Structure cristalline de SmZrF7. Relations structurales avec le type ReO3

The crystal structure of the fluorozirconate SmZrF₇ has been determined from single crystal X-ray data; the final R value is 0.028. In this compound, isotypic with all the fluorozirconates LnZrF₇ (Ln = rare earth), the symmetry of the cell is monoclinic (space group: P₂₁). The Zr atoms are surrounded by six fluorine atoms forming an almost perfect octahedron: the average distance ZrF is 2.006 Å. The Sm atoms are surrounded by a 8 fluorine atoms polyhedron, the distances SmF lie from 2.221 to 2.411 Å. The association of these two polyhedra by sharing corner forms slabs with an approximately ReO₃ type arrangement: the thickness of the slabs is two octahedra.These slabs with the composition M₂X₇ are held together so that the top fluorine atoms at the surface of a slab increase the coordination of the Sm atom of an adjacent slab. This MX_3.5 phase corresponds to a new structural type between MX₃ (ReO₃ type) and MX₄ (SnF₄ type).     

Ionic mobility and structure of fluorozirconates Rb2? x (NH4) x ZrF6 ( x > 1.5) by NMR, x-ray structure analysis, and impedance spectroscopy

The ionic mobility in the temperature interval 180 to 480 K, structure, and electrophysical properties of rubidium-ammonium hexafluorozirconates Rb_2−x (NH₄) _x ZrF₆ (1.5 ≤ x ≤ 2.0) are studied by methods of the ¹⁹F, ¹H NMR spectroscopy, x-ray structure analysis, differential thermal analysis, and impedance spectroscopy. Correlations between the composition of the cationic sublattice, the character of ionic motions, and the phase transition temperature (of the type order-disorder) are established in these compounds. The salient feature of the high-temperature modifications of these fluorozirconates with x ≥ 1.5 is the translation diffusion of ions inside the fluoride and ammonium sublattices and the ¹⁹F NMR spectra are characterized by monoaxial anisotropy of the magnetic shielding tensor of the fluorine nuclei. Fluorozirconates with x > 1.5 are shown to belong with the structural type (NH₄)₂ZrF₆. The rubidium cations isomorphically replace the ammonium cations. The electrophysical characteristics of the compounds are examined in the temperature interval 300 to 480 K. It is established that the electroconductivity of these compounds increases with x. Original Russian Text ? V.Ya. Kavun, A.V. Gerasimenko, A.B. Slobodyuk, N.A. Didenko, N.F. Uvarov, V.I. Sergienko, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 5, pp. 563–570. Based on the paper delivered at the 8th Meeting “Fundamental Problems of Solid-State Ionics”, Chernogolovka (Russia), 2006.     

Quantum-chemical analysis of the cluster models of coordination of imidazoline nitroxides on the silica surface

The results of ESR-spectroscopic and quantum-chemical studies of the coordination of 2,2,4,5,5-pentamethyl-3-imidazoline-N-oxyl and 2-phenyl-2,4,5,5-tetramethyl-3-imidazoline-N-oxyl with the surface acid sites (AS) of silica are analyzed. Cluster models of AS, corresponding to one- and two-point coordination and accepted in radiospectroscopy on the basis ofg-factors and constants of hyperfine splitting at the N nuclei in the resulting donor-acceptor complexes, are discussed. Within the framework of the unrestricted Hartree-Fock method using the MNDO approach, a comparative quantum-chemical analysis of the structural, spin, electrostatic, energy, and radiospectroscopic characteristics of the coordination of the model cluster AS to the imidazoline nitroxide probes is performed. For the two-center adsorption, three types of AS structures are considered, together with the paramagnetic surface complexes they form. A consistent semi-quantitative interpretation of the whole set of features found experimentally for the coordination of imidazoline nitroxides to the surface AS on silica is given. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1081–1086, June, 1998.     

Coordination polyhedra of hydration shells of Na+ and K+ cations in aqueous solutions

Simulation of the hydration of Na⁺ and K⁺ cations in dilute solution was performed by the Monte Carlo method. A novel approach to structural analysis of hydration shells of ions was developed. Specific sets of coordination polyhedra formed by water molecules of the first coordination sphere were found. Structural and energy characteristics of hydration were calculated. The effect of Na⁺ and K⁺ cations on the structure of the network of H-bonds and mobility of water molecules in hydration shells was studied. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 852–857, May, 1999.     

