Small silicon oxide clusters: chains and rings

We study several silicon oxide cluster series with different Si:O stoichiometries using photoelectron spectroscopy (PES) of size-selected anions: (SiO) _n ^? (n=3–5), (SiO₂) _n ^? (n =1–4), and Si(SiO₂) _n ^? (n = 2,3). The (SiO)_n clusters are shown to be closed-shell molecules and the HOMOLUMO gaps are observed from the PES spectra to decrease for larger n. These clusters are shown to have ring sturctures. Si₃O₄ is known to have a D_2d structure with two perpendicular Si₂O₂ rhombuses.The PES spectrum of Si₄O ₆ ^? is very similar to that of Si₃O ₄ ^? . It is concluded that Si₄O₆ has a similar structure with a chain of three Si₂O₂ rhombuses. The (SiO₂)_n clusters all exhibit high electron affinities and only one band is observed at 4.66 eV photon energy. These clusters are shown to have similar chain structures containing Si₂O₂ rhombuses, but the two terminal Si atoms are bonded to an extra 0 atom each. The possibility of using these clusters to provide structural models for oxygen-deficient defects in bulk silicon oxides is also discussed.     

Fragmentation of doubly charged noble metal clusters

The dissociation patterns of doubly charged noble metal clusters (M) _n ⁺⁺ to two singly charged clusters, (M) _m ^Emphasis>1/+ and (M) _m ^Emphasis>2/+ have been investigated using a double focusing mass spectrometer. They are compared with the dissociation patterns from singly charged clusters. The dissociation probabilities to (M) ₃ ⁺ and (M) ₉ ⁺ were large and the odd-even alternations were observed in both patterns.     

On a possible mechanism for Ar 4 + fragmentation

The full potential energy surface (PES) for the collinear Ar ₄ ⁺ cluster as a function of the three internuclear distances is computed at the post-Hartree-Fock level using Density Functional Theory (DFT) methods to treat dynamic correlation effects. The behaviour of the overall configuration energy minima as the central Ar ₂ ⁺ bond stretches is analysed as a function of the fragmentation coordinates of the wing atoms. The coupling between the stretching coordinate and the fragmentation coordinates is also analysed over the whole PES. The calculations suggest that large vibrational energy content in the core dimer ion causes localization of the coupling with either wing atoms which could in turn favour energetically the sequential fragmentation, while Ar ₄ ⁺ with a vibrationally cold core markedly lowers any energy barrier to fragment in a concerted fashion. Such suggestions provide further useful information for what has been found in some of the experimental studies on this ionic system (and on larger ionized argon clusters) and underline the possible role which the internal vibrational energy content of the ionic cluster can play in the fragmentation.     

Group VB transition metal oxide clusters M4O n −/0 (M = Nb, Ta; n = 8–11): structural evolution and chemical bonding

The structural and electronic properties of two series of Group VB transition metal oxide clusters, M₄O _n ^? and M₄O _n (M = Nb, Ta; n = 8–11), are investigated using density functional theory calculations. Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra. Large highest occupied molecular orbital–lowest unoccupied molecular orbital gaps are observed for these two stoichiometric M₄O₁₀ clusters and estimated to be 3.98 and 4.38 eV for M = Nb and Ta, respectively. The M₄O ₁₀ ^?/0 (M = Nb, Ta) clusters are polyhedral cage structures with high symmetry (T _d for the neutral and D _2d for the anion) in which each metal atom joints three bridging and one terminal O atoms. For the Nb oxide species, Nb₄O ₈ ^?/0 and Nb₄O ₉ ^?/0 can be viewed as removing two and one terminal O atoms from Nb₄O ₁₀ ^?/0 , respectively. The Ta species follow the same rule to the Nb species, except that the anionic Ta₄O₈ ^? is formed by removing one terminal and one bridging O atoms from Ta₄O₁₀ ^?. The Ta₄O₉ containing a localized Ta³⁺ site can readily react with O₂ to form the Ta₄O₁₁ which can also be viewed as replacing a terminal oxygen atom in Ta₄O₁₀ by a peroxo O₂ unit, whereas the added oxygen atom is found to be a bridging one in the O-rich clusters Nb₄O ₁₁ ^?/0 and the anionic Ta₄O₁₁ ^?. Molecular orbital analyses are performed to analyze the chemical bonding in the tetra-nuclear metal oxide clusters and to elucidate their structural and electronic evolution.     

