V2.38Nb10.7O32.7: A V2O5-Nb2O5 mixed oxide tunnel structure related to the tetragonal tungsten bronzes

A crystal structural model for the orthorhombic compound V_2.38Nb_10.7O_32.7, which is known as “V₂Nb₉O_27.5”, was developed by means of selected area electron diffraction (SAED), Rietveld refinement and high resolution electron microscopy (HREM). The metastable compound is obtained by thermal decomposition of freeze-dried precursors as chain-like agglomerated nanoparticles or by reaction of V₂O₅ with fresh-precipitated Nb₂O₅ as more compact micro-scaled crystals. With the latter, it was possible to identify its structure for the first time (space group Cmmm). The tetragonal tungsten bronze (TTB)-type structure shows high potential for ionic intercalation, since easily reducible [V⁵⁺₂O²⁻] units are implemented in the tunnels of a rigid niobium oxide framework.     

Ab initio calculation of the electronic structure and the barrier to pyramidal inversion for H3S+

The electronic structure of H₃S⁺ is examined by ab initio MO calculations (STO 3G) and is compared with those of other XH₃ type molecules. The barrier to pyramidal inversion and the proton affinity of H₂S are calculated to be 34.8 and 225.05 kcal/mol respectively. Computations are made with respect to a model A₃S⁺ which is employed to discuss the barrier to pyramidal inversion of H₃S⁺, where A corresponds to an electromagnetive substituent or an electron donating one.     

EPR and Spectrophotometric Studies of Free Radicals (O2°−, °OH, BPD-MA°−) and Singlet Oxygen (1O2) Generated by Irradiation of Benzoporphyrin Derivative Monoacid Ring A

Abstract— Benzoporphyrin derivative monoacid ring A (BPD-MA), a chlorin-type molecule, is a new photosensitizer currently in phase II clinical trials for the treatment by pho-todynamic therapy of cancerous lesions, psoriasis and pathologic neovascularization. The photochemistry (type I and/or II) of BPD-MA has been studied in homogeneous solution and in aqueous dispersions of unilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) using electron paramagnetic resonance and spectrophotometric methods. When oxygen-saturated solutions of BPD-MA were illuminated with 690 nm light, singlet oxygen (¹O₂), superoxide anion radical (O₂^?), hydroxyl radical (OH) and hydrogen peroxide (H₂O₂) were formed. The BPD-MA generates ¹O₂ with a quantum yield of ca 0.81 in ethanolic solution. The quantum yield does not change upon incorporation of BPD-MA into liposomes of DPPC. The superoxide anion radical was generated by the BPD-MA anion radical (BPD-MA^?) via electron transfer to oxygen, and this process was significantly enhanced by the presence of electron donors. The rate of production of 0₂ was also dependent on the concentration of BPD-MA used (3-100 μM). The quantum yield of O₂^?was found to be 0.011 and 0.025 in aqueous solution and DPPC liposomes, respectively. Moreover, O₂^_upon dis-proportionation can generate H₂O₂ and ultimately the highly reactive OH via the Fenton reaction. In anaerobic homogeneous solution, BPD-MA^?was predominantly photoproduced via the self-electron transfer between the excited- and ground-state species. The presence of an electron donor significantly promotes the reduced form of BPD-MA. These findings suggest that the photodynamic action of BPD-MA may proceed via both type I and type II mechanisms.     

Ionogenic equilibria of (φ3CCl + HgCl2) and ((pClC6H4)3CCl + HgCl2) in CH2Cl2

Binary ionogenic equilibria (RX + MtX_x ? R⁺MtX_{n + l}^? ? R⁺ + MtX_{n + 1}^?, R = φ₃C, (pClC₆H₄)₃C; X = Cl; Mt = Hg) are studied in CH₂Cl₂ by conductivity and u.v. spectroscopy. The importance of such studies to cationic polymerisation is emphasised. The equilibrium constants, the thermodynamic parameters and the molar conductivities of the individual ions are given and the results discussed both in terms of a comparison between the two systems studied and in terms of a comparison with similar data in the literature.     

