Stoichiometric Form Research Articles

ChemInform Abstract: Platinum‐Mediated Coupling of Alkynols and α‐Diazoesters. Pseudocatalytic Synthesis of Tetrahydrofuranylidene Esters via Alkyl Migration to a Coordinated Carbene.

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Platinum-Mediated Coupling of Alkynols and α-Diazoesters. Pseudocatalytic Synthesis of Tetrahydrofuranylidene Esters via Alkyl Migration to a Coordinated Carbene

A novel combination of metal-mediated transformations is reported, which together constitute a new organic reaction: the coupling of 3-alkynols and α-diazoesters to yield tetrahydrofuranylidene esters. The process involves the migratory insertion of a coordinated carbene into a metal−alkyl σ-bond and, in stoichiometric form, is efficient and pseudocatalytic, starting and ending with (dppe)PtCl2. The reaction, uniquely, can be made catalytic in platinum, but only poorly so.

Formation of radical oxygen species on hydroxyapatite

Hydroxyapatite of approximately stoichiometric composition ( Ca P molar ratio 1.65) evacuated at 600°C and exposed to O 2 at room temperature produces ESR spectra with g xx = 2.0013, g yy = 2.0102 and g zz = 2.0160, providing evidence for the formation of O − 2. Although the ESR signal attributed to Fs + centers is observable on both nonstoichiometric apatite ( Ca P molar ratio 1.61) and the stoichiometric form, no change in the ESR signal of the former sample takes place after adsorption of oxygen at room temperature. The formation of O − 2 suggests presence of O 2− of low coordination in the stoichiometric form.

INVESTIGATION OF CORROSION AND ION RELEASE FROM TITANIUM DENTAL IMPLANT

A thin passive titanium dioxide, in its stoichiometric form, has a very high corrosion resistance, but the same conclusion can not be made on corrosion resistance of a surface which is not stoichiometrically titanium dioxide, or even a surface which is a composition of various elements and oxides. In practice, the implants available on the market have an oxide surface contaminated with other elements. The aim of this paper is to correlate clinical observations that show the deterioration of Ti made implants after certain period of insertion in the patients, and in vitro corrosion resistance of Ti implants with surface passive oxide layer. For this purpose, surface analysis of the retrieved failed implants were performed and in vivo animal experiments with relation to ion release from implants were done. Finally, on the basis of the clinical observation, in vivo animal test, and in vitro electrochemical corrosion test, a model is proposed to explain the corrosion and ion release from the Ti implant.

Catalytic oxidation of carbon monoxide over stoichiometric and non-stoichiometric hydroxyapatites

Calcium hydroxyapatites [Ca10−z(PO4)6−z(HPO4)z(OH)2−z; (0 ⩽ z ⩽ 1)] catalyze the oxidation of CO at 600 °C. O2− radical oxygen species can be formed on the sample of approximately stoichiometric form (z = 0.10). It is believed that oxygen molecules are activated with O2− produced by the dehydration of the hydroxy groups in the structure. No radical oxygen species can be detected with the nonstoichiometric form (z = 0.34) on which the disproportionation of CO to CO2 and carbon takes place at 600 °C.

High-pressure synthesis and thermal decomposition of LaCuO 3

The high-pressure synthesis of the rhombohedral LaCuO 3 perovskite has been studied and optimized in terms of the applied pressure and temperature, the synthesis time and the amount of the oxidizer. Essentially single phase samples were obtained at temperatures above 1400 °C under an oxygen pressure of 5 GPa. The high oxygen pressures were generated using excess amounts of KClO 4 as an external oxidizing agent. Upon heating under ambient pressure the stoichiometric high-pressure form of LaCuO 3− y loses oxygen yielding various oxygen-deficient phases, and finally decomposes into La 2CuO 4 and CuO or Cu 2O at approximately 800 °C.