Primary processes in the thiacyanine dimer-photosensitized reduction of p-nitroacetophenone in water by

Primary processes in the reduction of p-nitroacetophenone (p-NAP) by ascorbic acid (AA) in water photosensitized by thiacyanine dimers M ₂ ^2? have been considered. For M ₂ ^2? , the quantum yields of fluorescence and intersystem crossing to the triplet state (M ₂ ^2? )_T increases in comparison to the monomers M^?. The dimers (M ₂ ^2? )_T enter into the reactions of both one-electron photoreduction by ascorbic acid to give AA^+· and M ₂ ^3? and one-electron photooxidation by p-nitroacetophenone to give p-NAP^?· and the dimeric radical anion M ₂ ^? which dissociates to M^? and M^· within 25–30 μs. The primary oxidative or reductive photosensitization in the ternary systems containing (M ₂ ^2? )_T, p-NAP, and AA affords p-NAP^?· and AA^+·.     

MB 8 2− (M = Be, Mg, Ca, Sr, and Ba): Planar octacoordinate alkaline earth metal atoms enclosed by boron rings

Complexes involving planar octacoordinate alkaline earth metal atoms in the centers of eight-membered boron rings have been investigated by two density functional theory (DFT) methods. BeB ₈ ^2? with D _8h symmetry is predicted to be stable, both geometrically and electronically, since a good match is achieved between the size of the central beryllium atom and the eight-membered boron ring. By contrast, the other alkaline earth metal atoms cannot be stabilized in the center of a planar eight-membered boron ring because of their large radii. By following the out-of-plane imaginary vibrational frequency, pyramidal C _8v MgB ₈ ^2? , CaB ₈ ^2? , SrB ₈ ^2? , and BaB ₈ ^2? structures are obtained. The presence of delocalized π and σ valence molecular orbitals in D _8h BeB ₈ ^2? gives rise to aromaticity, which is reflected by the value of the nucleus-independent chemical shift. The D _8h BeB ₈ ^2? structure is confirmed to be the global minimum on the potential energy surface.     

The nature of molecular oxygen binding and activation in Mn-O2 complex

Non-empirical calculations of CASSCF energies, electric dipole moments, Einstein coefficients, matrix elements of the operator of spin-orbital interaction between states of different multiplicity in a model complex ^6,8[Mn-O₂] of C _2v symmetry have been made in 3-21G, 6-31G, 6-31G** basis sets. The crosssections of the potential energy surface (PES) of the ground and excited states were built. It is found that oxygen bonding to manganese is possible when excited atoms of manganese collide with molecular oxygen, singlet oxygen with Mn[⁶ S _5/2] atoms, or in a close contact O₂[X³Σ _g ^? ] + Mn[⁶ S _5/2] and is determined by charge transfer states ^6,8CTS(Mn⁺O ₂ ^? ). Mechanisms of singlet oxygen activation/deactivation are determined by a considerably increased probability of electric dipole transitions b ¹Σ _g ⁺ ?a ¹Δ_g, a ¹Δ_g?X³Σ _g ^? , b ¹Σ _g ⁺ ?X³Σ _g ^? induced in oxygen in the collision process.     

Pressure and isotope effects on the proton jump of the hydroxide ion at 25°C

The limiting molar conductances Λ⁰ of potassium deuteroxide KOD in D₂O and potassium hydroxide KOH in H₂O were determined at 25°C as a function of pressure to disclose the difference in the proton-jump mechanism between an OH^? (OD^?) and a H₃O⁺ (D₃O⁺) ion. The excess conductance of the OD^? ion in D₂O λ _E ^O (OD ^-), as estimated by the equation $$\lambda _E^O (OD^ - ) = \Lambda ^O (KOD/D_2 O) - \Lambda ^O (KCl/D_2 O)$$ increases a little with pressure as well as the excess conductance of the OH^? ion in H₂O $$\lambda _E^O (OH^ - ) = \Lambda ^O (KOH/H_2 O) - \Lambda ^O (KCl/H_2 O)$$ However, their rates of increase with pressure are much smaller than those of the excess deuteron and proton conductances, λ _E ^O (D ⁺) and λ _E ^O (H ⁺). With respect to the isotope effect on the excess conductance, λ _E ^O (OH ^-)/λ _E ^O (D ⁺) decreases with presure as in the case of λ _E ^O (H ⁺)/λ _E ^O (D ⁺), but the value of λ _E ^O (OH ^-)/λ _E ^O (OD ^-) itself is much larger than that of λ _E ^O (H ⁺)/λ _E ^O (D ⁺) at each pressure. These results are ascribed to the difference in the pre-rotation of water molecules, which is brought about by the difference in the intial orientation of the rotating water molecule adjacent to the OH^? (OD^?) or the H₃O⁺ (D₃O⁺) ion.     