Ziegler-type catalysts in the system Cp2TiCl2/ethyl aluminium chloride with methylene chloride as solvent. Explanation of the “linear” uptake of ethylene

The paper gives an explanation for the “linear” uptake of ethylene by a polymerisation process, wrongly observed by Yildirim working on a Cp₂TiCl₂/Al₂Et₃Cl₃/CH₂Cl₂ system. A comment is also made on the interpretation of electron spin resonance spectra.     

Periodic trends in metal–metal bonding in edge-shared [M2Cl10] systems

Periodic trends in metal–metal interactions in edge-shared [M₂Cl₁₀]⁴⁻ systems, involving the transition metals from groups 4 through 8 and electronic configurations ranging from d¹d¹ through d⁵d⁵, have been investigated by calculating metal–metal bonding and spin-polarization (exchange) effects using density functional theory. The trends found in this study are compared with those for the analogous face-shared [M₂Cl₉]³⁻ systems reported in earlier work. Strong linear correlations between the metal–metal bonding and spin-polarization terms have been obtained for all groups considered. In general, spin polarization and electron localization are predominant in 3d–3d species whereas electron delocalization and metal–metal bonding are favoured in 5d–5d species, with more variable results observed for 4d–4d systems. As previously found for face-shared [M₂Cl₉]³⁻ systems, the strong correlations between the metal–metal bonding and spin polarization energy terms can be related to the fact that both properties appear to be similarly affected by the changes in the metal orbital properties and electron density occurring within the dⁿdⁿ groups. A significant difference between the face-shared and edge-shared systems is that while the 4d metals in the former show a strong tendency for delocalized metal–metal bonded structures, the edge-shared counterparts display much greater variation with both metal–metal bonded and weakly coupled complexes observed. The tendency for weaker metal–metal interactions can be traced to the inability of the edge-shared bridging structure to accommodate the smaller metal–metal distances required for strong metal–metal bonding.     

Energetics of hydride and electron pair attachment to EX3 (E = B, C, Al, Si and X = F, Cl, Br, I) and the study of bonding trends among EX3, EX3, and EX3H by use of ELF and NBO analyses

A theoretical gas-phase “ligand-free” or “electron pair affinity” (EPA) approach, based on CCSD(T)/(SDB-)cc-pVTZ//MP2/(SDB-)cc-pVTZ electronic structure calculations, is introduced as a possible means for determining Lewis acidity trends among planar EX₃^0/+ (E = B, C, Al, Si; X = F, Cl, Br, I) species. In this treatment, the free electron pair is considered to be an extreme Lewis base. The calculated EPA values are compared with experimental Lewis acidities, previously calculated fluoride ion affinity (FIA) and hydride ion affinity (HA) trends, and are found to exhibit reasonable correlations in all cases. The bonding in the planar and trigonal pyramidal conformations of EX₃^0/+ and of the trigonal pyramidal Lewis base EX₃^2−/− anions are assessed by use of natural bond orbital (NBO) and natural resonance theory (NRT) analyses. The NBO charges of the CX₃⁺ (X = Cl, Br, I, OTeF₅) cations are shown to correlate with the cation-anion and cation-solvent contacts in the recently determined crystal structures of [CCl₃][Sb(OTeF₅)₆], [CBr₃][Sb(OTeF₅)₆]·SO₂ClF, [CI₃][Al(OC(CF₃)₃)₄], and [C(OTeF₅)₃][Sb(OTeF₅)₆]·3SO₂ClF and known fluoro-carbocation structures. Topological electron localization function (ELF) basin lobe isosurfaces and volumes are used to rationalize the Lewis acidity trends and bond ionicities of the EX₃^0/+ species, and Lewis basicities of the EX₃^2−/− species.     

