Superconductivity and Chemical Bonding in Mercury

Band structure and Fermi surface calculations were performed for the metal mercury, for which superconductivity was discovered 87 years ago. The electronic properties of mercury were analyzed to find the origin of singlet electron pair formation and condensation which leads to superconductivity.     

High response smart gels: synthesis and application

Stimuli-responsive hydrogels change their volume and elasticity in dependence on the properties of the liquid phase. The amount of solvent uptake is dependent upon the chemical nature of the gel and the nature of its environment, e.g. solvent composition, temperature, pH, etc. The swelling/deswelling process is time consuming. For many applications the response time must be short. According to Tanaka's equation the rate of equilibration is inversely proportional to the square of gel size and proportional to the mutual (or cooperative) diffusion of the network and the solvent. D_coop is determined by the properties of the cross-linked polymer and can not be changed for a given chemical system. By decreasing the characteristic dimension of the gel, it is possible to reduce the time for equilibration. The synthesis, characterization of thin layers and small particles of T-sensitive polymers and their application in a micro-valve is described.     

Polymerization with metallocene catalysts. Hydrooligomerization and NMR investigations concerning the microstructure of poly(cyclopentenes)

Hydrooligomerizations and polymerizations of cyclopentene were carried out using C_S, C₂ and C_2v symmetric zirconocenes and methylaluminoxane as catalyst. The structure of the hydrotrimers was investigated by one- and twodimensional NMR spectroscopy and correlated to the structure of the polymers. The assignments for the ¹³C-NMR spectrum of the cis-1,3-enchained hydrotrimer made by Collins [3] are corrected. Cis-1,3-enchainment of the configurational base units in the poly(cyclopentenes) is confirmed. rac-[Me₂Si(Ind)₂]ZrCl₂/MAO catalysis features crystalline polymers and the erythrodiisotactic hydrotetramer indicating highly stereoselective insertion. Cp₂ZrCl₂/MAO and [Ph₂C(Fluo)(Cp)]ZrCl₂/MAO show none or low stereoselectivity respectively yielding amorphous polymers and both of the diastereomeric hydrotetramers.     

A Porous Media Model to Describe the Behaviour of Brain Tissue

The human brain is a very sensitive organ. Even small changes in the cranium cavity can cause life–threatening effects. In case of medical intervention, biomechanics can assist the therapy decisions by simulating the physical behaviour of brain tissue, e.g., the coupled interaction of the fluid motion and the deformation of the brain tissue. In the context of the Theory of Porous Media (TPM), a convenient model of the brain is introduced, which is able to simulate essential mechanical effects in the porous structure of the brain material. The fluid–saturated brain can be treated as an immiscible binary mixture of constituents. In this macroscopic biphasic model, the mixture consists of a solid phase (brain tissue) and a fluid phase (interstitial fluid or blood plasma). Both constituents are assumed to be materially incompressible. The resulting set of coupled partial differential equations is then spatially discretised using mixed finite elements with a backward Euler time integration. Numerical examples are presented illustrating the fundamental effects on the brain tissue under heart–rate dependent pulsative pressure variations. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental investigation of the auto-ignition of a transient propane Jet-in-Hot-Coflow

Auto-ignition is a complex process which is extremely sensitive to boundary conditions such as local temperature, mixture or strain rate and occurs on very short time-scales. Therefore, measurement techniques with high spatio-temporal resolution have to be applied to test cases with well-defined boundary conditions in order to generate high-quality validation data for numerical simulations. In the current paper, the auto-ignition of a transient propane jet-in-hot coflow was studied with high-speed OH* chemiluminescence imaging and high-speed Rayleigh scattering for the simultaneous determination of mixture fraction, mixture temperature and scalar dissipation rate immediately prior to the onset of auto-ignition. A variation of the coflow temperature showed a pronounced temperature dependence of the auto-ignition location and time, and the temperature sensitivity was higher than for a comparable methane test case from the literature. This is explained by the lower sensitivity of propane ignition delay times to the local strain rate in comparison to methane. The Rayleigh measurements however showed that the formation mechanism of auto-ignition kernels is similar for propane and methane. Ignition kernels were found to form upstream of bulges of the inflowing jet at locations with locally low scalar dissipation rate.     

Cornforth–Evans Transition States in Stereocontrolled Allylborations of Epoxy Aldehydes

Allylboration reactions rank among the most reliable tools in organic synthesis. Herein, we report a general synthesis of trifunctionalized allylboronates and systematic investigations of their stereocontrolled transformations with substituted aldehyde substrates, in order to efficiently access diverse, highly substituted target substrates. A peculiar transition in stereocontrol was observed from the polar Felkin–Anh (PFA) to the Cornforth–Evans (CE) model for alkoxy- and epoxy-substituted aldehydes. CE-type transition states were uniformly identified as minima in advanced, DFT-based computational studies of allylboration reactions of epoxy aldehydes, conforming well to the experimental data, and highlighting the underestimated relevance of this model. Furthermore, a mechanism-based rationale for the substitution pattern of the epoxide was delineated that ensures high levels of stereocontrol and renders α,β-epoxy aldehydes generally applicable substrates for target synthesis.