Photoelectron spectroscopy of transition metal-sulfur cluster anions

Metal (M)-sulfur cluster anions (M = Ag, Fe and Mn) have been studied using photoelectron spectroscopy (PES) with a magnetic-bottle type time-of-flight electron spectrometer. The M_nS _m ^? cluster anions were formed in a laser vaporization cluster source. For Ag-S, the largest coordination number of Ag atoms (n _max) is generally expressed as n _max =2m ? 1 in each series of the number of S atoms (m). For Fe?S and Mn?S, it was found that the stable cluster ions are the ones with compositions of n=m and n=m±1. Their electron affinities were measured from the onset of the PES spectrum. For Ag?S, the EAs of Ag₁S_m are small and around 1 eV, whereas those of Ag_nS_m (_n ≥ 2) become large above 2 eV. The features in the mass distribution and PES suggest that Ag₂S unit is preferentially formed with increasing the number of Ag atoms. For Fe?S and Mn?S, the PES spectra of Fe_nS _m ^? /Mn_nS _m ^? show a unique similarity at n ≥ m, indicating that the Fe/Mn atom addition to Fe_nS _n ^? /Mn_nS _n ^? has little effect on the electronic property of Fe_nS_n/Mn_nS_n. The PES spectra imply that the Fe_nS_n cluster is the structural framework of these clusters, as similarly as the determined structure of the Fe_nS_n cluster in nitrogenase enzyme.     

Crystal structure of (C2H7N4O)2[(UO2)2)(OH)2(C2O4)(CHO2)2]

An X-ray diffraction study of the single crystals of (C₂H₇N₄O)₂[(UO₂)₂(OH)₂(C₂O₄)(CHO₂)₂] was carried out. The compound crystallizes in the triclinic system, space group $P\bar 1$ , Z = 2, a = 5.5621(8) Å, b = 8.1489(10) Å, c = 11.8757(16) Å, α = 88.866(7)°, β = 82.204(6)°, γ = 87.378(6)°, V = 532.7(1) ų, ρ_calcd = 2.988 g/cm³. The main structural units in the crystal are the [(UO₂)₂(OH)₂(C₂O₄)(CHO₂)₂)]^2? chains corresponding to the crystal chemical group A₂M ₂ ² K⁰²M ₂ ¹ (A = UO ₂ ²⁺ , M² = OH^?, K⁰² = C₂O ₄ ^2? , M¹ = CHO ₂ ^? ) of uranyl complexes. The chains are united into a three-dimensional framework through the electrostatic interaction and hydrogen bonds involving uranyl, oxalate, and hydroxyl groups, formate ions, and 1-carbamoylguanidinium cations.     

High-temperature X-ray diffraction studies of compounds in the M 2 I O-Ga2O3-TiO2 system

Compounds that are formed in the M ₂ ^I O-Ga₂O₃-TiO₂ system and crystallize in three structural types were prepared by solid-phase reactions. The M ₂ ^I Ga₂Ti₆O₁₆ (M^I = Na, K, Rb, Cs) compounds were prepared for the first time. The thermal expansion coefficients of LiGaTiO₄, Na₂Ga₂Ti₆O₁₆, K₂Ga₂Ti₆O₁₆, Rb₂Ga₂Ti₆O₁₆, and Cs₂Ga₂Ti₆O₁₆ were determined by high-temperature X-ray diffraction. Some tendencies of thermal distortions in M ₂ ^I A ₂ ^III Ti₆O₁₆ and LiA^IIITiO₄ (M^I = Na, K, Rb, Cs; A^III = Al, Cr, Fe, Ga) were disclosed.     

Study of ionic equilibria and computation of thermodynamic characteristics of heteropoly anion formation in GdW10O 36 9− solution

Interaction in GdW₁₀O ₃₆ ^9? -H⁺(OH^?)-H₂O system ( \(C_{GdW_{10} O_{36}^{9 - } } \) = 1 × 10^?3 mol/L) was studied by pH potentiometry at 25 ± 0.1°C, and a model that describes equilibrium processes in acid and alkaline regions was selected. Logarithms of concentrational and thermodynamic constants, values of Gibbs energy of monomeric ions reactions, and standard Gibbs energies of formation (ΔG _f ^o ) of heteropoly anions H _n GdW₁₀O ₃₆ ^(9?n)? and H _m GdW₅O ₁₈ ^(3?n)? were calculated. A series-parallel scheme of ion transitions was pro-posed, ion distribution diagrams in aqueous solutions were built, the regions of preferable anion content were found, and heteropoly salts were synthesized.     