Potential curves and molecular properties of Na2+

A model potential method in which a molecule is described as a single electron moving in the field of two polarizable cores is used to calculate the potential energy curves and the wavefunctions of the lowest six electronic states of the molecular ion Na₂⁺. The ground X²Σ_g state has a dissociation energy of 0.98 eV at an equilibrium separation of 3.3 Å and the excited ²Π_u state has a dissociation energy of 0.23 eV at an equilibrium separation of 5.2 Å. Various molecular properties of these two bound states are calculated. An analysis of the long range behaviour of all the six states is presented.     

Modified electron transfer model for excitation functions of the type M + RX → MX + R and M+ + RX− (M = alkali,R = alkyl group,X = halogen) and M′ + N2O → MO* + N2 (M′= Ba OR Sm)

A one-dimensional model is described for the excitation functions of reactions that are initiated by an electron transfer at close range. The process is governed by a barrier in the entrance channel, the abortive reflection of trajectories at higher energies and by the competition of an adiabatic and a diabatic channel for the reactive flux. The model is fitted to measured cross sections for the (K.Rb)+CH₃I, K+C₂H₅Br and (Ba.Sm)÷ N₂O reactions and the electron transfer cross section for K+CH₃I → K⁺ + CH₃I^- is successfully predicted from the fitting parameters of the reactive channel.     

New salts of graphite,C12+HF2− & C24+SiF5− and the treshold for the oxidative intercalation of graphite

Third row transition metal hexafluorides (MF₆) for which the electron affinity exceeds 130 kcal/mole (M = Os, Ir, Pt) have been found to intercalate graphite with electron oxidation of the host lattice, whereas those with inferior electron affinities (M = W, Re) do not intercalate¹. This behavior can be rationalized on kinetic or thermodynamic grounds; arguing for the latter, a simple Born-Haber cycle may be used which suggests an electron affinity threshold of 120–130 kcal/mole for the MF₆ intercalation reaction. For the general case of intercalation reactions by metal fluorides (with or without added fluorine), wherein the graphite lattice is oxidized, the threshold is determined by the free energy of the half-reaction which produces the intercalating fluoro-anion. The lattice energy of the graphite salt must also be taken into account when comparing free energy thresholds for large (e.g., MF₆) and small (e.g. HF₂^?) intercalating species.We have evaluated the free energy of formation of a number of fluoro-anions from the heats of formation and lattice energies of salts which contain them. These studies indicate a threshold free energy of ca. 110 kcal/mole for graphite intercalation. Two ‘borderline’ second stage compounds, C₂₄⁺SiF₅^? and C₁₂⁺HF₂^?, have been synthesized.     

四角锥形结构化合物BeB4X4(X=H，F，Cl)，HBB4H4与BB4H4+中B4平面环的半芳香性

选用6-311++G(3df,2p)基组,在二级微扰的理论下,对四角锥形结构化合物BeB4X4(X=H,F,Cl),HBB4H4与BB4H4+的分子振动频率,及原子间的相互作用进行了计算,作用能的计算使用了CCSD(T)方法。结果显示HBB4H4与BB4H4+是违反韦德规则的另两个特例,它们表现稳定的原因与芳香性有关。     

L3/L2 white-line intensity ratios in the electron energy-loss spectra of 3d transition-metal oxides

The L₃/L₂ white-line intensity ratio in transition-metal oxides deviates widely from the statistical value of 2 : 1 but shows interesting systematics. In a series of oxides of a given metal, the ratio reaches a maximum for the d⁵ configuration (e.g. MnO) and a minimum for the d⁰ configuration (e.g. KMnO₄). In a series of monoxides, sesquioxides and dioxides of different metals, the ratio is again a maximum at the d⁵ configuration and decreases as the configuration changes towards d⁰ or d¹⁰. Our results, obtained by electron energy-loss spectroscopy, carried out in an electron microscope, are interpreted on an atomic mechanism involving spin-spin coupling. According to this model, the L₂ transition probability decreases in the progression d⁰ to d⁵ whereas the L₃ transition probability decreases beyond d⁵.     

