Primary Amine‐Initiated Polymerizations of α–Amino Acid N‐Thiocarbonic Acid Anhydrosulfide

The N‐thiocarbonic acid anhydrosulfides NTAs of D,L‐leucine, D,L‐phenylalanine and sarcosine were polymerized in dioxane by addition of n‐hexylamine as initiator. Despite variation of the monomer‐initiator ratio (M/I) only low yields of oligopeptides were obtained from D,L‐Leu‐ and D,L‐Phe‐NTA. Both yields and molecular weights were almost twice as high for polymerizations of Sar‐NTA. MALDI‐TOF mass spectra confirmed that the isolated oligo‐and polypeptides possess the expected structure with one reactive amino end group. Therefore, it is surprising that the polymerizations stopped at low conversions. Two hypotheses explaining this phenomenon are discussed.     

Carbocyclization of E,E-1,4-Diphenyl-1,3-butadiene with Dichloroalkanes Mediated by Rieke Metals

The Rieke metal complexes of barium and strontium readily react with E,E-1,4-diphenyl-1,3-butadiene to form metal-diene reagents. Upon treatment with l .n-dichloroalkanes, these metallocycles are transformed into ring derivatives in excellent chemical yield.     

Light‐Driven Coordination‐Induced Spin‐State Switching: Rational Design of Photodissociable Ligands

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin‐state switching to isolated molecules at room temperature (light‐driven coordination‐induced spin‐state switching, or LD‐CISSS). Whereas bistability and hysteresis in conventional spin‐crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD‐CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel–porphyrin as the transition‐metal complex and 3‐phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni–porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72 % paramagnetic Ni–porphyrin after irradiation at 365 nm, 32 % paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD‐CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light‐controlled magnetic levitation are conceivable applications.     

Global attractors for the three-dimensional Navier-Stokes equations

In this paper we show that the weak solutions of the Navier-Stokes equations on any bounded, smooth three-dimensional domain have a global attractor for any positive value of the viscosity. The proof of this result, which bypasses the two issues of the possible nonuniqueness of the weak solutions and the possible lack of global regularity of the strong solutions, is based on a new point of view for the construction of the semiflow generated by these equations. We also show that, under added assumptions, this global attractor consists entirely of strong solutions.     

Über den Anstieg von Flüssigkeiten in vertikalen makroskopischen Modellrohren unter Weltraumbedingungen

übersicht Der Anstieg einer Flüssigkeitssäule in makroskopischen kreiszylindrischen und konischen Rohren unter verminderter Schwereeinwirkung wird theoretisch und experimentell (Höhenforschungsrakete) untersucht. Das Verhalten von Flüssigkeiten unter Weltraumbedingungen (Restschwere, Mikrogravitation) ist von besonderem Interesse in der Raumfahrttechnik. Unter Berücksichtigung aller wesentlichen Kräfte ergibt sich trotz verschiedener Vereinfachungen in den theoretischen Ansätzen eine gute übereinstimmung zwischen numerischen Resultaten und den Meßergebnissen.

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On the rise of liquid columns into vertical tubes subjected to zero-gravity

Summary Theoretical and experimental information is given on the capillary rise of liquid into macroscopic vertical round tubes subjected to reduced gravity. The response in a low gravitational environment is of particular interest in studying the behaviour of liquid systems for space vehicles. An approximate analysis is presented to predict the transient rise and to interpret the experimental data. Agreement between numerical and experimental results is very good.

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Herrn Professor em. Dr. Dr.-Ing. E. h. Eberhard Mettler zur Vollendung des 75. Lebensjahres gewidmet     

Cesium vapor laser with transverse pumping by multiple laser diode arrays

We have demonstrated a Cs vapor laser that utilizes fifteen laser diode arrays for transverse pumping of the gain medium. A maximum output power of 28 W was achieved with a total optical to optical efficiency of 14% and a slope efficiency of 15%. Transverse pumping allows scaling of such a laser system to a higher power level by simple increasing of the gain medium volume and the number of pump sources.