Factors affecting metal-metal bonding in the face-shared d(3)d(3) bioctahedral dimer systems,MM'Cl(9)(5-) (M,M' = V,Nb, Ta)

Density functional theory (DFT) calculations have been used to investigate the d(3)d(3) bioctahedral complexes, MM'Cl(9)(5-), of the vanadium triad. Broken-symmetry calculations upon these species indicate that the V-containing complexes have optimized metal-metal separations of 3.4-3.5 A, corresponding to essentially localized magnetic electrons. The metal-metal separations in these weakly coupled dimers are elongated as a consequence of Coulombic repulsion, which profoundly influences (and destabilizes) the gas-phase structures for such dimers; nevertheless, the intermetallic interactions in the V-containing dimers involve significantly greater metal-metal bonding character than in the analogous Cr-containing dimers. These observations all show good agreement with existing experimental (solid state) results for the chloride-bridged, face-shared dimers V(2)Cl(9)(5-) and V(2)Cl(3)(thf)(6)(+). In contrast to the V-containing dimers, complexes featuring only Nb and Ta have much shorter intermetallic distances (approximately 2.4 A) consistent with d-electron delocalization and formal metal-metal triple bond formation; again, good agreement is found with available experimental data. Calculations on the complexes V(2)(mu-Cl)(3)(dme)(6)(+), Nb(2)(mu-dms)(3)Cl(6)(2-), and Ta(2)(mu-dms)(3)Cl(6)(2-), which are closely related to compounds for which crystallographic structural data exist, have been pursued and provide an insight into the intermetallic interactions in the experimentally characterized complexes. Analysis of the contributions from d-orbital overlap (E(ovlp)) stabilization, as well as spin polarization (exchange) stabilization of localized d electrons (E(spe)), has also been attempted for the MM'Cl(9)(5-) dimers. While E(ovlp) clearly dominates over E(spe) as a stabilizing factor in those dimers containing only Nb and Ta metal atoms, detailed assessment of the competition between E(ovlp) and E(spe) for V-containing dimers is obstructed by the instability of triply bonded V-containing dimers against Coulombic explosion. On the basis of the periodic trends in E(ovlp) versus E(spe), the V-triad dimers have a greater propensity for metal-metal bonding than do their Cr-triad or Mn-triad counterparts.     

Herstellung von 6,14-Ethenomorphinan-Derivaten

Zusammenfassung Dihydronorthevinon (2b) wurde aus Dihydrothevinon (2a) mit Azodicarbonsäurediethylester (DEAD) hergestellt und zu einigen neuen N-substituierten Dihydronorthevinon-Derivaten (2c–2g) umgesetzt. Es wurdenGrignard-Reaktionen dieser Verbindungen mit Methylmagnesiumiodid bzw.tert-Butylmagnesiumchlorid durchgeführt. O-Demethylierungen von3a–3j ergaben die entsprechenden N-substituierten Buprenorphin- bzw. Diprenorphin-Analoga4a–4j.

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Preparation of 6,14-ethenomorphinan derivatives

Summary Dihydronorthevinone (2b) was prepared from dihydrothevinone (2a) with diethyl azodicarboxylate (DEAD) and transformed into a number of new N-substituted dihydronorthevinone derivatives (2c–2g).Grignard reactions of the new compounds with methylmagnesium iodide andtert-butyl-magnesium chloride were studied. O-Demethylation of3a–3j resulted in the corresponding N-substituted buprenorphine and diprenorphine analogs4a–4j.

     

Thermal analysis and X-ray studies of chiral ferroelectric liquid crystalline materials and their binary mixtures

Summary Thermal properties of a homologous series of ferroelectric liquid crystals S-(-)-[4-(2-n-alkoxy-propionyloxy)]biphenyl-4'-[n-alkoxy-(3,5-dimethyl)]benzoate have been investigated by polarizing optical microscopy and differential scanning calorimetry. The mesophases were identified and confirmed by X-ray too. Three binary mixtures were prepared from the individual homologues. In one of the mixtures (Mix1), the ferroelectric SmC* phase has broadened and became enantiotropic. This mesophase remained monotropic in the other two mixtures (Mix2, Mix3). The chiral nematic N* phase did not appear in Mix1, but remained monotropic for the other two mixtures. Two molecular parameters, the layer spacing and the average intermolecular distance have been calculated from the X-ray results for the homologues and their mixtures. An intercalated tail-to-tail packing of molecules was found both in the single compounds and their mixtures resulting in the layer spacing about half of the molecular length of the single compounds.     