Structural studies of some body-centered cubic phases of mixed oxides involving Bi2O3: The structures of Bi25FeO40 and Bi38ZnO60

The structures of the compounds initially reported to be 7·Bi₂O₃·ZnO and 96·Bi₂O₃·4Fe₂O₃, have been determined by X ray methods. Three dimensional, absorption corrected diffractometer data were used and atomic parameters were refined by least-squares procedures. The structures are isomorphous, cubic witha = 10.194(3)and10.179(3)A?, respectively, and space group I23. Each Bi³⁺ ion is surrounded by five oxygen atoms that form an incomplete octahedral arrangement with BiO distances ranging from 2.07–2.60A?. The6s²inert electron pair completes the octahedron. The Bi³⁺ ions are vibrating anisotropically. Tetrahedral sites in the structures contain 61 and 46 electrons, respectively. These values are consistent with a statistical distribution of Zn²⁺ and Bi⁵⁺ ions or Fe³⁺ and Bi⁵⁺ ions on these sites. Molar ratios are derived that agree with the observed distributions of electron density and give rise to perfectly stoichiometric systems, devoid of cationic or anionic vacancies. The compositions studied correspond to Bi³⁺₂₄Bi⁵⁺Fe³⁺O₄₀ and Bi³⁺₃₆Bi⁵⁺₂ZnO₆₀ and they are optical enantiomorphs.It is proposed that a reduction in the percentage composition of Bi₂O₅ leads to metastable phases, in which all atomic positions remain fully occupied but some tetrahedral sites contain Bi³⁺ ions. The end product of the series is γ-Bi₂O₃ in which 50% of these sites contain Bi³⁺ and the remainder Bi⁵⁺ ions. We believe that γ-Bi₂O₃ is Bi³⁺₂₅Bi⁵⁺O₄₀.     

The application of 13C and 1H NMR spectroscopy to the investigation of the dinitrogen fixation process in the system (η-C5H5)2TiCl2-Mg

The N₂ reduction reaction in the system (η-C₅H₅)₂TiCl₂-Mg in tetrahydrofuran was examined. The ¹³C and ¹H NMR results as well as the chemical properties of the products formed revealed that the reaction yielded a mixture of compounds in which the titanium atom was bonded both to the μ-(η⁵: η⁵-fulvalene) ligand and to the cyclopentadienyl ligands. In this system dinitrogen undergoes reduction to N^3?, which then forms M₃N bridges (M = Ti, Mg). The nitride nitrogen may readily be oxidized to imide nitride N^?1, which may react further, e.g. with carbon monoxide to produce isocyanates, or, with excess oxidizing agent N₂. THF in this system undergoes polymerisation. In addition, a ?OC₄H₉ alkoxy group is formed which makes the substitution of the cyclopentadienyl group bonded to the titanium atoms possible.     

p-Type electron transport in La1−xSrxFeO3−δ at high temperatures

Electrical conductivity, thermopower and oxygen content were measured for La_1−xSr_xFeO_3−δ (x=0.2, 0.5, 0.9) within the oxygen partial pressure range 10⁻⁴-0.5 atm and at temperatures 750-950 °C. The dominating charge carriers under these experimental conditions are electron holes. The results of oxygen nonstoichiometry measurements are used to estimate the concentration of holes and to analyze data on conductivity and thermopower. The changes in thermopower are described by the model assuming that the number of sites accessible for migration of holes is independent on oxygen content. The mobility of electron holes is calculated, and it is found to be virtually independent on temperature in the compositions with x<0.5 while compositions with x>0.5 exhibit thermally activated mobility and mobility values about 0.1 cm² V⁻¹ s⁻¹ or smaller. It is suggested that the main contribution to the composition dependent variations in electron hole mobility are associated with Coulomb interactions at small x's, whereas at high degrees of doping the mobility of holes is most strongly affected by the increasing amount of oxygen vacancies.     

