Phosphine‐Stabilized Diiododiborenes: Isolable Diborenes with Six Labile Bonds

The lability of B=B, B?P, and B–halide bonds is combined in the syntheses of the first diiododiborenes. In a series of reactivity tests, these diiododiborenes undergo cleavage of all six of their central bonds in different ways, leading to products of B=B hydrogenation and dihalogenation as well as halide exchange.     

Densities and isothermal compressibilities of ionic liquids—Modeling and application

Two corresponding-states forms have been developed for direct correlation function integrals in liquids to represent pressure effects on the volume of ionic liquids over wide ranges of temperature and pressure. The correlations can be analytically integrated from a chosen reference density to provide a full equation of state for ionic liquids over reduced densities from 1.5 to more than 3.6. One approach is empirical with 3 parameters, the other is a 2-parameter theoretical form which is directly connected to a method for predicting gas solubilities in ionic liquids. Parameters for both methods have been obtained for 28 different ionic liquid systems by reduction of carefully validated PρT data. The results show excellent agreement with the experimental data for nearly all substances over all ranges of conditions. Group contributions to parameters are also explored, leading to an entirely predictive method for ambient pressure densities and densities of compressed ionic liquids. Extensive comparisons are made with other techniques.     

Stereochemical effects in supramolecular self-assembly at surfaces: 1-D versus 2-D enantiomorphic ordering for PVBA and PEBA on Ag(111)

We present investigations on noncovalent bonding and supramolecular self-assembly of two related molecular building blocks at a noble metal surface: 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)]benzoic acid (PEBA). These rigid, rodlike molecules comprising the same complementary moieties for hydrogen bond formation are comparable in shape and size. For PVBA, the ethenylene moiety accounts for two-dimensional (2-D) chirality upon confinement to a surface; PEBA is linear and thus 2-D achiral. Molecular films were deposited on a Ag(111) surface by organic molecular beam epitaxy and characterized by scanning tunneling microscopy. At low temperatures (around 150 K), both species form irregular networks of flat lying molecules linked via their endgroups in a diffusion-limited aggregation process. In the absence of kinetic limitations (adsorption or annealing at room temperature), hydrogen-bonded supramolecular assemblies form which are markedly different. With PVBA, enantiomorphic twin chains in two mirror-symmetric species running along a high-symmetry direction of the substrate lattice form by diastereoselective self-assembly of one enantiomer. The chirality signature is strictly correlated between neighboring twin chains. Enantiopure one-dimensional (1-D) supramolecular nanogratings with tunable periodicity evolve at intermediate coverages, reflecting chiral resolution in micrometer domains. In contrast, PEBA assembles in 2-D hydrogen-bonded islands, which are enantiomorphic because of the orientation of the supramolecular arrangements along low-symmetry directions of the substrate. Thus, for PVBA, chiral molecules form 1-D enantiomorphic supramolecular structures because of mesoscopic resolution of a 2-D chiral species, whereas with PEBA, the packing of an achiral species causes 2-D enantiomorphic arrangements. Model simulations of supramolecular ordering provide a deeper understanding of the stability of these systems.     

Synthesis of C-8 Deuterated Glycosides of 3-Deoxy-D-manno-oct-2-ulosonic Acid (Kdo) Related to Chlamydial Lipopolysaccharides

Summary.  Methyl glycosides of Kdo and a (2→8)-linked Kdo disaccharide were prepared which contain a deuterium label at C-8 of the reducing unit. The label was introduced in fair diastereoselectivity upon reduction of an aldehyde group using a chiral borane complex derived from N-benzyloxycarbonyl-(S)-proline which produced the 8-(S)-deuterated derivative as the major isomer. Further coupling with a Kdo bromide gave the α-(2→8)-linked disaccharide in good yield. The deprotected disaccharide serves as a model for NMR spectroscopic studies on the side chain conformation of a carbohydrate epitope from the bacterial pathogen Chlamydia. Received September 17, 2001. Accepted October 17, 2001     

Ionene–surfactant complexes: temperature and humidity sensitive materials

Solid stoichiometric complexes of [3,12]-ionene and dodecyl sulfate form upon reaction of the bromide of the ionene and the silver salt of dodecyl sulfate in methanol. IR, DSC, and TG investigations indicated that the solid complexes are stable between 30 and 120 °C. TG and DSC also showed that the complexes easily take up water at ambient conditions. These samples are optically isotropic. When exposed to an increased humidity they exhibit optical anisotropy, i.e., birefringence, which is caused by the formation of a hexagonal mesogenic phase. Mesogenicity is necessarily accompanied by a further uptake of water (4–5 H₂O molecules per ionic unit), which is dependent on the relative humidity. The phase behavior as a function of temperature and controlled relative humidity was studied using birefringence measurements and polarizing microscopy.     

