The dramatic effect of NH3 co-ligation on the Fe+-assisted activation of carbon dioxide in the gas phase: from bare metal ions to complexes

The catalytic efficiency of Fe(+) ion over the CO(2) decomposition in the gas phase has been extensively investigated with the help of electronic structure calculation methods. Potential-energy profiles for the activation process Fe(+) + CO(2) --> CO + FeO(+) along two rival potential reaction paths, namely the insertion and addition pathways, originating from the end-on kappa(1)-O and kappa(2)-O,O coordination modes of CO(2) with the metal ion, respectively, have been explored by DFT calculations. For each pathway the potential energy surfaces of the high-spin sextet (S = 5/2) and the intermediate-spin quartet (S = 3/2) spin-states have been explored. The complete energy reaction profile calculated by a combination of ab initio and density functional theory (DFT) computational techniques reveals a two-state reactivity, involving two spin inversions, for the decomposition process and accounts well for the experimentally observed inertness of bare Fe(+) ions towards CO(2) activation. Furthermore, the coordination of up to three extra ancillary NH(3) ligands with the Fe(+) metal ion has been explored and the geometric and energetic reaction profiles of the CO(2) activation processes Fe(+) + n x NH(3) + CO(2) --> [Fe(NH(3))(n)(CO(2))](+) --> [Fe(NH(3))(n)(O)(CO)](+) --> CO + [Fe(O)(NH(3))(n)](+) (n = 1, 2 or 3) have thoroughly been scrutinized for both the insertion and the addition mechanisms. Inter alia, the geometries and energies of the various states of the [Fe(NH(3))(n)(CO(2))](+) and [Fe(NH(3))(n)(O)(CO)](+) complexes are explored and compared. Finally, a detailed analysis of the coordination modes of CO(2) in the cationic [Fe(NH(3))(n)(CO(2))](+) (n = 0, 1, 2 and 3) complexes is presented.     

Deciphering the bonding mode of the trihalide ligands in a series of halogen carrier homo- and hetero-trihalide Cu(II) Schiff base complexes

Electronic structure calculation techniques (DFT) have been used to decipher the bonding of the trihalide ligands in a series of homo- and hetero-trihalide Cu(II) Schiff base complexes formulated as [Cu(RdienR)(X)(XY₂)] (RdienR = Schiff base; R = furan, thiophene or pyrrol; X = Cl or Br; Y = Cl, Br or I). The association of the incoming Y₂ halogen molecule with one of the halide X ligands of the precursor [Cu(RdienR)(X)₂] complexes alters their distorted trigonal bipyramidal stereochemistry which is transformed to a distorted square pyramidal geometry. The bonding mechanism between the halogen Y₂ molecule and the halide X ligand was thoroughly explored by means of various electronic parameters and charge decomposition analysis techniques. The bond dissociation energy of the Cu–XY₂ bond, BDECu–XY₂${\text{BDE}}_{\text{Cu}''–''{\text{XY}}_2}$, was estimated in the range of 61.9–68.4 kcal/mol, while the bond dissociation energy of the X–Y₂ bond, BDECu–XY₂${\text{BDE}}_{\text{Cu}''–''{\text{XY}}_2}$, was found in the range of 10.6–12.5 kcal/mol. It was found that the X?Y₂ interactions correspond to weak hyperconjugative donor–acceptor interactions between a non-bonding n(X) molecular orbital (donor orbital) localized on the coordinated halide X ligand and an antibonding σ^∗(Y–Y) molecular orbital (acceptor orbital) localized on the Y₂ halogen molecule. The n(X) → σ^∗(Y–Y) donor–acceptor interactions are associated with a second-order perturbation stabilization energy, ΔE(2) of 34.5–52.5 kcal/mol. The loose association of the halogen molecules with the coordinated halide ligand renders the [Cu(RdienR)(X)(XY₂)] complexes good halogen carrier molecules.     

Surface diffusion,migration, and conjugated processes at heterophase interfaces between WO<Subscript>3</Subscript> and MeWO<Subscript>4</Subscript> (Me = Ca,Sr, Ba)