Synthesis of various LaCuO 3−y phases by a high-pressure technique and subsequent post-annealing treatments

The formation of the highly oxidized LaCuO 3 perovskite is promoted by high oxygen pressures. In the present contribution the high oxygen pressures were generated in a cubic-anvil-type high-pressure apparatus using excess amounts of KClO 4 as an external oxidizing agent. The high-pressure synthesis of the rhombohedral LaCuO 3 was optimized in terms of the applied pressure and temperature, the synthesis time and the amount of the oxidizing agent. Essentially single-phase samples were obtained at 5 GPa and 1400°C. Upon heating under ambient pressure the stoichiometric high-pressure form of LaCuO 3−y loses oxygen yielding various oxygen-deficient phases as intermediates, and finally decomposes to La 2CuO 4 and CuO or Cu 2O around 800°C. The tetragonal, monoclinic and orthorhombic forms of oxygen-deficient LaCuO 3−y could be isolated as pure phases by annealing in argon at temperatures of 280, 400 and 500°C, respectively.

Studies on oxygen dissociation pressure of LnMnO3 (Ln = rare earth) with the e.m.f. technique

The oxygen dissociation pressure of LnMnO3 (Ln  La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y or Sc) was measured at high temperature (1170–1400 K) by means of e.m.f. using stabilized ZrO2 as a solid electrolyte. Assuming that the decomposition reaction proceeds in a stoichiometric form, a standard Gibbs free energy change of decomposition was determined from the oxygen dissociation pressure. The variation of the free energy change with change in identity of Ln was investigated. It was found that the energy change depends strongly upon both the crystal structure and the ionic radius of Ln3+. The standard Gibbs free energy change of formation for LnMnO3 compounds was obtained from the standard Gibbs free energy change for the decomposition reactions and those for the formation of the decomposition materials.

Effects of fine structure changes of strontium hydroxyapatites on ion-exchange properties with divalent cations

The ion exchange of various strontium hydroxyapatites [SrHAp; stoichiometric form, Sr10(PO4)6(OH)2] of different Sr/P ratios with divalent cations has been investigated and compared with those between the corresponding cations and calcium hydroxyapatites (CaHAp). SrHAp has superior exchange properties for divalent cations in comparison with those of CaHAp and the former solid with Sr/P equal to 1.73 has the highest capacity for ion exchange of the compositions examined. The addition of HCl to the exchanging solution enhanced the exchange of SrHAp with Pb2+, apparently influenced by the formation of strontium and lead chlorapatite during the exchange process. The extended X-ray absorption fine structure (EXAFS) analyses revealed that the ion-exchange characteristics are strongly influenced by the nearest-neighbour distances of the Sr—O bond of SrHAp or the Ca—O bond of CaHAp.

π-Bonded alkene and arene complexes of silver(I): an electrospray mass spectrometric study

Electrospray mass spectra have been observed for a number of alkene and arene complexes of Ag(I) formed by the interaction of AgNO 3 and the organometallic ligand in water/methanol solution. The ES mass spectra show that almost all the alkene and arene ligands in stoichiometric excess form labile 1:2 cationic complexes with Ag(I) which are easily decomposed by collisional activation to the 1:1 species. However, with a deficiency of organic ligand polymeric species are observed. The cation [Ag(cod) 2] + (cod=1,5-cyclooctadiene) was reacted with a variety of other potential ligands, such as PPh 3, AsPh 3, PhSCH 2SPh etc. In most cases, mixed complexes [Ag(cod)(ligand)] + were observed, and excess ligand usually produced [Ag(ligand) 2] +.

The effect of carbon monoxide and methane on the electrical conductivity of TiO2single crystals

Abstract Polycrystalline n-type semiconducting metal oxides such as ZnO and SnO2 have been used in commercial gas-sensing devices for many years, the mode of operation being based on the decrease in surface electrical resistance in the presence of reducing gases. Although the polycrystalline materials are highly sensitive to these gases, a major disadvantage is their lack of selectivity. Studies of single crystals of ZnO and SnO2 have shown that the single crystals are selective to carbon monoxide (CO) over methane (CH4). In this paper we describe the results of electrical studies performed on single crystals of TiO2 in the presence of gas mixtures. It is found that these crystals are also highly selective to CO in air but only in their stoichiometric form and the effect is to decrease the conductivity. The results can be explained in terms of the crystals behaving as extrinsic p-type semiconductors at the temperatures where they respond to the gases.

