Biblioteca Digital

Synthesis, characterization, and thermal behavior of transition metal oxamates, M(NH(2)C(2)O(3))(2)center dot nH(2)O (M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II)), as well as the thermal behavior of oxamic acid and its sodium salt (NaNH(2)C(2)O(3)) were investigated employing simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC), experimental and theoretical infrared spectroscopy, TG-DSC coupled to FTIR, elemental analysis and complexometry. The results led to information about the composition, dehydration, thermal stability, thermal decomposition, as well as of the gaseous products evolved during the thermal decomposition of these compounds in dynamic air and N(2) atmospheres.

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Solid-state M(L)(2)center dot nH(2)O compounds, where M stands for bivalent transition metals (Mn, Fe, Co, Ni, Cu and Zn), L is nicotinate and n = 0-4.5, have been synthesized. Characterization and thermal behaviour of these compounds were investigated employing elemental analysis based on the mass losses observed in the TG-DTA curves, complexometry, X-ray diffractometry, infrared spectroscopy (FTIR), simultaneous thermogravimetric and differential thermal analysis (TG-DTA) and TG-DSC coupled to FTIR. The thermal behaviour of nicotinic acid and its sodium salt was also investigated. For the hydrated transition metal compounds, the dehydration and thermal decomposition of the anhydrous compounds occur in a single step. For the sodium nicotinate, the final residue up to 765 degrees C is sodium carbonate and for the transition metal nicotinates, the final residues are Mn3O4, Fe2O3, Co3O4, NiO, CuO and ZnO. The results also provided information concerning the thermal stability, thermal decomposition and identification of the gaseous products evolved during the thermal decomposition of the compounds. (C) 2013 Published by Elsevier B.V.

Toluene is a very noxious organic compound used for the preparation of many compounds. Liquid phase photo-oxidation of toluene was carried out using poly crystalline samples of transition metal oxide doped TiO2 containing different phases of titania, namely anatase, rutile and mixture of anatase and rutile. The main objective was to find out the conversion of toluene to benzoic acid and how the phase modification of doped TiO2 affects the reaction rate. The different phases of TiO2 were determined using powder X-ray diffraction. Other methods such as chemical analysis, Surface area measurements and Crystallite size calculation were also used for the characterization of the samples. Anatase doped with transition metal oxides gives higher yield than doped rutile. Mixture of anatase and rutile has no significant effect.

Two transition-metal compounds derived from 2,4-dinitroimidazole, {[Ni(DNI)2(H2O)3][Ni(DNI)2 (H2O)4]}·6H2O, 1, and Pb(DNI)2(H2O)4, 2, were characterized by elemental analysis, FT-IR, TG-DSC and X-ray single-crystal diffraction analysis. Crystal data for 1: monoclinic, space group C2/c, a = 26.826(3), b = 7.7199(10), c = 18.579(2) Å, β = 111.241(2)° and Z = 4; 2: monoclinic, space group C2/c, a = 6.5347(6), b = 17.1727(17), c = 14.1011(14) Å, β = 97.7248(10) and Z = 4. Compound 1 contains two isolated nickel centers in its structure, one being six-coordinate and another five-coordinate. The structure of 2 contains a lead (II) center surrounded by two chelating DNI ligands and four water molecules in distorted square-antiprism geometry. The abundant hydrogen bonds in two compounds link the molecules into three-dimensional network and stabilize the molecules. The TG-DSC analysis reveals that the first step is the loss of water molecules and the final residue is the corresponding metal oxides and carbon.

Thesis (Ph.D.1979) from the Dept. of Physical and Inorganic Chemistry, University of Adelaide; Title on spine: Transition metal complexes; 173 leaves : ill., tables, graphs ; 30 cm.; Title page, contents and abstract only. The complete thesis in print form is available from the University Library.