Substituent-dependent reactivity in the photodimerization of N-substituted dibenz[b,f]azepines

The photoprocesses of a series of N-substituted dibenz[b,f]azepines (iminostilbenes) were studied by absorption and emission spectroscopy, by laser flash photolysis, and by preparative irradiation with NMR analysis. In solutions, 2pi+2pi photodimers of N-cyano and N-acyl dibenzazepines are formed via the triplet state upon acetone- or benzophenone-sensitized energy transfer. T-T absorption spectra were measured and absorption coefficients were determined. The triplet energy transfer is equally efficient for N-alkyl dibenzazepines, which do not dimerize. Excited states of npi* character in the latter cases are discussed to rationalize the different reactivities. In spite of negligible intersystem crossing of 21 dibenzazepine derivatives, photodimers of N-acyl and N-cyano dibenzazepines are formed upon direct excitation in concentrated solutions (0.01-0.1 mol dm(-3)) as well as in the solid state. A selective anti-configuration of the photodimers was found throughout.     

Raman and cathodoluminescence spectroscopic investigations on Permian fossil wood from Chemnitz--a contribution to the study of the permineralisation process

Samples of different three-dimensionally preserved fossil plants (Medullosa sp., Dadoxylon sp., Calamodendron striatum, Psaronius sp.) from the Lower Permian petrified forest of Chemnitz were examined with regard to their chemical composition and structural order. Raman spectroscopy and cathodoluminescence microscopy are shown to be powerful tools for such investigations. Silicified wood from Chemnitz-Hilbersdorf generally shows yellow cathodoluminescence (CL) of the cell walls and only weak yellow-brownish CL of the cell lumina. By time-resolved cathodoluminescence spectroscopy, a secondary mineralisation of hydrothermal origin was recognized. The latter is shown by short-lived blue CL at the cell walls extinguishing the yellow signal. Therefore, after the primary silicification step a secondary mineralisation step initiated by hydrothermal processes, seems to have taken place at probably slightly higher temperatures. The resulting silica matrix consists of phanerocrystalline and microcrystalline alpha-quartz as well as microcrystalline moganite, both partially associated with iron oxides. Dadoxylon sp. is a prominent example for parallel permineralisation by alpha-quartz and fluorspar, which is outstanding for the Chemnitz Petrified Forest. CL on this samples shows parallel silicification and fluoritisation, followed by infiltration of iron oxides. Permineralised samples show very low percentage of original organic remains. The seed fern Medullosa, for example, shows dispersed carbon, which is mainly restricted to the centres of the typical star-shaped vascular bundles. Raman spectroscopy revealed that these carbonaceous particles are of an anthracite structure. For experimental confirmation coal samples of different rank, especially anthracite from different geological times and localities, were studied by means of Raman spectroscopy. The remaining pith of the vascular bundles is white-coloured and consists of alpha-quartz and moganite, whereas surrounding tracheides exhibit white and reddish coloured parts. The reddish parts, mainly found in the rays, additionally contain alpha-Fe2O3 and Fe3O4 among the SiO2 polymorphs of alpha-quartz and moganite. Sometimes iron oxides could have dominated permineralisation processes as the peak intensities of distinct parts of the samples suggest.     

Methane activation by silica-supported Zr(IV) hydrides: the dihydride [(triple bond)SiO)2ZrH2] is much faster than the monohydride [(triple bond)SiO)3ZrH

The silica-supported Zr(iv) dihydride [(triple bond)SiO)2ZrH2] reacts quickly and completely with methane to yield [(triple bond)SiO)2ZrMe2] through the intermediate [(triple bond)SiO)2ZrHMe], while its monohydride analogue [(triple bond)SiO)3ZrH] yields the monomethylated product [(triple bond)SiO)3ZrMe] slowly and incompletely.