Spironolactone-cyclodextrin complexes: phase solubility and ultrafiltration studies

Phase-solubility and ultrafiltration techniques, coupled with UV spectrophotometric analyses, have been employed to demonstrate the formation and to determine the stoichiometry of soluble and solid-state complexes in spironolactone : cyclodextrin systems. α-, β- and -γ-cyclodextrins (α, β, γ-CDX) all formed complexes with spironolactone in aqueous solution with apparent mole ratios of 1:1 for the α- and β-CDX complexes. The γ-CDX system formed more than one stoichiometric form in solution. Apparent stability constants, K, reflecting the spatial compatibility of guest molecules, show the β-CDX : spironolactone soluble complex to be the most stable. Solid-state complexes of mole ratio 3:1 and 2 : l CDX : spironolactone were formed for the β- and γ-CDX systems, respectively, and these gave 8- and 17-fold corresponding increases in aqueous solubility for spironolactone.

Photoemission and low-energy-electron-diffraction study of clean and oxygen-dosed Cu2O (111) and (100) surfaces.

The geometric and electronic structure of clean and oxygen-dosed ${\mathrm{Cu}}_{2}$O single-crystal surfaces was studied with x-ray and ultraviolet photoelectron (UPS) spectroscopies and low-energy electron diffraction. The nonpolar (111) surface can be prepared in a nearly stoichiometric (1\ifmmode\times\else\texttimes\fi{}1) form by ion bombardment and annealing in vacuum. Oxygen adsorbs molecularly on the stoichiometric (111) surface at 300 K, but adsorbs dissociatively on a defective (111) surface prepared by ion bombardment. For the polar ${\mathrm{Cu}}_{2}$O(100) face it was possible to prepare a reconstructed, Cu-terminated surface with a (3\ensuremath{\surd}2 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 )R45\ifmmode^\circ\else\textdegree\fi{} periodicity by ion bombardment and annealing in vacuum. Preparation of an unreconstructed, (1\ifmmode\times\else\texttimes\fi{}1), O-terminated (100) surface was possible by large (${10}^{9}$-L) oxygen exposures. UPS investigations of the O-terminated (100) surface suggest a mixture of incorporated (i.e., lattice) oxygen and adsorbed atomic oxygen (i.e., adatoms) in the terminating layer. The annealing behavior of the ${\mathrm{Cu}}_{2}$O(100) surface was history dependent. Early in the sample history, bulk lattice oxygen diffused to the surface at temperatures above 800 K giving domains of (\ensuremath{\surd}2 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 )R45\ifmmode^\circ\else\textdegree\fi{} periodicity associated with half a terminating layer of oxygen atoms. After repeated ion bombardment and annealing cycles, heating above 800 K gave only a Cu-terminated surface, apparently because of a depletion of bulk lattice oxygen.

Irreversible Changes Occur in Chromatin Structure Upon Dissociation Of Histone H1

The role of histone H1 in the actual interactions bringing about chromatin folding is investigated by studying the reversibility of its dissociation. H1 was dissociated by increase of the NaCl concentration and reassociated by dialysis, without removal from the dialysis bag. To scrutinize the fidelity of this stoichiometric form of chromatin reconstitution, we use circular dichroism, nuclease digestion, thermal denaturation and the sensitive electric birefringence method. No alteration of the repeat length and no nucleosomal sliding are observed upon the reassociation procedure. However, under all the different conditions investigated, the original value of the positive electric birefringence is never recovered, indicating an irreversible change of structure. CD and melting profiles confirm that DNA-protein interactions are modified, and orientational relaxation time measurements indicate that these structural perturbations affect the salt-induced transition of polynucleosomal fibers. The striking conclusion of these studies is that variations of ionic concentration are sufficient to induce irreversible structural alterations affecting the higher-order folding of chromatin. It is of interest that the only sample which exhibits behavior upon reassociation comparable to that of native chromatin is the one which experienced the fastest salt transitions. We suggest that these conformational changes arise from the unbinding to DNA of certain basic tails of histone(s), and that a competition for DNA binding locations exists upon the reassociation. These results are then additional arguments (Mazen, A., Hacques, M.F. and Marion, C.,J. Mol. Biol. 194, 741-745 (1987)), to suggest that dissociation of H1 might modify a direct interaction between basic tails of core histones and H1.