Die vorliegende Arbeit untersucht binäre Übergangsmetalloxide. Im Speziellen werden sowohl ultradünne und epitaktische Manganoxid-Schichten (MnO) auf einkristallinen Silbersubstraten als auch Grenzflächen zwischen Übergangsmetallen und Oxiden betrachtet. Strukturelle, elektronische und magnetische Eigenschaften von Übergangsmetalloxiden in dünnen Schichtsystemen sind von grundlegendem Interesse. Für Anwendungen im Bereich der magnetischen Datenspeicherung oder in der Entwicklung von Spintronik-Bauteilen ist vor allem Nickeloxid (NiO) ein viel untersuchtes Oxid. Die binären Übergangsmetalloxide dienen dabei auch als Referenzsysteme für das Verständnis von komplexeren Oxiden. Wichtige Fragestellungen sind dabei das Ausmaß von Grenzflächenreaktionen im Schichtsystem sowie die Veränderung der Eigenschaften durch die abnehmenden Dimensionen und durch die auftretenden Grenzflächenspannungen. Zur Klärung dieser Fragestellungen wurden Photoelektronenspektroskopie (PES, XPS), Photoelektronenbeugung (PED, XPD), polarisationsabhängige Röntgenabsorptionsspektroskopie (XAS, NEXAFS), Elektronenbeugung (LEED) und theoretische Methoden eingesetzt.; Binary transition metal oxides are studied in this thesis. Ultrathin and epitaxial manganese oxide layers (MnO) on silver single crystals as well as interfaces of transition metals and oxides are investigated. Structural...

The increased availability of synchrotron x-ray sources has facilitated the development of new diffraction techniques based on x-ray resonant exchange scattering (XRES). Resonant electric dipole (E1) and quadrupole (E2) scattering involving virtual transitions between occupied core and empty valence levels is sensitive to the splitting and filling of the valence shells, thus providing information about the spin and orbital distributions of the valence electrons. This sensitivity to electronic properties makes x-ray resonant exchange scattering (XRES) a useful probe of rare earth magnetism as well as effects due to crystal fields (or molecular orbitals) in transition metal compounds. In spiral antiferromagnets, such as holmium, the magnetic sensitivity results in a series of off-Bragg magnetic diffraction peaks. Resonant scattering calculations provide good predictions of the experimental observations, including diffraction conditions, intensities, lineshapes, and polarization dependence. Coefficients giving the magnitude of the scattering are computed for the rare earths. These are then related to effective scattering operators which can be expressed in terms of the angular momentum J in rare earths, making it possible to extend the results of calculations at zero temperature and zero crystal field to finite temperatures and crystal fields. Resonant scattering from transition metal ions in crystal fields includes contributions sensitive to the chemical environment of the ion. The amplitude for E1 resonances exhibits 1- and 2-fold azimuthal patterns in C$sb1$ and C$sb2$ symmetries respectively...

Kohlenhydratderivate sind als Vertreter des sog. ''chiral pools'' nicht nur Ausgangsstoffe für die Synthese chiraler Verbindungen, sondern finden in den letzten Jahren gehäuft Anwendung als chirale Liganden in der Chemie der Übergangsmetalle. Seit ihrer Entdeckung durch Fischer und Öfele in den 1950er Jahren haben Aren-Chromtricarbonylkomplexe eine breite Anwendung in der organischen Synthese gefunden. Die Komplexierung mit einem Chromtricarbonylfragment erlaubt Reaktionen, wie beispielsweise die nucleophile aromatische Substitution oder die Deprotonierung in benzylischer Position, die ohne die Komplexierung nicht oder nur sehr schwer möglich wären. Ausgenutzt wurde dies zur Synthese von diversen Naturstoffen, insbesondere solchen mit einem hochsubstituierten Benzol- oder Cyclohexangrundgerüst. Ziel dieser Arbeit war die Synthese neuartiger kohlenhydratsubstituierter Übergangsmetallkomplexe. Dabei wurden drei verschiedene Systeme als Liganden für Übergangsmetalle verwendet: Zur spezifischen Komplexierung mit Cr(CO)3 wurden verschiedene Phenylglycoside zu den entsprechenden kohlenhydratsubstituierten Aren-Chromtricarbonyl-Komplexen umgesetzt, welche meist kristalline Verbindungen darstellen, die sich zur Einkristallröntgenstrukturanalyse eigneten. In diesen Kristallstrukturen wurden zahlreiche sog. nicht-klassische Wasserstoffbrückenbindungen zwischen den Sauerstoffatomen der Carbonyleinheiten am Chrom und Wasserstoffatomen sowohl von benachbarten Molekülen als auch innerhalb desselben Moleküls gefunden. Auch konnte in einer Kristallstruktur eine Wasserstoffbrücke zwischen einem Ringsauerstoffatom eines Kohlenhydratrestes und einem Wasserstoffatom eines benachbarten komplexierten Aromaten nachgewiesen werden. Ausgehend vom Glucosamin wurden zweizähnige Liganden synthetisiert...