沉淀煅烧法合成(CuAg)0.15In0.3Zn1.4S2光催化剂及可见光下的制氢性能

研究了沉淀煅烧法制备的纳米级(CuAg)_0.15In_0.3Zn_1.4S₂光催化剂以及产氢效率,并采用X射线衍射、透射电镜、扫描电镜、N₂吸附-脱附、紫外-可见吸收光谱等手段对催化剂进行了表征. 结果表明,(CuAg)_0.15In_0.3Zn_1.4S₂的结晶度、比表面积和吸收可见光能力与煅烧温度和煅烧时间有关. 在可见光下,以KI为电子给体,考察了不同条件对光催化产氢能力的影响. 发现在600℃煅烧5h时所制的(CuAg)_0.15In_0.3Zn_1.4S₂光催化剂产氢活性最高,产氢速率为1750 μmol g^-1 h^-1,量子效率在420±5nm达到12.8%,比未煅烧催化剂的光催化活性提高了约6倍.     

Reactions of the dications C7H6, C7H7, and C7H8 with methane: Predominance of doubly charged intermediates

The reactions of methane with the dications C₇H₆²⁺, C₇H₇²⁺, and C₇H₈²⁺ generated by electron ionization of toluene are studied using mass-spectrometry tools. It is shown that the reactivity is dominated by the formation of doubly charged intermediates, which can either eliminate molecular hydrogen to yield doubly charged products or undergo charge-separation reactions leading to the formation of a methyl cation and the corresponding C₇H_n+1⁺ monocation. Typical processes observed for dications, like electron transfer or proton transfer, are largely suppressed. The theoretically derived mechanism of the reaction between C₇H₆²⁺ and CH₄ indicates that the formation of the doubly charged intermediate is kinetically preferred at low internal energies of the reactants. In agreement, the experimental results show a pronounced hydrogen scrambling and dominant formation of the doubly charged products at low collision energies, whereas direct hydride transfer prevails at larger collision energies.     

Enthalpies of polymerisation of SbF5, NbF5 and TaF5

The enthalpies of polymerisation of SbF_5(g), NbF_5(g) and TaF_5(g) have been estimated from molecular weight data to be ?18.5, ?25.9 and ?25.7 kJ mol^?1 respectively assuming mixtures of monomer and tetramer. Estimates of the entropy changes have also been made.     

Proton hyperfine couplings in VO(H2O)2+5: The validity of the point-dipole approximation

A double-zeta wavefunction has been used to calculate the dipolar electron—proton hyperfine interactions in VO(H₂O)²⁺₅ using numerical integration. The results indicate that a point-dipole model is a good approximation for this ion.     

Λ-doublet populations in CH(A2Δ) produced in the 193 nm multiphoton dissociation of (CH3)2CO, (CD3)2CO, (CH3)2S and CH3NO2

Unequal intensities of the Λ-doublet components were observed in the CH(A²Δ-X²Π) emission following the multiphoton dissociation of (CH₃)₂CO, (CH₃)₂S and CH₃NO₂ by an ArF laser (193 nm). The power dependence of the emission intensity was estimated to be cubic (3.1±0.2) when the laser power was below ≈ 8×10¹⁷ photons cm^?2 pulse^?1. The Λ-doublet populations depended on the rotational quantum number N′ and the preferred level changed at N′ = 20. A similar behavior was observed for the CD(A²Δ) from (CD₃)₂CO. Rotational distributions show bimodal behavior, having a hump around N′ = 13 in CH(A²Δ) and N′ = 11 in CD(A²Δ). These trends indicate that the CH(A²Δ) is produced through multiple processes where stepwise mechanisms are operative via either CH₂ or CH₃, or both radicals as intermediates.