Ab initio potential energy surface for Ar 3 +

The potential energy surface (PES) of linear Ar ₃ ⁺ is calculated at the MP4/6-31G* level including all single, double, triple and quadruple excitations. The results show that the PES of the linear Ar ₃ ⁺ has a very flat valley along the asymmetric stretching vibration normal mode, ν₃. A higher level quadratic configuration interaction calculation including single, double and triple substitutions QCISD (T) along this flat valley suggests that an asymmetric geometry energy minimum reported earlier based on MP2 [1] is due to symmetry breaking in UHF. The global minimum of the PES is found to be for the symmetric geometry atR _ab =R _bc =2.66±0.01 Å, which is in good agreement with the MRD-CI calculation [2] and expectations from our earlier photodissociation experiments [3]. The calculational results are compared with other theoretical calculations, and are discussed in the context of the photodissociation and dynamics of dissociation experiments conducted on Ar ₃ ⁺ .     

Synthesis and X-ray diffraction study of K4[(UO2)2(C2O4)3(NCS)2] · 4H2O

Single crystals of K₄[(UO₂)₂(C₂O₄)₃(NCS)₂] · 4H₂O(I) have been synthesized and studied by X-ray diffraction. The crystals are monoclinic with the unit cell parameters a = 8.0226(7) Å, b = 14.9493(11) Å, c = 11.1670(9) Å, β = 98.299(3)°, space group P2₁/n, Z = 2, V = 1325.26(19) ų, R = 0.0186. The main structural units of the crystals of structure I are discrete binuclear groups [(UO₂)₂(C₂O₄)₃(NCS)₂]^4? belonging to the crystal-chemical group A₂K⁰²B ₂ ⁰¹ M ₂ ¹ (A =UO ₂ ²⁺ , K⁰² =C₂O ₄ ^2? , B⁰¹ =C₂O ₄ ^2? , M¹ = NCS^?) of the uranyl complexes. The uranium-containing complexes are linked into a three-dimensional framework through the potassium ions and a system of hydrogen bonds involving the outer-sphere water molecules.     

Guided ion beam studies of the reactions of O+, O 2 + , N+ and N 2 + with silane

Guided ion beam mass spectrometry is used to measure the cross sections as a function of kinetic energy for reaction of SiH₄ with O⁺(⁴S), O ₂ ⁺ (²Π_g,v=0), N⁺(³P), and N ₂ ⁺ (²Σ _g ⁺ ,v=0). All four ions react with silane by dissociative charge-transfer to form SiH _m ⁺ (m=0?3), and all but N ₂ ⁺ also form SiXH _m ⁺ products where (m=0?3) andX=O, O₂ or N. The overall reactivity of the O⁺, O ₂ ⁺ , and N⁺ systems show little dependence on kinetic energy, but for the case of N ₂ ⁺ , the reaction probability and product distribution relies heavily on the kinetic energy of the system. The present results are compared with those previously reported for reactions of the rare gas ions with silane [13] and are discussed in terms of vertical ionization from the 1t ₂ and 3a ₁ bands of SiH₄. Thermal reaction rates are also provided and dicussed.     

Adsorption studies of the removal of anions from aqueous solutions onto an adsorbent prepared from wheat straw

Modified wheat straw (MWS) was prepared by the grafting of epichlorohydrin, triethylamine and ethylenediamine onto WS. The characteristics of MWS and its adsorption capacity for NO ₃ ^? , PO ₄ ^3? and Cr₂O ₇ ^2? were investigated. The results indicate that amine groups with positive charge have been introduced into the structure of MWS, and significantly increased its anion adsorption property. The functions of MWS dosage, the solution pH, the contact time and temperature have significant influence on the adsorption process, and the adsorption is well fitted with the Langmuir equation and pseudo second-order model. The maximum adsorption capacity of MWS for NO ₃ ^? , PO ₄ ^— (P) and Cr₂O ₇ ^2? (Cr) is 53.5, 62.4 and 386.2 mg g^?1, respectively.     