Oxide/fluoride ordering in three families of 'disordered' transition metal oxyfluoride compounds is carefully investigated and used to obtain insight into why the constituent inherently acentric octahedral units do not usually order in a long range ordere

We analyze the role of local geometry in the spin and orbital interaction in transition metal compounds with orbital degeneracy. We stress that the tendency observed for the most studied case (transition metals in O$_6$ octahedra with one common oxygen -- common corner of neighboring octahedra and with $\sim 180^{\circ}$ metal--oxygen--metal bonds), that ferro-orbital ordering renders antiferro-spin coupling, and, {\it vice versa}, antiferro-orbitals give ferro-spin ordering, is not valid in general case, in particular for octahedra with common edge and with $\sim 90^{\circ}$ M--O--M bonds. Special attention is paid to the ``third case'', neighboring octahedra with common face (three common oxygens) -- the case practically not considered until now, although there are many real systems with this geometry. Interestingly enough, the spin--orbital exchange in this case turns out to be to be simpler and more symmetric than in the first two cases. We also consider, which form the effective exchange takes for different geometries in case of strong spin--orbit coupling.; Comment: 31 pages, 9 figures, submitted to JETP

First-principles calculations within the Local Density Approximation (LDA) or Generalized Gradient Approximation (GGA), though very successful, are known to underestimate redox potentials, such as those at which lithium intercalates in transition metal compounds. We argue that this inaccuracy is related to the lack of cancellation of electron self-interaction errors in LDA/GGA and can be improved by using the DFT+$U$ method with a self-consistent evaluation of the $U$ parameter. We show that, using this approach, the experimental lithium intercalation voltages of a number of transition metal compounds, including the olivine Li$_{x}$MPO$_{4}$ (M=Mn, Fe Co, Ni), layered Li$_{x}$MO$_{2}$ ($x=$Co, Ni) and spinel-like Li$_{x}$M$_{2}$O$_{4}$ (M=Mn, Co), can be reproduced accurately.; Comment: 19 pages, 6 figures, Phys. Rev. B 70, 235121 (2004)

We have studied the effects of electron correlation on Van Vleck susceptibility ($\chi_{\rm{VV}}$) in transition metal compounds. A typical crossover behavior is found for the correlation effect on $\chi_{\rm{VV}}$ as sweeping spin-orbit interaction, $\lambda$. For a small $\lambda$, orbital fluctuation plays a dominant role in the correlation enhancement of $\chi_{\rm{VV}}$; however, the enhancement rate is rather small. In contrast, for an intermediate $\lambda$, $\chi_{\rm{VV}}$ shows a substantial increase, accompanied by the development of spin fluctuation. We will discuss the behavior of $\chi_{\rm{VV}}$ in association with the results of Knight-shift experiments on Sr$_2$RuO$_4$ and an anomalously large magnetic susceptibility observed for $5d$ Ir compounds.; Comment: 5 pages, 3 figures, to appear in J. Phys. Soc. Jpn

We study the classical version of the 120-degree model. This is an attractive nearest-neighbor system in three dimensions with XY (rotor) spins and interaction such that only a particular projection of the spins gets coupled in each coordinate direction. Although the Hamiltonian has only discrete symmetries, it turns out that every constant field is a ground state. Employing a combination of spin-wave and contour arguments we establish the existence of long-range order at low temperatures. This suggests a mechanism for a type of ordering in certain models of transition-metal compounds where the very existence of long-range order has heretofore been a matter of some controversy.; Comment: 40 pages, 1 eps fig; a revised version correcting a bunch of small errors

We study the classical 120-degree and related orbital models. These are the classical limits of quantum models which describe the interactions among orbitals of transition-metal compounds. We demonstrate that at low temperatures these models exhibit a long-range order which arises via an "order by disorder" mechanism. This strongly indicates that there is orbital ordering in the quantum version of these models, notwithstanding recent rigorous results on the absence of spin order in these systems.; Comment: 7 pages, 1 eps fig

We found that magnetic ground state of one-dimensional atomic chains of carbon-transition metal compounds exhibit half-metallic properties. They are semiconductors for one spin-direction, but show metallic properties for the opposite direction. The spins are fully polarized at the Fermi level and net magnetic moment per unit cell is an integer multiple of Bohr magneton. The spin-dependent electronic structure can be engineered by changing the number of carbon and type of transition metal atoms. These chains, which are stable even at high temperature and some of which keep their spin-dependent electronic properties even under moderate axial strain, hold the promise of potential applications in nanospintronics.; Comment: 11 pages, 3 figures, 1 tables