With the aid of a complex of methods it is demonstrated that at heterophase interfaces between WO₃ and MeWO₄ (Me = Ca, Sr, Ba) there occurs penetration of components WO₃ and MeWO₄ into one another under spontaneous conditions and after the imposition of an electric field. Experimental data concerning the electrosurface migration in potentiostatic and galvanostatic regimes are compared. It is demonstrated that the amount of WO₃ transported onto inner surface of MeWO₄ is defined by the magnitude of the electric charges passed through the system but does not depend on the I–U parameters of experiment. It is established that the magnitude of the faradaic efficiency of the WO₃ transport in an electric field at 900°C is close for all compounds of the type MeWO₄ (Me = Ca, Sr, Ba) and amounts to 0.42 ± 0.02 for a galvanostatic regime of the process. Methods of x-ray diffraction analysis, x-ray-fluorescence analysis, XPS, and electron microscopy are employed to explore the properties and compositions of regions adjacent to the WO₃|MeWO₄ interface after experiments in spontaneous and field-induced regimes. Data are obtained that confirm the reality of formation of nonautonomous phase MeW-s and its crucial role in the origin and mechanism of processes that occur at the heterophase interface WO₃|MeWO₄. The real architecture of the interface may be portrayed by the scheme WO₃?MeW-s|MeW-s?MeWO₄, which reflects penetration of MeW-s into both initial briquettes. The reasons for the loss of weight of briquettes of MeWO₄ when annealed in contact with WO₃ under spontaneous conditions are analyzed. It is shown that the weight loss may be caused by congruent sublimation of the MeW-s phase, which is directly connected with its low surface energy and relatively low sublimation energy.     

A PIC method for solving a paraxial model of highly relativistic beams

Solving the Vlasov–Maxwell problem can lead to very expensive computations. To construct a simpler model, Laval et al. [G. Laval, S. Mas-Gallic, P.A. Raviart, Paraxial approximation of ultrarelativistic intense beams, Numer. Math. 69 (1) (1994) 33–60] proposed to exploit the paraxial property of the charged particle beams, i.e the property that the particles of the beam remain close to an optical axis. They so constructed a paraxial model and performed its mathematical analysis. In this paper, we investigate how their framework can be adapted to handle the axisymmetric geometry, and its coupling with the Vlasov equation. First, one constructs numerical schemes and error estimates results for this discretization are reported. Then, a Particle In Cell (PIC) method, in the case of highly relativistic beams is proposed. Finally, numerical results are given. In particular, numerical comparisons with the Vlasov–Poisson model illustrate the possibilities of this approach.     

Molecular and electronic structure, magnetotropicity and absorption spectra of benzene-trinuclear copper(I) and silver(I) trihalide columnar binary stacks

The molecular and electronic structures, stabilities, bonding features, magnetotropicity and absorption spectra of benzene-trinuclear Cu(I) and Ag(I) trihalide columnar binary stacks with the general formula [c-M(3)(μ(2)-X)(3)](n)(C(6)H(6))(m) (M = Cu, Ag; X = halide; n, m ≤ 2) have been investigated by means of electronic structure calculation methods. The interaction of c-M(3)(μ(2)-X)(3) clusters with one and two benzene molecules yields 1:1 and 1:2 binary stacks, while benzene sandwiched 2:1 stacks are formed upon interaction of two c-M(3)(μ(2)-X)(3) clusters with one benzene molecule. In all binary stacks the plane of the alternating c-M(3)(μ(2)-X)(3) and benzene components adopts an almost parallel orientation. The separation distance between the centroids of the benzene and the proximal c-M(3)(μ(2)-X)(3) metallic cluster found in the range 2.97-3.33 ? at the B97D/Def2-TZVP level is indicative of a π···π stacking interaction mode, for the centroid separation distance is very close to the sum of the van der Waals radii of Cu···C (3.10 ?) and Ag···C (3.44 ?). Energy decomposition analysis (EDA) at the SSB-D/TZP level revealed that the dominant term in the c-M(3)(μ(2)-X)(3)···C(6)H(6) interaction arises from dispersion and electrostatic forces while the covalent interactions are predicted to be negligible. On the other hand, charge decomposition analysis (CDA) illustrated very small charge transfer from C(6)H(6) toward the c-M(3)(μ(2)-X)(3) clusters, thus reflecting weak π-base/π-acid interactions which are further corroborated by the respective electrostatic potentials and the fact that the total dipole moment vector points to the center of the metallic ring of the c-M(3)(μ(2)-X)(3) cluster. The absorption spectra of all aromatic columnar binary stacks simulated by means of TD-DFT calculations showed strong absorptions in the UV region. The main features of the simulated absorption spectra are thoroughly analyzed, and assignments of the contributing electronic transitions are given. The magnetotropicity of the binary stacks evaluated by the NICS(zz)-scan curves indicated an enhancement of the diatropicity of the inorganic ring upon interaction with the aromatic benzene molecule. Noteworthy is the slight enhancement of the diatropicity of the benzene ring, particularly in the region between the interacting rings, probably due to the superposition (coupling) of the diamagnetic ring currents of the interacting aromatic ring systems.     

