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An electron energy loss spectroscopic study of the formic acid dimer has shown bands centred around 7.2, 8.5, 9.8, and 11.1 eV, of which the first and the third bands are assigned to n–π* transitions and the other two to π–π* transitions; similar transitions are found in the acetic acid dimer.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
No abstract is provided for this article.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
With a view to investigate the effect of small size of A-site cations on the magnetic properties of the rare earth cobaltates, Ln 1− xAx CoO3 (Ln=rare earth, A=alkaline earth), we have investigated Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3 in detail. For this purpose, we have carried out low-field DC magnetization and ac susceptibility measurements including a study of magnetic relaxation and memory effects. Both Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3 show frequency-dependent transitions at 70 and 55K respectively in the ac susceptibility data, due to the onset of spin-glass like behavior. Their relaxation behavior exhibits aging effects. In addition, memory effects are found in the magnetization behavior. These characteristics establish spin-glass behavior in both these cobaltates, a behavior that is distinctly different from that of La0.7Ca0.3CoO3 and La0.5Sr0.5CoO3 which show well-defined ferromagnetic transitions, albeit without long-range ordering.
No abstract is provided for this article.
Chemical methods of synthesis play a crucial role in designing and discovering new inorganic materials and also in providing better and less cumbersome methods for preparing known materials. We discuss here the various methods along with several examples, paying special attention to soft chemistry routes. Besides the conventional ceramic method, the methods examined are the precursor method, topochemical reactions, intercalation reactions, ion-exchange method, alkali flux method and electrochemical methods. We discuss cuprate superconductors as a case study in chemical synthesis.
An aspect of metal oxides, colossal magnetoresistance exhibited by certain manganese oxides, in particular rare-earth manganates of perovskite structure, has received much attention in recent years. Some of these oxides show 100 per cent magnetoresistance, and have potential for technological applications. This text begins with a review of the topics of colossal magnetoresistance, charge ordering and related phenomena exhibited by oxides, and presents contributions covering the present status of the subject.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
The structure, packing, and charge distribution in molecules of nonlinear optical materials have been analysed with reference to their counterparts in centrosymmetric structures based on low temperature X-ray measurements. The systems studied are the centric and noncentric polymorphs of 5-nitrouracil as well as the diamino, dithio, and thioamino derivatives of 1,1-ethylenedicarbonitrile; the latter possesses a noncentric structure. The molecular structure of 5-nitrouracil is invariant between the two forms, while the crystal packing is considerably different, leading to dimeric N-H·∙∙O rings in the centric polymorph and linear chains in noncentric one. There is an additional C-H·∙∙O contact in the centric form with a significant overlap of the electrostatic potentials between the alkenyl hydrogen atom and an oxygen atom of the nitro group. The dipole moment of 5-nitrouracil in the noncentric form is much higher (μ=9 D) than in the centric form (≈6 D). Among the 1,1-ethylenedicarbonitriles, there is an increased charge separation in the noncentric thioamino derivative, leading to an enhanced dipole of 15 D compared to the centric diamino (5 D) and dithio (6 D) derivatives. The effect of the crystal field is borne out by semiempirical AM1 calculations on the two systems. Dipole moments calculated for the molecules in the frozen geometries match closely with those obtained for centric crystals from the experimental charge densities. The calculated values of the dipole moment in the frozen or optimized geometries in the noncentric structures are, however, considerably lower than the observed value. Furthermore, the conformation of the S-CH(3) group in the noncentric crystal is anti with respect to the central C=C bond while the syn conformation is predicted for the free molecule in the optimized geometry.
No abstract is provided for this article.
No abstract is provided for this article.
Chemistry of high-temperature oxide superconductors The role of crystal chemistry for generation of high-Twc oxide superconductors Ba(Pb,Bi)Ow3 superconductors and their relationship to the copper oxide based superconductors Chemistry and high-Twc superconductivity in the La-Ba-Cu-O system Synthesis, structural chemistry and properties of YBAw2Cuw3Ow7-x High-Twc superconductivity in LnBaw2Cuw3Ow7 (Ln = La-Yb, Y) compounds Chemistry-Twc relations in oxide superconductors Thermal decomposition studies of YBAw2CUw3Ow7-x using beam modulation mass spectrometry Kinetics of oxygen uptake in YBaw2Cuw3 Owx The role of oxygen in YBaw2Cuw3Ow7-(SY) Are one-dimensional structural features essential for superconductivity at 90 K Isoelectronic analogs of the high temperature oxide superconductor Substitutional chemistry of the 1:2:3 phase and studies of the Yw2O w3-SrO-CuO phase diagram Theory of superconductivity in systems with strong coulomb correlations, with applications to the organic and oxide superconductors Raman scattering studies of the copper-oxygen vibrations and oxygen order in La, Y-Ba, Sr-Cu oxides A simple method to prepare high-Twc superconducting thin films: pulsed laser deposition High-temperature superconductivity in bismuth and thallium cuprates.
Some interesting aspects of the structure and stoichiometry of YBa 2 Cu 3 O 7+δ and other high- T c oxide superconductors are discussed. The nature of Cu and the importance of the Cu 3+ /Cu 2+ ratio in these copper oxides are pointed out. Marginal metallicity of the non-superconducting phase in these oxides as an important factor in the mechanism is indicated. Effect of Y substitution in YBa 2 Cu 3 O 7+δ by other rare earths is commented upon.
Carbon monoxide adsorbed on Al-promoted Ni and Cu surfaces undergoes facile dissociation at 300 K or below. CO adsorbed on an Al-promoted Ni surface shows a larger separation (3.5 eV) between the 5σ and 4σ levels compared to that on a clean Ni surface (3.0 eV), besides exhibiting a low C-O stretching frequency. CO adsorbed on an Al-promoted Cu surface shows a similar large 5σ-4σ separation; this is accompanied by the disappearance of the satellites in the valence band and O(1s) regions. These features of CO adsorbed on Al-promoted Ni and Cu surfaces characterize the precursor to dissociation. Molecular orbital calculations suggest that CO bonds parallel to the Al-promnoted Cu surface.