Effect of pH, fulvic acid and temperature on the sorption of uranyl on ZrP2O7

The sorption of UO ₂ ²⁺ onto ZrP₂O₇ was studied using the batch technique and the point of zero charge of ZrP₂O₇ was obtained through mass titration. The results indicated that sorption of UO ₂ ²⁺ onto ZrP₂O₇ was strongly affected by pH, solid-to-liquid ratio (m/V), the species of electrolyte in solution and fulvic acid (FA), but was insensitive to ionic strength. The sorption of UO ₂ ²⁺ increased with increasing pH and m/V. The presence of FA enhanced UO ₂ ²⁺ sorption onto ZrP₂O₇ at low pH. The presence of phosphate or sulfate caused opposite effects on the sorption of UO ₂ ²⁺ onto ZrP₂O₇. Addition of citrate also significantly affected UO ₂ ²⁺ sorption. The sorption of UO ₂ ²⁺ increased as the temperature of the system increased. The Langmuir and Freundlich models were used to simulate the sorption isotherms of UO ₂ ²⁺ onto ZrP₂O₇ at different temperatures. The results indicated that the Freundlich model described UO ₂ ²⁺ sorption better than the Langmuir model. Thermodynamic parameters for the sorption process were calculated from the temperature dependent sorption isotherms. The results suggested that the sorption process of UO ₂ ²⁺ onto ZrP₂O₇ is spontaneous and endothermic. The desorption process of UO ₂ ²⁺ from ZrP₂O₇ was also investigated and it was found that sorption onto ZrP₂O₇ was irreversible.     

The thermodynamic parameters of oxygen nonstoichiometry defects in lanthanum cobaltite doped with acceptor impurities (Sr and Ni)

The oxygen nonstoichiometry δ of lanthanum cobaltite doped with acceptor impurities (Sr and Ni), La_{1 ? x} Sr_xCo_0.9Ni_0.1O_{3 ? δ} (x = 0.1, 0.3), was studied by high-temperature thermogravimetry over the temperature and pressure ranges 723 K ≤ T ≤ 1373 K and 10^?3 atm ≤ $p_{O_2 } $ ≤ 1 atm. The partial replacement of cobalt with nickel and lanthanum with strontium increased the oxygen nonstoichiometry δ. The partial molar enthalpies $\Delta \bar H^\circ _O $ and entropies $\Delta \bar S^\circ _O $ of solution of oxygen in the solid phase were calculated. Models of point defect formation were suggested and analyzed. The equilibrium constants of formation and concentrations of predominant point defects, ionized oxygen vacancies V _o ^.. , holes Me _Co ^. (Co _Co ^. and Ni _Co ^. ), and electrons Me _Co ^′ (Co _Co ^′ and Ni _Co ^′ ) localized on 3d transition metals, were determined by nonlinear regression from the experimental and theoretical logp $p_{O_2 } $ ?δ dependences.     

Shell correction study of fission of doubly charged silver clusters

Fission of doubly charged silver clusters is investigated by the method of shell corrections. The following fission events are considered: Ag ₂₂ ²⁺ → Ag _n ⁺ + Ag _{22 ?n} ⁺ , (n=11, 10, 9, 8); Ag ₂₁ ²⁺ → Ag _n ⁺ + Ag _{21 ?n} ⁺ , (n=10, 9, 8, 7); Ag ₁₈ ²⁺ → Ag _n ⁺ + Ag _{18 ?n} ⁺ , (n=9, 8, 7, 6). It is found that the shell correction energy is comparable to or larger than the deformation energy of the liquid drop. Threshold energies for the fission events are calculated and compared with the experimental abundance spectra obtained by Katakuse et al. (1990). Correspondence between the calculated threshold energies with the shell corrections and the experimental abundance is very good, showing products from lower threshold fission channels yield more abundance. The threshold energies without the shell corrections are almost constant irrespective of the fission channels and cannot explain the experimental abundance. Abundance of some products are too small to be accounted for only by the threshold energies. The low abundance of those products may be explained by the presence of competing fission channels that have similar minimal energy paths. It is found in fission of Ag ₁₈ ²⁺ that the shell correction overwhelms the Coulomb energy and the fission channel to Ag₈ + Ag ₁₀ ²⁺ is preferred over the fission channel to Ag ₈ ⁺ + Ag ₁₀ ⁺ .     

Interaction of neptunyl with goethite (α-FeOOH), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) in water as probed by X-ray photoelectron spectroscopy

The sorption behavior is studied and the physicochemical neptunium species existing on the surface of goethite (α-FeOOH), maghemite (γ-Fe₂O₃), and hematite (α-Fe₂O₃) are determined. Solvent extraction and X-ray photoelectron spectroscopy (XPS) are used to determine the neptunium surface species. The ion and elemental composition of the surface of the minerals and surface neptunyl NpO ₂ ⁺ complexes is determined using these data. Compounds containing neptunium(IV) or neptunium(VI) ions do not appear; rather, neptunyl (Np(V)O ₂ ⁺ group is complexed with surface hydroxide groups of α-FeOOH, γ-Fe₂O₃, and α-Fe₂O₃. Presumably, the oxygen atoms of iron oxides and water and/or carbonate (CO ₃ ^2- ) or nitrate (NO ₃ ^- ) group lie in the equatorial plane of the neptunyl (NpO ₂ ⁺ ) group.