Mixed conductivity of gadolinium titanate-based pyrochlore ceramics: The grain boundary effects

In order to reveal the role of grain boundaries on the ionic and electronic conduction processes, the transport properties of Gd_2−xGa_xTi₂O_7−δ (x=0.10–0.14) pyrochlore ceramics, pure and with SiO₂ additions, were studied at 700–950 °C by impedance spectroscopy and faradaic efficiency measurements. The oxygen ion transference numbers of “pure” materials in air vary in the range of 0.95–0.97, increasing when temperature decreases. For silica-containing ceramics having, as expected, highly resistive grain boundaries, the ion transference numbers were considerably lower, 0.76–0.88, and increase with temperature. This behavior suggests that grain boundaries in these oxygen ion-conducting ceramics have a larger limiting effect on ionic transport than on electronic conduction. Increasing boundary resistivity may increase the relative role of electronic conductivity in solid oxide electrolytes, thus preventing their potential use in electrochemical cells at low temperatures. Also, the presence of even small electronic contributions to the total conductivity may lead to significant errors in the grain-boundary resistance values estimated from impedance spectroscopy data. The evaluation of the grain boundary exact contribution should be based on a clear knowledge of the magnitude of transference numbers. Paper prestented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Electrical Conductivity,Thermal Expansion and Electrochemical Properties of Perovskites PrBaFe<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5 + δ</Subscript>

In order to evaluate applicability of mixed-conducting PrBaFe_2–xNi _x О_{5 + δ} perovskites for cathodes of solid oxide fuel cells (SOFCs), their crystal structure, thermal and chemical expansion, electrical conductivity and electrochemical behavior were studied. The solubility limit of nickel in PrBaFe₂O_{5 + δ} corresponds to x = 0.8. At x > 0.2, the disordered cubic phase transformed into the tetragonal phase. The maximum level of conductivity (50–120 S/cm) at the operating temperatures of SOFC was found for the composition with the maximum nickel content, PrBaFe_1.2Ni_0.8О_{5 + δ}. This material is also characterized by moderate thermal and chemical expansion relative to other ferrite-nickelates. The polarization resistance of a porous PrBaFe_1.2Ni_0.8О_{5 + δ} cathode in a cell with a protective Ce_0.6La_0.4O_2–δ layer and a solid electrolyte (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3–δ was ~0.9 Ohm cm² at a temperature of 1073 K, atmospheric oxygen pressure, and current density of–120 mA cm^–2.     

Structure, energetics, and bonding of first row transition metal pentazolato complexes: a DFT study

Quantum chemical calculations with gradient-corrected (B3LYP) density functional theory for the mono- and bispentazolato complexes of the first row transition metals (V, Cr, Mn, Fe, Co, and Ni), the all-nitrogen counterparts of metallocenes, were performed, and their stability was investigated. All possible bonding modes (e.g. eta1, eta2, eta3, and eta5) of the pentazolato ligand to the transition metals have been examined. The transition metal pentazolato complexes are predicted to be strongly bound molecules. The computed total bond dissociation enthalpies that yield free transition metal atoms in their ground states and the free pentazolato ligands were found in the range of 122.0-201.9 (3.7-102.3) kcal mol(-1) for the bispentazolato (monopentazolato) complexes, while those yielding M2+ and anionic pentazolato ligands were found in the range of 473.2-516.7 (273.6-353.5) kcal mol(-1). The electronic ground states of azametallocenes along with their spectroscopic properties (IR, NMR, and UV-vis) obtained in a consistent manner across the first transition metal series provide means for discussion of their electronic and bonding properties, the identification of the respective azametallocenes, and future laboratory studies. Finally, exploring synthetic routes to azametallocenes it was found that a [2 + 3] cycloaddition of dinitrogen to a coordinated azide ligand with nickel(II) does not seem to provide a promising synthetic route for transition metal pentazolato complexes while the oxidative addition of phenylpentazole and fluoropentazole to Ni(0) bisphosphane complexes merits attention for the experimentalists.     

Turn‐On Luminescence Sensing and Real‐Time Detection of Traces of Water in Organic Solvents by a Flexible Metal–Organic Framework

The development of efficient sensors for the determination of the water content in organic solvents is highly desirable for a number of chemical industries. Presented herein is a Mg²⁺ metal–organic framework (MOF), which exhibits the remarkable capability to rapidly detect traces of water (0.05–5 % v/v) in various organic solvents through an unusual turn‐on luminescence sensing mechanism. The extraordinary sensitivity and fast response of this MOF for water, and its reusability make it one of the most powerful water sensors known.