Boron- versus Nitrogen-Centered Nucleophilic Reactivity of (Cyano)hydroboryl Anions: Synthesis of Cyano(hydro)organoboranes and 2-Aza-1,4-diborabutatrienes

Cyclic alkyl(amino)carbene-stabilized (cyano)hydroboryl anions were synthesized by deprotonation of (cyano)dihydroborane precursors. While they display boron-centered nucleophilic reactivity towards organohalides, generating fully unsymmetrically substituted cyano(hydro)organoboranes, they show cyano-nitrogen-centered nucleophilic reactivity towards haloboranes, resulting in the formation of hitherto unknown linear 2-aza-1,4-diborabutatrienes.     

Isomer-specific influences on the composition of reaction intermediates in dimethyl ether/propene and ethanol/propene flame

This work provides experimental evidence on how the molecular compositions of fuel-rich low-pressure premixed flames are influenced as the oxygenates dimethyl ether (DME) or ethanol are incrementally blended into the propene fuel. Ten different flames with a carbon-to-oxygen ratio of 0.5, ranging from 100% propene (phi = 1.5) to 100% oxygenated fuel (phi = 2.0), are analyzed with flame-sampling molecular-beam mass spectrometry employing electron- or photoionization. Absolute mole fraction profiles for flame species with masses ranging from m/z = 2 (H2) to m/z = 80 (C6H8) are analyzed with particular emphasis on the formation of harmful emissions. Fuel-specific destruction pathways, likely to be initiated by hydrogen abstraction, appear to lead to benzene from propene combustion and to formaldehyde and acetaldehyde through DME and ethanol combustion, respectively. While the concentration of acetaldehyde increases 10-fold as propene is substituted by ethanol, it decreases as propene is replaced with DME. In contrast, the formaldehyde concentration rises only slightly with ethanol replacement but increases markedly with addition of DME. Allyl and propargyl radicals, the dominant precursors for benzene formation, are likely to be produced directly from propene decomposition or via allene and propyne. Benzene formation through propargyl radicals formed via unsaturated C2 intermediates in the decomposition of DME and ethanol is negligibly small. As a consequence, DME and ethanol addition lead to similar reductions of the benzene concentration.     

Tunable blue laser based on intracavity frequency doubling with a fan-structured periodically poled LiTaO(3) crystal

We introduce a new concept for a wavelength-tunable frequency-doubled laser diode with a single control parameter. The concept is based on intracavity frequency doubling in an external resonator geometry with spatial separation of the spectral components. The use of a fan-structured periodically poled LiTaO(3) crystal permits tuning of both the fundamental and the second harmonic simultaneously with one aperture. We demonstrate tunability over more than 10 nm in the blue (480.4 to 490.6 nm) with output powers of the order of 50 nW.     

Light-induced Photoswitching of 4-(Phenylazo)benzoic Acid on Au(111)

Photochromic molecules can undergo a reversible conversion between two isomeric forms upon exposure to external stimuli such as electromagnetic radiation. A significant physical transformation accompanying the photoisomerization process defines them as photoswitches, with potential applications in various molecular electronic devices. As such, a detailed understanding of the photoisomerization process on surfaces and the influence of the local chemical environment on switching efficiency is essential. Herein, we use scanning tunneling microscopy to observe the photoisomerization of 4-(phenylazo)benzoic acid (PABA) assembled on Au(111) in kinetically constrained metastable states guided by pulse deposition. Photoswitching is observed at low molecular density and is absent in tight-packed islands. Furthermore, switching events were noted in PABA molecules coadsorbed in a host octanethiol monolayer, suggesting an influence of the surrounding chemical environment on photoswitching efficiency.     

Isomer-specific fuel destruction pathways in rich flames of methyl acetate and ethyl formate and consequences for the combustion chemistry of esters

The influences of fuel-specific destruction pathways on flame chemistry are determined for two isomeric ester fuels, methyl acetate, CH3(CO)OCH3, and ethyl formate, H(CO)OC2H5, used as model representatives for biodiesel compounds, and their potential for forming air pollutants is addressed. Measurements are presented of major and intermediate species mole fractions in premixed, laminar flat flames using molecular-beam sampling and isomer-selective VUV-photoionization mass spectrometry. The observed intermediate species concentrations depend crucially on decomposition of the different radicals formed initially from the fuels. The methyl acetate structure leads to preferential formation of formaldehyde, while the ethyl formate isomer favors the production of acetaldehyde. Ethyl formate also yields higher concentrations of the C2 species (C2H2 and C2H4) and C4 species (C4H2 and C4H4). Benzene concentrations, while larger for ethyl formate, are at least an order of magnitude smaller for both flames than seen for simple hydrocarbon fuels (ethylene, ethane, propene, and propane).     

On a kinetic model for shallow water waves

The system of shallow water waves is one of the classical examples for non-linear, two-dimensional conservation laws. The paper investigates a simple kinetic equation depending on a parameter ? which leads for ? → 0 to the system of shallow water waves. The corresponding ‘equilibrium’ distribution function has a compact support which depends on the eigenvalues of the hyperbolic system. It is shown that this kind of kinetic approach is restricted to a special class of non-linear conservation laws. The kinetic model is used to develop a simple particle method for the numerical solution of shallow water waves. The particle method can be implemented in a straightforward way and produces in test examples sufficiently accurate results.     

Control of Crystallization Processes

In this paper an optimal control problem for polymer crystallization is investigated. The crystallization is described by a non–isothermal Avrami–Kolmogorov model. The boundary temperature serves as control variable. The cost functional takes the spatial variation of the crystallinity and the final degree of crystallization into account. The resulting boundary control problem for a parabolic equation coupled with two ordinary differential equations is treated by an adjoint variable approach. Numerical simulations are carried out for different scenarios to illustrate the applicability and performance of the optimization algorithm. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Regional brain activity after prolonged cholinergic enhancement in early Alzheimer's disease

This functional magnetic resonance imaging (fMRI) study examined changes in brain activation after prolonged (20 weeks) and stabilized treatment with the cholinesterase inhibitor galantamine in a small group of patients with very mild Alzheimer's disease (AD). Two cognitive activation paradigms were chosen: one requiring semantic association and the other relying on attention and requiring target detection. A group of age- and education-matched healthy controls was also scanned for comparison. A modest (but not statistically significant) improvement in behavioral scores after treatment was observed in both fMRI tasks. There were brain activation increases in the semantic association task after treatment, and the differences in brain activation present in the comparison of AD patients' baseline images with those of controls were not detectable after treatment. In the target detection task, regions that were activated in the elderly controls but not in the baseline images of the AD group also showed significant activation after treatment. Overall, however, the increases were modest and might reflect the heterogeneity of clinical response to treatment in this small group. Future pharmacological fMRI studies should include clinical response as a factor in the analysis of cholinergic enhancement effects in AD patients.     

Naturfarbstoffe: Farben mit Geschichte

Up to the introduction and industrial use of synthetic colouring materials, natural dyestuffs like woad, purple and madder were commonly used. Natural dyestuffs require different dyeing methods like vat‐, direct‐ or mordant‐dyeing. The fundamental principles of these methods are still in use in modern dyeing mills. The article gives an idea of cultural and historical connections during the development from textile handicraft to the first industrial organisation. By the example of madder the influence of mordants on colour is presented.