Cluster models in the quantum-chemical analysis of coordination of nitroxide probes to Lewis acid sites on the surface of Al2O3

The results of ESR-spectroscopic and quantum-chemical investigations of the coordination of 2,2,6,6-tetramethylpiperidineN-oxyl, 2,2,3,4,5,5-hexamethyl-3-imidazolidineN-oxyl, 2,2,4,5,5-pentamethyl-3-imidazolineN-oxyl, 2,4,5,5-tetramethyl-2-phenyl-3-imidazolineN-oxyl, and 2,4,5,5-tetramethyl-2-octyl-3-imidazolineN-oxyl to Lewis acid sites (LAS) on alumina surface are described systematically and analyzed. The cluster models of LAS accepted in radiospectroscopy and based on experimental data on g-factors and hyperfine coupling constants with N and Al nuclei in the corresponding donor-acceptor complexes are discussed. Within the framework of the unrestricted Hartree—Fock (UHF) method using the STO-3G, STO-6G, 3–21G, and 6–31G basis sets and also in terms of semiempirical MNDO, AM1, and PM3 procedures, comparative quantum-chemical analysis of the structural, spin, electrostatic, energy, and radiospectroscopic characteristics of the coordination of the model cluster LAS to the simplest representative of nitroxides is performed. Three illustrative types of structures of the resulting surface complex are considered. A semiquantitative interpretation of the whole set of features found experimentally for the coordination of nitroxide probes to the surface LAS on alumina is given. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1743–1756, October, 1997.     

Quantum chemical study of model fluorozirconate clusters

A number of fluorozirconate clusters having the general formula {fx41-01} were investigated by the DV-X_α quantum chemical method. The results of calculations are used to explain the existence of polyhedm with coordination numbers of the central atom 6, 7, and 8 in crystalline fluomzirconates. In the title series of clusters, the [ZrF₆]^2- octahedron and the dimer of edge-sharing seven-vertex polyhedm [Zr₂F₁₂]^4-s are highly stable. Translated from Zhumal Struktumoi Khimii, Vol. 41, No. 1, pp. 49-57, January–February, 2000.     

Structural Features of ONS-Donor Salicylidene Schiff Base Complexes

This review article summarizes the structural features of complexes of salicylidene Schiff bases containing, in addition to the phenolic-OH and the azomethine (–RC=N–) groups, a thiole group, and/or a sulfur atom participating in coordination. Structural aspects of metal complexes of salicylidene-2-aminothiophenol, salicylidene-3-aminothiophenol, salicylidenethiosemicarbazone, salicylidenedithiocarbazates, salicylidenedithiocarbazates, salicylideneaminopropyleneaminocyclopentenedithiocarboxylates, salicylideneimidazoles, and salicylidene-thiosalicylidene-1,3-propanediamine are reported.     

Structural Features of ONS-Donor Salicylidene <Emphasis Type="Italic">Schiff</Emphasis> Base Complexes

Summary. This review article summarizes the structural features of complexes of salicylidene Schiff bases containing, in addition to the phenolic-OH and the azomethine (–RC=N–) groups, a thiole group, and/or a sulfur atom participating in coordination. Structural aspects of metal complexes of salicylidene-2-aminothiophenol, salicylidene-3-aminothiophenol, salicylidenethiosemicarbazone, salicylidenedithiocarbazates, salicylidenedithiocarbazates, salicylideneaminopropyleneaminocyclopentenedithiocarboxylates, salicylideneimidazoles, and salicylidene-thiosalicylidene-1,3-propanediamine are reported.     

Influence of addition of cobalt oxide on microstructure and electrochemical capacitive performance of nickel oxide

Ni–Co oxide nanocomposite was prepared by thermal decomposition of the precursor obtained via a new method—coordination homogeneous coprecipitation method. The synthesized sample was characterized physically by X-ray diffraction, scanning electron microcopy, energy dispersive spectrum, transmission electron microscope, and Brunauer–Emmett–Teller surface area measurement, respectively. Electrochemical characterization of Ni–Co oxide electrode was examined by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance measurements in 6-mol L⁻¹ KOH aqueous solution electrolyte. The results indicated that the addition of cobalt oxide not only changed the morphology of NiO but also enhance its electrochemical capacitance value. A specific capacitance value of 306 F g⁻¹ of Ni–Co oxide nanocomposite with n _Co = 0.5 (n _Co is the mole fraction of Co with respect to the sum of Co and Ni) was measured at the current density of 0.2 A g⁻¹, nearly 1.5 times greater than that of pure NiO electrode. Lower resistance and better rate capability can also be observed.     