Chiral Compounds Synthesized by Biocatalytic Reductions [New Synthetic Methods (51)]

It has been known for many decades that chiral compounds can be obtained by stereospecific biocatalytic reduction. Further significant methodological developments in this field have, however, only been made during the past ten years; they include the application of previously unused microorganisms and electron donors, the discovery of additional substrates for the known reductases, the development of methods for regenerating reduced pyridine nucleotides, and the discovery of new reductases which were sought for specific preparative purposes. Many chiral compounds can now be synthesized by microbial hydrogenation using H₂ and hydrogenase-containing microorganisms as well as by electromicrobial or electroenzymatic reduction. In the two latter methods, anaerobic or aerobic organisms are supplied with electrons from electrochemically reduced, artificial mediators, e.g., methyl viologen. Reductases that do not require pyridine nucleotides and can accept electrons directly from reduced viologens are especially useful. Two examples of this type of enzyme are described which are of preparative interest. Many cells contain methyl viologen-dependent NAD(P) reductases, a large number of which have still not been characterized. A productivity number is proposed which allows different methods of bioconversion with microorganisms to be compared. The productivity numbers of compounds synthesized by the methods described in this review are often 10- to 100-fold higher than those of substances obtained by conventional techniques.     

A Simple proof of the generalized electrostatic Israel theorem

A simple proof of a generalization of the theorem of Israel concerning the uniqueness of the Reissner-Nordström black holes is presented. The present method is also applied to show, in the bosonic sector ofN=4-supergravity, the uniqueness of a static black hole solution found by Gibbons.Work supported by Fonds zur Förderung der wissenschaftlichen Forschung in Österreich, Project No. 4961.     

Gravitational field strength and generalized Komar integral

We define a gravitational field strength in theories of the Einstein-Cartan type admitting a Killing vector. This field strength is a second rank, antisymmetric, divergence-free tensor, whose (Komar) integral over a closed 2-surface gives a physically meaningful quantity. We find conditions on the Lagrange density of the theory which ensure the existence of such a tensor, and show that they are satisfied forN=2-supergravity and for a special case of the bosonic sector ofN=4-supergravity. We discuss a possible application of the generalized Komar integral in the theory of stationary black holes. We also consider the field strength problem in Kaluza-Klein theory, where the application to black holes is particularly interesting.     

Analytical Detectors Based on Microplasma Spectrometry

Miniaturizing all dimensions of apparatus, such as electronics and computers, is the current trend followed by scientists in various fields. The idea of Lab-on-a-Chip has significantly expanded and found its broad applications in analytical chemistry. Microplasmas can act as a sample excitation source and are the miniaturized versions of full-sized plasmas. These can be created in various forms, such as direct current, microwave induced, capacitively coupled and inductively coupled plasmas. Scaling down the size would reduce the amount of gases, liquids and consumables required, as well as the sample analysis time, which in turn would decrease the operating costs. Therefore, several research groups are involved in the development of microplasmas for utilisation in analytical instruments.     

Raman- und infrorofspektroskopische Untersuchungen on Alkylderivaten der Arsensäure. V1 Die Schwingungsspektren der Dimethyl- und Diäthylarsinsäure sowie deren Reaktionsprodukte mit Chlorwasserstoff

RAMAN and IR Spectroscopic Investigation on Alkyl Derivatives of Arsenic-Acid. V. Vibrational Spectra of Dimethyl and Diethyl Arsinic Acid and their Reaction Products with HCl The RAMAN and IR spectra of (CH₃)₂AsO₂H–partially deuterated–and (C₂H₅)₂AsO₂H and of the reaction products of these acids with HCl (solid and in concentrated aqueous solution) are discussed. The symmetry of the R₂AsO₂H skeleton is C_s. of the [R₂As(OH)₂]+ ion very probably C_2v. Whereas (CH₃)₂AsO₂H gives with HCl only a compound (CH₃)₂ACO₂H · HCl (connected by H bonds), the weaker (C₂H₅)₂AsO₂H is able to form a salt [(C₂H₅)₂As(OH)₂]Cl. The H bonds in the substances are discussed.