⋅3H₂O, (NH₄)₃Al(C₂O₄)₃⋅2H₂O and the anhydrous salt. The X-ray powder diffraction profiles are reported for each of the three phases. This revised version was published online in August 2006 with corrections to the Cover Date.     

The spectral characteristics of chlorine-containing water clusters in the presence of ozone and oxygen

The infrared absorption and Raman spectra were calculated by the molecular dynamics method for water clusters with chlorine ions in a medium of water and ozone or oxygen molecules. The intensity of IR absorption spectra of clusters with absorbed oxygen increased and that of clusters with absorbed ozone decreased as the number of chlorine ions grew. Excitation strengthening caused by an increase in the number of Cl⁻ ions weakened the intensity of Raman spectra when either oxygen or ozone was absorbed; for ozone, this weakening was more noticeable.     

Modeling of magic water clusters (H2O)20 and (H2O)21H+ with the PM3 quantum-mechanical method

The PM3 quantum-mechanical method is able to model the magic water clusters (H₂O)₂₀ and (H₂O)₂₁H⁺. Results indicate that the H₃O⁺ ion is tightly bound within the (H₂O)₂₀ cluster by multiple hydrogen bonds, causing deformation to the symmetric (H₂O)₂₀ pentagonal dodecahedron structure. The structures, energetics, and hydrogen bond patterns of six local minima (H₂O)₂₁H⁺ clusters are presented. © John Wiley & Sons, Inc.     

Thermal Investigations of M[La(C2O4)3]⋅xH2O (M=Cr(III) and Co(III))

The complexes M[La(C₂O₄)₃]⋅xH₂O (x=10 for M=Cr(III) and x=7 forM=Co(III)) have been synthesized and their thermal stability was investigated. The complexes were characterized by elemental analysis, IR, reflectance and powder X-ray diffraction (XRD) studies. Thermal investigations using TG, DTG and DTA techniques in air of chromium(III)tris(oxalato)lanthanum(III)decahydrate, Cr[La(C₂O₄)₃]⋅10H₂O showed the complex decomposition pattern in air. The compound released all the ten molecules of water within ∼170°C, followed by decomposition to a mixture of oxides and carbides of chromium and lanthanum, i.e. CrO₂, Cr₂O₃, Cr₃O₄, Cr₃C₂, La₂O₃, La₂C₃, LaCO, LaCrO_x (2<x<3) and C at ∼1000°C through the intermediate formation of several compounds of chromium and lanthanum at ∼374, ∼430 and ∼550°C. Thecobalt(III)tris(oxalato)lanthanum(III)heptahydrate, Co[La(C₂O₄)₃]⋅7H₂O becomes anhydrous around 225°C, followed by decomposition to Co₃O₄, La₂(CO₃)₃ and C at ∼340°C and several other mixture species of cobalt and lanthanum at∼485°C. The end products were identified to be LaCoO₃, Co₃O₄, La₂O₃, La₂C₃, Co₃C, LaCO and C at ∼ 2>1000°C. DSC studies in nitrogen of both the compounds showed several distinct steps of decomposition along with ΔH and ΔSvalues. IR and powder XRD studies have identified some of the intermediate species. The tentative mechanisms for the decomposition in air are proposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fe2P2O7(H2O)2

The compound diiron diphosphate dihydrate, Fe₂P₂O₇(H₂O)₂, was synthesized hydro­thermally and crystallizes in the monoclinic space group P2₁/n. The compound has a somewhat open framework made up of edge‐sharing iron(II) octahedra that form chains connected by five bridging diphosphates. The remaining octahedral site of each iron is occupied by coordinated water. The H atoms of the water molecules all point into a common channel.     

Phase Analysis and Crystallographic Properties of the Phosphate Systems MO-ZrO2-P2O5 (M=Mg,Ca, Sr or Ba)

Phase formation in the systems MO-ZrO₂-P₂O₅ (M=Mg, Ca, Sr or Ba) with various ratios of M to Zr cations and within the temperature interval from 20 to 1200°C was investigated by means of DTA, TG, XRD and IR spectroscopy. The orthophosphate phases M_0.5xZr_2.25−0.25x(PO₄)₃ with x=0−1, 3 and 7 were synthesized. Concentration and temperature limits of phase existence were found for phosphates belonging in the NaZr₂(PO₄)₃ structural family. They exist within the regions with M to Zr ratios of 0≤x≤1 (with the exception of the Mg phases) and in the temperature interval from room temperature to 900–1700°C. This revised version was published online in August 2006 with corrections to the Cover Date.     

Competition Between O2 and H2O2 in the Oxidation of Fe(II) in Natural Waters

The oxidation rates of nanomolar levels of Fe(II) in seawater (salinity S = 36.2) by mixtures of O₂ and H₂O₂ has been measured as a function of pH (5.8–8.4) and temperature (3–35∘C). A competition exists for the oxidation of Fe(II) in the presence of both O₂ (μ mol⋅L⁻¹ levels) and H₂O₂ (nmol⋅L⁻¹ levels). A kinetic model has been applied to explain the experimental results that considers the interactions of Fe(II) with the major ions in seawater. In the presence of both oxidants, the hydrolyzed Fe(II) species dominate the Fe(II) oxidation process between pH 6 and 8.5. Over pH range 6.2–7.9, the FeOH⁺ species are the most active, whereas above pH 7.9, the Fe(OH)⁰₂ species are the most active at the levels of CO²⁻₃ concentration present in seawater. The predicted Fe(II) oxidation rate at [Fe(II)]₀ = 30nmol⋅L⁻¹ and pH = 8.17 in the oxygen-saturated seawater with [H₂O₂]₀ = 50nmol⋅L⁻¹ (log ₁₀ k = −2.24s⁻¹) is in excellent agreement with the experimental value of log ₁₀ k = −2.29s⁻¹ ([H₂O₂]₀ = 55nmol⋅L⁻¹, pH = 8).