Computational insight into the effect of C(19) substituents on [1,7]-hydrogen shift in previtamin D

 B3LYP calculations in conjunction with natural bond orbital population analysis have been performed for a previtamin D model and corresponding transition structures for the [1,7]-hydrogen migration. In addition the 19,19-difluoro, 19-methoxy and 19-fluoro substituted analogs were investigated. The calculated activation barriers decrease in the following order: CHF₂>CH₃>CH₂OCH₃ (24.8, 23.5 and 20.1 kcal/mol). This is in qualitative agreement with experiments. It has been suggested that a decrease of the barrier by a 19-methoxy substituent and its increase by a 19,19-difluoro substituent are phenomena of different origin. In the case of 19-methoxy substitution, the effect is due to the charge redistribution in the triene system and the decrease of the C(19)–H bond energy. The effect of two fluorine substituents at C-19 on the activation barrier is suggested to originate from the combination and balance of several factors: electrostatic repulsion between the negative fluorine atom and the π-electron cloud over the conjugated system, an increase of the HOMO–LUMO gap, and geminal difluoro substitution affecting C–F and C–C bond energies. Received: 17 May 2002 / Accepted: 11 September 2002 / Published online: 14 February 2003     

Energetic heterogeneity of the surface of a micro/mesoporous zeolite adsorbent

A complex approach based on treatment of calorimetric data using the Guggenheim–Andersen–de Boer and modified Jovanović equations was developed to evaluate the energetic and structural heterogeneity of microporous and mixed micro/mesoporous zeolite adsorbents. The calculated dependence of the hexane adsorption energy distribution function for the Y-US-Ex micro/mesoporous zeolite on the adsorbate pressure is in good accord with the calorimetric curves for the differential heats of adsorption, which indicates the suitability of this theoretical approach.     

A comparative study of the viscosity of ion conducting polymers based on the bond strength-coordination number fluctuation model and other models

According to the bond strength-coordination number fluctuation (BSCNF) model of viscosity proposed some years ago by one of the authors, viscosity is controlled by the relaxation of structural units that form the melt, and is described in terms of the average bond strength, coordination number, and their fluctuations of the structural units. In the present paper, a comparative study of the viscosity of ion conducting polymeric system is presented. The analysis has been done though the BSCNF model and other models widely used in the literature such as Vogel–Fulcher–Tamman and Williams–Landel–Ferry equations. The result reveals that the three models describe well the temperature dependence of the viscosity reported experimentally. As a gross trend, the fragility of the polymeric system in consideration increases with the increase in the concentration of salts. Based on the analysis with the BSCNF model, such a behavior has been interpreted to arise from the disruption of the network that results by the addition of salts. The comparative study also reveals that the BSCNF model could provide a physical interpretation to the empirical parameters used in other models.     

Conformational landscape of platinum(II)-tetraamine complexes: DFT and NBO studies

The potential energy surfaces of chiral tetraamine Pt(II) coordination complexes were computed at the B3LYP/LANL2DZ level of theory by a systematic variation of two dihedral angles: C12–C15–C34–C37 (θ) and C24–C17–C31–C48 (ψ) employing a grid resolution of 30°. Potential energy surfaces calculated using density functional theory methods and Boltzmann-derived populations revealed strong preference for one diasteromer of each series studied. In addition, natural bond orbital analysis show that the minima are stabilized predominantly by a combination of electronic interactions between two phenyl groups, the phenyl groups and the Pt²⁺ ion, as well as with the amine groups. Additional experimental characterization of the diasteroisomers studied here is in progress and will permit further molecular modeling studies with the appropriate stereochemistry.     

Coordination of nitroxyl radical probes to the Lewis acid sites on the surface of gallium oxide: A quantum-chemical analysis

The results of ESR investigations and quantum-chemical studies on the coordination of 2,2,6,6-tetramethylpiperidine-N-oxyl to the Lewis acid sites (LAS) on the surface of gallium oxide are reported and analyzed. The structural, spin, electrostatic, and energy characteristics of the coordination of the simplest nitroxyl radical, H₂NO^., to the Ga(OH)₃ model cluster-type LAS were calculated by the unrestricted Hartree—Fock method in the 6–31G basis set and using the semiempirical PM3 approximation. The properties of gallium-containing nitroxyl surface complexes and their aluminium-containing analogs are compared. According to the results of cluster quantum-chemical calculations, the LAS on the surface of gallium oxide are stronger than those on the alumina surface. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 996–1001, June, 2000.     