Nonstoichiometry effects on microstructure in amorphous CdGeAs2

The mechanism of nucleation and growth in amorphous semiconducting CdGeAs2has been demonstrated with the aid of thermal analysis, TEM, and XRD to be: 1) germanium precursor formation, 2) nonstoichiometric chalcopyrite structure crystal growth from the germanium precursor, and 3) solid state transformation of the precursor to the chalcopyrite structure. This crystallization sequence does not follow the pseudo-binary phase diagram which indicates that only in the case of germanium content in excess of stoichiometric CdGeAs2shouldαsolid solution form in equilibrium with chalcopyrite structured CdGeAs2(see Fig. 1). To establish the extent of the germanium precursor nucleation mechanism, samples of deficient (CdGe0.8As2), stoichiometric, and excess (CdGe1.2As2) germanium content were prepared in the bulk amorphous form.Samples were fabricated by water-quenching the molten compound, vacuum sealed in carbon coated (by pyrolysis) fused silica tubes. The amorphous ampoules were broken out of the tubes, sliced to fit the hermetically sealing sample capsule for the differential scanning calorimeter (DSC) and heat treated at 0.5°C/min to various points along the exotherms. The partially devitrified samples were mechanically thinned and ion-milled using a liquid nitrogen cold stage, for examination in the TEM.

Raman study of the formation of transition alumina single crystal from protonic ?/?? aluminas

Transition aluminas are usually obtained by thermal treatment of aluminium hydroxides between 450 and 1000 ° C. X-ray diffraction [1] but also IR spectroscopy [2] have been used to characterize the various phases (7, 0, &, x . . . ) occurring before crystallization in corundum (e-alumina). However, samples prepared this way are in the form of small crystallites and, aluminium oxides being poor scatterers, the use of Raman scattering was not successful. In previous work [3, 4], we have shown by X-ray diffraction and electron paramagnetic resonance how a protonic /3 (or /3") alumina single crystal can be transformed by heating into an e-alumina oriented polycrystal through the formation of a transition alumina single crystal. To our knowledge, this is the unique method to prepare easily large single crystals (a few tens of mm 2) of transition aluminas. These crystals are suitable for special studies, such as those of surface properties. The structure of/?-alumina consists in spinel blocks separated by a loosely packed plane where conducting iotas diffuse rapidly. The bridging between spinel blocks is made by two tetrahedra sharing the same oxygen ion. The difference between/3 and/?" phases resides in the nature of the conducting plane: only in the /3 phase is there a mirror plane from a crystallographic point of view. A protonic/?-alumina single crystal with composition equal to 1.25M20 • 11A1203 (M + = NH4 + or H+(H20),) is transformed above 400°C into a stoichiometric/?-alumina single crystal (composition close to (I-I30)20 • 1 IA1203). This transformation takes place from the reaction of interstitial oxygens ensuring the charge compensation mechanism, with the protons liberated by the desolvation of protonic ions [5-7]. Further heating above 800°C involves the diffusion of the remaining protons in the spinel blocks where they are trapped, Stacking faults are created by the collapse of some loosely packed conducting planes in between the spinel blocks. The samples remain in the form of single crystals; the /3-alumina unit cell is multiplied by three or even six along the a direction of the hexagonal plane (Fig. 1), whereas complex superstructures and diffusions are observed along the c direction [7]. Finally a topotactic growth of e-alumina crystallites is observed above 1100 ° C [4, 7, 8]. Similar transformations are observed for ion-rich /?and fl"-alumina single crystals with composition close to Allo.33Mgo.66017M1.66 (M = NH2, H+(H20)~). However, due to the new charge compensation mechanism (Mg2+/A13+ ion substitution) the stoichiometric form is not obtained and stacking faults occur at lower temperature with a drastic collapse of spinel blocks, above 320 and 400°C for ion-rich/3 and/3" phase, respectively [9]. A transition alumina phase with spinel-like structure is formed. Above 670°C syntactic nucleation of nonstoichiometric spinel occurs [9, 10]. Finally, separation into e-alumina and MgAI204 spinel is observed above 1000°C [10]. This quasi-continuous evolution of relatively well-ordered spinel-like structures into disordered ones provides a good opportunity to state the vibrational fingerprinting evolution of spinel-like materials. However, such structures are found for many solid-state reactions in ceramics (e.g. mullite