We show that in transition metal compounds containing structural metal dimers there may exist in the presence of different orbitals a special state with partial formation of singlets by electrons on one orbital, while others are effectively decoupled and may give e.g. long-range magnetic order or stay paramagnetic. Similar situation can be realized in dimers spontaneously formed at structural phase transitions, which can be called orbital-selective Peierls transition. This can occur in case of strongly nonuniform hopping integrals for different orbitals and small intra-atomic Hund's rule coupling JH. Yet another consequence of this picture is that for odd number of electrons per dimer there exist competition between double exchange mechanism of ferromagnetism, and the formation of singlet dimer by electron on one orbital, with remaining electrons giving a net spin of a dimer. The first case is realized for strong Hund's rule coupling, typical for 3d compounds, whereas the second is more plausible for 4d-5d compounds. We discuss some implications of these phenomena, and consider examples of real systems, in which orbital-selective phase seems to be realized.; Comment: to be published in PRB Rapid Com

3 pages, 3 figures.-- X-Ray Absorption Fine Structure-XAFS13: 13th International Conference.-- et al.; We report on a transition metal contribution to the rare-earth L2-edge XMCD spectrum of intermetallic rare-earth - transition metal compounds (R-T). The shape and location of this contribution is independent on both the specific rare-earth and the specific 3d metal in the compound. The evolution of the signal with the 3d content indicates that the signal is proportional to the number of T neighboring atoms around the absorbing R atom. In addition, comparison between Fe and Co Laves compounds indicates that the signal also resembles the magnetic moment of the T metal. Comparison with other R:T series shows that the presence of a T contribution is not a particularity of Laves phases but a general result.; This work has been partially financed by the Spanish CICYT research projects MAT2005-06806- C04-04. M.A. Laguna-Marco acknowledges MEC for Ph.D. grant.; Peer reviewed

Homoleptic derivatives of formula [M(C(6)X(5))(n)](z-) (X = F, Cl) have been prepared and isolated for every first-row transition metal as well as for several of the heavier ones. The stoichiometry attained in each case (n ranging between 2 and 6) can be understood considering the tendency of a given metal ion to compensate its coordinative and electronic unsaturation, while not incurring severe interligand repulsive effects. The molecular structures associated with each stoichiometry seem, in turn, to be governed by electronic rather than steric factors. Most of these [M(C(6)X(5))(n)](z-) compounds are unsaturated, open-shell organometallic species, not fulfilling the 18-electron (or Effective Atomic Number) rule. This behaviour can be attributed to the absence of pi stabilising ligands (such as CO, phosphines, alkenes, alkynes, a variety of substituted aromatic rings, and so on) which are otherwise ubiquitous in organotransition-metal chemistry. The magnetic properties of the [M(C(6)X(5))(n)](z-) species have been determined by EPR spectroscopy and/or bulk magnetisation measurements. Excellent correlation between molecular geometry and magnetic properties (whether diamagnetic or paramagnetic) has been observed. Many of these compounds undergo chemically- or electrochemically-induced electron exchange processes. These redox reactions proceed without alteration in the stoichiometry of the [M(C(6)X(5))(n)](z-) compound...

High resolution experimental data for resonant soft x-ray scattering of transition metal compounds are shown. The compounds studied are the ionic transition metal di-fluorides, ionic and covalent orthovanadates and members of the La1-xSr xCoO3 perovskite family. In all compounds we studied the transition metal L2,3 edge and also the ligand (oxygen or fluorine) K edge. For the ionic compounds the transition metal data are in good agreement with atomic multiplet ligand field calculations that include charge transfer effects. Density functional calculations give very useful information to interpret the ligand x-ray emission data. The experimental metal Lα emission data show that the region between valence and conduction bands in the di-fluorides has several d-excited states. At the L2edge of the ionic orthovanadates we found the signature of a fast Coster-Kronig decay process that results in a very localized emission peak. Changes in the oxidation sate in the La1-xSr xCoO3 compounds are observed at both the metal L2,3 edge and the oxygen K edge absorption spectra.

We present results of x-ray absorption and x-ray emission spectroscopies of transition metal oxides and fluorides. The absorption is studied in the vicinity of the transition metal L edge. At selected values of the excitation energy we also show emission spectra. The data are interpreted in terms of the multiplet structure of the transition metal ion in a crystal field, considering also configuration interaction effects in the solid such as charge transfer. The data are compared with results of free-ion calculations that allow a direct interpretation of the absorption spectra. The free-ion calculations also indicate that some of the main emission features correspond to d to d excitations in the compound, with the emission peaks with higher energy losses resulting from production of charge transfer states