Non-isothermal degradation-based solid state kinetics study of copper (II) Schiff base complex,at different heating rates

The coordination complex of Cu (II) with the Schiff base derived from 4-chloroaniline with salicylaldehyde have been synthesized and characterized by micro analytical data; FT–IR, UV–Vis, FAB-mass and thermal analysis studies. Thermal data show degradation of complexes. We carried out thermal analysis at three different heating rates viz. 5, 10 and 20 °C per min. The activation thermodynamic parameters, such as activation energy (E*), entropy of activation (ΔS*), enthalpy of activation (ΔH*) and Gibbs free energy (ΔG*) have been calculated with the help of TG, DTA and DTG curves using Coats–Redfern method. The stoichiometry of the complexes are in 1:2 (M:L) molar ratio. Synthesized complex has been tested for their reactivity and substitution behaviour.     

A theory of spin-Peierls transitions in chains of exchange clusters

A theoretical approach to analysis of magnetic structural phase transitions of chain polymer heterospin complexes is proposed. The approach is based on the model for spin-Peierls phase transition of chain exchange clusters. The type of the phase transition depends on the elastic constant of the chain. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 849–852, April, 2008.     

First principle study of CrF n (n = 1–7) nano clusters: An investigation of superhalogen properties

Quantum chemical calculations using gradient corrected density functional theory at B3LYP level reveals the unusual properties of a chromium (Cr) atom interacting with fluorine (F) atoms. Up to seven F atoms are bound to a single Cr atom, which results in increase of electron affinities as successive fluorine atoms are attached, reaching a peak value of 7.14 eV for CrF₆. The large HOMO–LUMO energy gap, both in neutral and anionic form, further provide evidence of their stability. These unusual properties brought about by involvement of inner shell 3d-electrons, which not only allow CrF _n (n = 1–7) clusters to belong to the class of superhalogens but also show that its valence can exceed the nominal value of 2.     

Non-destructive profiling of fluorine in teeth to large depths

A new method for non-destructive depth profiling of fluorine has been developed which extends the profiling range to much larger depths than hitherto possible. In this method the thick-target yield of 6–7 MeV gammas from the reaction of ¹⁹F(p,αγ) ¹⁶O in the tooth sample, was measured as a function of the incident energy from threshold to up to 2.7 MeV and the yield curve plotted. This curve was compared to the calculated yield curve of 6–7 MeV gammas from the same reaction but on an enamel matrix containing uniformly distributed fluorine. The difference in the shape of the two curves was only due to the non-uniform distribution of fluorine in the tooth sample, which could then be calculated. By making use of this method F-depth profiles in teeth of monkeys were determined non-destructively to a depth of 14 μm. This method is also applicable for profiling other elements through appropriate resonant or non-resonant nuclear reactions.     

Digenite Cu2?xS: thermodynamic analysis of sulfur activities

Abstract  

Gibbs energy modeling of the intermediate copper–sulfur phase digenite is performed at a total pressure of 1 bar using experimental sulfur activity data from the literature. A two-sublattice approach used in the framework of the compound energy formalism takes into account structural considerations. A limited set of optimized model quantities is obtained, which allows reproduction of all sulfur activity data available in the literature over a wide homogeneity and temperature range of the phase. Phase equilibria calculations applying the modeled Gibbs energy for digenite give very satisfactory phase relations in accordance with experimental data. A partial phase diagram within the composition range 0.0 ≤ x _S ≤ 0.7 up to elevated temperatures is calculated.     

Recoordination of a metal ion in the cavity of a crown compound: a theoretical study 2.* Effect of the metal ion— solvent interaction on the conformations of calcium complexes of arylazacrown ethers

The effect of the local interaction of a metal ion with the solvent on the conformations of calcium complexes of arylazacrown ethers and an azacrown-containing dye was studied using the density functional method with the PBE and B3LYP functionals. The structures were studied and the interaction energies were determined for the calcium complexes with n = 1–12 water or acetonitrile molecules. It was found that the inner coordination sphere of the free Ca²⁺ cation contains six H₂O or seven MeCN molecules. The cation—acetonitrile interaction energy is higher than the cation—water interaction energy up to the moment the second solvation shell of the cation is almost complete (n = 11). The inner coordination sphere of Ca²⁺ in the macrocycle cavity contains at most three water molecules, while the fourth one is displaced to the second coordination sphere. Taking into account the local interaction with the solvent (H₂O or MeCN), the conformers of the calcium complexes of arylazacrown ethers and the azacrown-containing dye were studied. It was shown that the presence of two to four water molecules in the coordination sphere of the cation reduces the relative energies of the conformers with broken metal—nitrogen bond, thus favoring ground-state metal recoordination. For Part 1, see Ref. 1. Dedicated to Academician A.L. Buchachenko on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1981–1992, September, 2005.