Chemistry of polar intermetallic compounds. Study of two zirconium-antimony (Zr5Sb3) phases, hosts for a diverse interstitial chemistry

A variety of synthesis routes, some phase properties, and two single-crystal x-ray studies have established something of the nature of the Zr-Sb system in the neighborhood of 65 atom % zirconium. Samples prepared by vapor-phase transport or by annealing either elemental powders or arc-melted products at 1000-1100/degree/C yield the Mn/sub 5/Si/sub 3/ form of Zr/sub 5/Sb/sub 3+x/. This is true binary phase that is nonstoichiometric over the range of 0.0/sub 0/ /le/ x /le/ 0.4/sub 2/. Study of a single crystal with x = 0.16 (1) from an arc-melted sample has established that the extra antimony in this structure is bonded in an interstitial site in the center of a chain of confacial zirconium octahedra (space group P6/sub 3//mcm, Z = 2, a = 8.4267 (5) /angstrom/, c = 5.7856 (6) /angstrom/, R = 2.7%, R/sub w/ = 3.4%). Many other atoms may also be bound at this site. Binary Zr-Sb samples in the range 0 /le/ x /le/ /approx/ 0.2 melt incongruently to form the foregoing phase plus a second, new, high-temperature, stoichiometric form of Zr/sub 5/Sb/sub 3/ in the Y/sub 5/Bi/sub 3/ structure (space group Pnma, Z = 4, a = 7.465 (1) /angstrom/, b = 8.801 (1)more » /angstrom/, c = 10.865 (2) /angstrom/, R = 2.8%, R/sub w/ = 2.9%). The two structure types are further described and compared. 22 references, 6 figures, 5 tables.« less

Infrared spectra of species adsorbed on dispersed systems

The applications of IR Spectroscopy to the investigation of the species formed upon chemisorption at the surface of dispersed solids are briefly discussed. In particular two examples are described in more detail i.e: the surface proper ties of dispersed α-Cr 2O 3, (both under stoichiometric and reduced form) and copper dispersed on silica. Both examples are utilized for discussing a few problems concerning the metal-support interaction.

A Program for Fitting Rate Constants in Gas Phase Chemical Kinetics Models

This paper describes a program for developing gas phase chemical kinetics mechanisms. Program input is a proposed chemical reaction mechanism specified in the usual stoichiometric form and associated thermodynamic data. The program generates and solves the corresponding rate equations and adjusts reaction rate constants to fit a user-provided data base of experimental values of concentrations of certain chemical species. Examples are given.

Normal coordinate analysis of NS β Al 2O 3

Preliminary results on normal coordinate analysis of non stoichiometric Na + and Tl + β aluminates are presented. Different kindsof cation configurations have been tested. The compensating O = i oxygen atoms and the Al F atoms associated with the Frenkel defect were considered in our calculations. The calculated eigenvectors of the Non stoichiometric compounds were projected on those ofthe Stoichiometric form in order to appreciate the role played by O i, Al F and the excess conducting cations on the spectra.