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By heating imidazolium bromide-based ionic liquids with lead(II) salts under ionothermal conditions, we have been able to obtain two imidazolium bromoplumbates, (EMIm)PbBr3, I, and (BMIm)2PbBr4, II, where EMIm and BMIm stand for 1-ethyl-3-methyl and 1-butyl-3-methyl imidazolium cations respectively. Interestingly, both these compounds exhibit unusual supramolecular organization wherein dialkylimidazolium cations arrange themselves in a cylindrical fashion giving rise to channel structures. The bromoplumbate anions reside in the channels. The lead(II) cation exhibits either hemi- or holodirected coordination geometry.
Effects of non-polar, polar and proton-donating solvents on the n → π* transitions of CO, CS, NO2 and NN groups have been investigated. The shifts of the absorption maxima in non-polar and polar solvents have been related to the electrostatic interactions between solute and solvent molecules, by employing the theory of McRAE. In solvents which can donate protons the solvent shifts are mainly determined by solute-solvent hydrogen bonding. Isobestic points have been found in the n → π∗ bonds of ethylenetrithio-carbonate in heptane-alcohol and heptane-chloroform solvent systems, indicating the existence of equilibria between the hydrogen bonded and the free species of the solute. Among the different proton-donating solvents studied water produces the largest blue-shifts. The blue-shifts in alcohols decrease in the order 2,2,2-trifluoroethanol, methanol, ethanol, isopropanol and t-butanol, the blue-shift in trifluoroethanol being nearly equal to that in water. This trend is exactly opposite to that for the self-association of alcohols. It is suggested that electron-withdrawing groups not merely decrease the extent of self-association of alcohols, but also increase the ability to donate hydrogen bonds. The approximate hydrogen-bond energies for several donor-acceptor systems have been estimated. In a series of aliphatio ketones and nitro compounds studied, the blue-shifts and consequently the hydrogen bond energies decrease with the decrease in the electron-withdrawing power of the alkyl groups. It is felt that electron-withdrawing groups render the chromophores better proton acceptors, and the alcohols better donors. A linear relationship between n → π∗ transition frequency and the infrared frequency of ethylenetrithiocarbonate has been found. It is concluded that stabilization of the electronic ground states of solute molecules by electrostatic and/or hydrogen-bond interactions determines the solvent shifts.
Transition-metal oxides at the metal–insulator boundary, especially those belonging to the perovskite family, exhibit fascinating phenomena such as insulator–metal transitions controlled by composition, high-temperature superconductivity and giant magnetoresistance (GMR). Interestingly, many of these marginally metallic oxides obey the established criteria for metallicity and have a finite density of states at the Fermi level. The perovskite manganates exhibiting GMR, on the other hand, are unusual in that they possess very high resistivities in the 'metallic' state and show no significant density of states at the Fermi level. Marginal metallicity in oxide systems is a problem of great complexity and contemporary interest and its understanding is of crucial significance to the diverse phenomena exhibited by these materials.
The field of inorganic open-framework materials is dominated by aluminosilicates and phosphates. The metal carboxylates have emerged as an important family in the last few years. This family includes not only mono- and dicarboxylates of transition, rare-earth, and main-group metals, but also a variety of hybrid structures. Some of the carboxylates possess novel adsorption and magnetic properties. Dicarboxylates and related species provide an effective means of designing novel hybrid structures with porous and other properties. In some of these structures, the dicarboxylate acts as a linker between two inorganic units. Hybrid nanocomposites are also of particular note, for example, cadmium oxalate host lattices that can accommodate extended alkali-metal halide structures. This Review discusses the synthesis, structure, and properties of various types of open-framework metal carboxylates.
Some aspects of the properties of oxides of perovskite and K2 NiF4 structures are presented. Some of the interesting aspects discussed are intergrowths, orthorhombicity of superconducting cuprates and importance of holes on oxygen.
For Abstract see ChemInform Abstract in Full Text.
A variety of nanostructures of carbon and inorganic nanomaterials possessing different dimensionalities have been synthesized and characterized in the last few years. Several of these nanostructures are found to have properties of utility with potential applications. Using the nanostructures in many situations requires their dispersions in suitable solvents. This can be done in most instances by appropriate functionalization of the nanostructures. In this contribution, we provide an account of the covalent and noncovalent methods of functionalization of carbon and inorganic nanostructures and their subsequent solubilization in nonpolar, polar, and aqueous media.
Two three-dimensional open-framework metal dicarboxylates possessing channels with a hydrophobic environment have been synthesized. One is a malonate of the formula, [Cd(O2C-CH2-CO2)(H2O)]·H2O, I, and the other a glutarate of the formula [Mn(O2C-(CH2)3-CO2)], II. The three-dimensional structure of I is attained through infinite Cd–O–Cd corner-linkages. On the other hand, II, contains Mn–O–Mn layers cross-linked by MnO6 octahedra and glutarate moieties.
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.
Many solids undergo transformations from one crystal structure to another as the temperature or pressure is varied and this phenomenon is popularly referred to as polymorphism. Whereas polymorphism normally refers to phase transitions involving changes in the atomic configurations in crystals, there are also transitions where the electronic or spin configuration undergoes changes. The subject of phase transitions is not only of great academic interest but also of technological importance. Phase transitions are exhibited by a wide variety of systems (Table 4.1) from simple metals and alloys to complex inorganic and organic materials. The subject has grown enormously in the last two decades with new types of transitions as well as new approaches to explain the phenomena. Traditionally, the subject has been of vital concern to metallurgists (Porter & Easterling, 1981) but there are many aspects of great importance to solid state chemistry (Rao, 1984). Varied aspects of phase transitions such as critical phenomena, soft modes, mechanisms and changes in properties at phase transitions have been treated in a unified manner by Rao & Rao (1978). We shall deal with some highlights of the subject and examine some classes of transitions in this chapter.
No abstract is provided for this article.
In rare earth cobaltites, the cobalt ions are present mainly in the diamagnetic low‐spin CoIII state at low temperatures. The CoIII ions transform to high‐spin Co3+ ions with increase in temperature. At higher temperatures, there is electron‐transfer from Co3+ to CoIII ions producing intermediate states. We see spin‐state transitions in these cobaltites in the range 250–870 K. At very high temperatures, the cobaltites show evidence for localizeditinerant electron transitions. In La1−xSrxCoO3, there is onset of ferromagnetism at x > 0.125, at which point there is a structural discontinuity and electrons become itinerant. The composition with x = 0.5 is metallic and Tc = 230 K. The ferromagnetic component in La1−xSrxCoO3, increases with x in the range 0.125–0.50. In LaCo1−xFexO3, the iron ions are always present in the high‐spin state and the d‐electrons get more localized with increase in x. Effect of substitution by ions like Th4+ and Ni3+ in LaCoO3 have also been examined. Catalytic properties of rare earth cobaltites appear to be related to the spin state equilibria.
