Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Band structure and electronic properties of lithium azide (LiN3)

Ab initio Hartree–Fock band structure and molecular calculations have been performed to study the electronic structure of LiN₃ in a monoclinic C 2/m crystal structure. The total energy, band structure, density of states, and charge densities are computed. The calculated lattice energy (energy to separate the ions infinitely apart) of 8.6 eV agrees very well with 8.45 eV deduced from Madelung and London polarizability energies. The calculated split of the N 1s core bands of 5.0 eV compares favorably with the experimental X-ray photoelectron value of 4.4 eV. This good agreement is not contributed to crystalline environment effects as proposed in earlier MO studies of N where the best values obtained were 5.1, 5.8, and 6.3 eV, but to the quality of the nitrogen core basis set. The calculated valence density of states supports one of two competing interpretations that peak III observed in the X-ray photoelectron spectrum arises from contaminations or other extrinsic states.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Ergebnisse der umsetzung von carbonylcobaltaten mit chlordifluoracetylchlorid

We have been able to synthesize chlorodifluomethyltetracarbonyl- cobalt (1) by the reaction of chlorodifluoroacetyl chloride and sodium tetracarbonylcobaltate(-I) at low temperatures. (1) decomposes at normal temperature to yield μ-difluoromethylene-μ-carbonyl-bis(tricarbonylcobalt) (Co-Co) (2), di-μ-difluoromethylene-bis (tricarbonylcobalt) (Co-Co) (3) and μ-difluormethylene-μ-tetrafluoroethylidene-bis (tricarbonylcobalt) (Co-Co) (4). (2) proved to be an intermediate in the formation of the cluster fluoromethinyl-enneacarbonyltricobalt (5).Na[Co(CO)₃P(C₆H₅)₃] (or Na[Co(CO)₄] in presence of triphenylphosphine) and ClCF₂COCl react to form the acetyl derivative ClCF₂ CO·Co(CO)₃P(C₆H₅)₃ (6) which cannot be decomposed to give the corresponding methyl compound.     

Molecules with Large Cavities in Supramolecular Chemistry

Supramolecular chemistry is a new area of research that has rapidly developed from pure synthetic chemistry, and its novelty has led to interdisciplinary cooperation between organic and inorganic chemistry, biochemistry, physical and theoretical chemistry, and physics. Whereas molecular chemistry essentially deals with the covalent bonding of atoms, Supramolecular chemistry is predominantly involved in the study of the weaker intermolecular interactions resulting in the association and self-organization of several components to form larger aggregates (supramolecules). The first crown ether discovered by the subsequent Nobel prizewinner Pedersen was more the fortuitous reaction product of an impurity, but nowadays, some twenty-five years later, chemists are able to tailor host molecules to special requirements. Host compounds having a cyclophane skeleton make an important contribution, since their aromatic structural units ensure the necessary rigidity of the molecular structures and thereby improve the preorganization of the coordination sites for the cooperative binding of the guests. During the course of the rapid development of Supramolecular chemistry such a large number of synthetic hosts has been developed and their interaction with guests studied in such depth that we must restrict ourselves here to a discussion of a particular group of host compounds, namely cavity-supporting macrobicyclic and macrooligocyclic phanesu, which bear a similar relation to open-chain and monocyclic hosts as the metal-complexing cryptands to the podands and crown ethers. The molecular architecture of these three-dimensionally bridged macrooligocycles is a challenge for synthetic chemistry. (Not only the size and shape of the intramolecular cavity, but also the provision of the latter with suitable coordination centers have to be included in the synthesis strategy.) The capacity for the envelopment of guests from all sides and the expedient endo functionalization often also produce a particularly strong binding of host and guest, outstanding selectivities with regards to molecular recognition, and special properties of the Supramolecular complexes.     

19F-Kernmagnetische resonanz-untersuchungen zur frage der assoziation des Schwefeltetrafluorids

On coolilng sulphur-fluorine compounds with 3- or 4-coordinated sulphur atoms, changes occur in the chemical shifts of fluorine atoms and in spin coupling constants which suggest the formation of fluorine bridges. In solutions of sulphur tetrafluoride, the presence of associated species is suggested by a decrease in the distance between the two triplets.