Versus memoriales.

Accompanies "Programm"--Gymnasium, Oldenburg.Mode of access: Internet

Triggered Exchange of Anionic for Neutral Guests inside a Cationic Coordination Cage

Molecular encapsulation processes under the control of an external trigger play a major role in biological signal transduction processes and enzyme catalysis. Here, we present an artificial mimic of a controllable host system that forms via self-assembly from a simple bis-monodentate ligand and Pd­(II) cations. The resulting interpenetrated double cage features three consecutive pockets which initially contain one tetrafluoroborate anion, each. Activation of this host system with two halide anions triggers a conformational change that renders the central pocket susceptible to the uptake of small neutral guest molecules. Thereby, the pentacationic cage expels the central anion and replaces it with a neutral molecule to give a hexacationic species. The cage structures prior and after the halide triggered binding of benzene were examined by X-ray crystallography, ESI MS, and NMR techniques. The kinetics and thermodynamics of the encapsulation of benzene, cyclohexane, and norbornadiene are compared

Triggered Exchange of Anionic for Neutral Guests inside a Cationic Coordination Cage

Molecular encapsulation processes under the control of an external trigger play a major role in biological signal transduction processes and enzyme catalysis. Here, we present an artificial mimic of a controllable host system that forms via self-assembly from a simple bis-monodentate ligand and Pd­(II) cations. The resulting interpenetrated double cage features three consecutive pockets which initially contain one tetrafluoroborate anion, each. Activation of this host system with two halide anions triggers a conformational change that renders the central pocket susceptible to the uptake of small neutral guest molecules. Thereby, the pentacationic cage expels the central anion and replaces it with a neutral molecule to give a hexacationic species. The cage structures prior and after the halide triggered binding of benzene were examined by X-ray crystallography, ESI MS, and NMR techniques. The kinetics and thermodynamics of the encapsulation of benzene, cyclohexane, and norbornadiene are compared

Triggered Exchange of Anionic for Neutral Guests inside a Cationic Coordination Cage

Molecular encapsulation processes under the control of an external trigger play a major role in biological signal transduction processes and enzyme catalysis. Here, we present an artificial mimic of a controllable host system that forms via self-assembly from a simple bis-monodentate ligand and Pd­(II) cations. The resulting interpenetrated double cage features three consecutive pockets which initially contain one tetrafluoroborate anion, each. Activation of this host system with two halide anions triggers a conformational change that renders the central pocket susceptible to the uptake of small neutral guest molecules. Thereby, the pentacationic cage expels the central anion and replaces it with a neutral molecule to give a hexacationic species. The cage structures prior and after the halide triggered binding of benzene were examined by X-ray crystallography, ESI MS, and NMR techniques. The kinetics and thermodynamics of the encapsulation of benzene, cyclohexane, and norbornadiene are compared

Thermodynamics of CeO₂ Thermochemical Fuel Production

In this work the thermodynamics of thermochemical fuel production using a CeO₂ redox cycle are studied. The need to reduce the oxygen partial pressure in order to improve efficiency is investigated, with both sweep gas and vacuum pumping considered as methods of achieving this. At ambient pressure the cycles can be maximized with respect to the temperature swing, the minimum oxygen partial pressure, and the extent of the oxidation reaction. For reduction at 1500 °C the maximum efficiency was found to be 4.5%, which is significantly lower than the values found in previous studies. In addition isothermal operation had very low efficiency (less than 2%) under all of the conditions considered. If the system is operated at lower than ambient pressure, the pumping efficiency will depend on the pressure. From an investigation of commercially available pumps the pressure dependence was given an analytical expression. The results showed the cycles have an optimal operating pressure and that using sweep gas, as well as pumping, only reduced the overall efficiency. The efficiency was maximized with respect to the temperature swing, the reduction pressure, and the extent of oxidation, giving a peak efficiency of 7.5% for a reduction temperature of 1500 °C. Reducing the pressure during reduction could also be beneficial due to improved reaction kinetics at lower pressure and an increased yield due to lower oxygen partial pressures. Recovering heat from both the high temperature ceria and the oxidation reaction, and using it as process heat, was also considered. With 60% of this heat being recovered, the peak efficiency for the 1500 °C pumped cycle increased to 11%. Finally the practicality of the cycles, in terms of the quantity of ceria required to maintain continuous operation, are considered, and some suggestions for improving the cycle are given

One-Electron-Mediated Rearrangements of 2,3-Disiladicarbene

A disiladicarbene, (Cy-cAAC)2Si2 (2), was synthesized by reduction of Cy-cAAC:SiCl4 adduct with KC8. The dark-colored compound 2 is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190 °C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a SiSi double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that 2 can quasi-reversibly accept an electron to produce a very reactive radical anion, 2•–, as an intermediate species. Thus, reduction of 2 with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of 2•–

One-Electron-Mediated Rearrangements of 2,3-Disiladicarbene

A disiladicarbene, (Cy-cAAC)₂Si₂ (<b>2</b>), was synthesized by reduction of Cy-cAAC:SiCl₄ adduct with KC₈. The dark-colored compound <b>2</b> is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190 °C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a SiSi double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that <b>2</b> can quasi-reversibly accept an electron to produce a very reactive radical anion, <b>2</b>^{<b>•–</b>}, as an intermediate species. Thus, reduction of <b>2</b> with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of <b>2</b>^{<b>•–</b>}

Gene Cluster of Rhodothermus marinus High-Potential Iron-Sulfur Protein:Oxygen Oxidoreductase, a caa(3)-Type Oxidase Belonging to the Superfamily of Heme-Copper Oxidases

The respiratory chain of the thermohalophilic bacterium Rhodothermus marinus contains an oxygen reductase, which uses HiPIP (high potential iron-sulfur protein) as an electron donor. The structural genes encoding the four subunits of this HiPIP:oxygen oxidoreductase were cloned and sequenced. The genes for subunits II, I, III, and IV (named rcoxA to rcoxD) are found in this order and seemed to be organized in an operon of at least five genes with a terminator structure a few nucleotides downstream of rcoxD. Examination of the amino acid sequence of the Rcox subunits shows that the subunits of the R. marinus enzyme have homology to the corresponding subunits of oxidases belonging to the superfamily of heme-copper oxidases. RcoxB has the conserved histidines involved in binding the binuclear center and the low-spin heme. All of the residues proposed to be involved in proton transfer channels are conserved, with the exception of the key glutamate residue of the D-channel (E(278), Paracoccus denitrificans numbering). Analysis of the homology-derived structural model of subunit I shows that the phenol group of a tyrosine (Y) residue and the hydroxyl group of the following serine (S) may functionally substitute the glutamate carboxyl in proton transfer. RcoxA has an additional sequence for heme C binding, after the Cu(A) domain, that is characteristic of caa(3) oxidases belonging to the superfamily. Homology modeling of the structure of this cytochrome domain of subunit II shows no marked electrostatic character, especially around the heme edge region, suggesting that the interaction with a redox partner is not of an electrostatic nature. This observation is analyzed in relation to the electron donor for this caa(3) oxidase, the HiPIP. In conclusion, it is shown that an oxidase, which uses an iron-sulfur protein as an electron donor, is structurally related to the caa(3) class of heme-copper cytochrome c oxidases. The data are discussed in the framework of the evolution of oxidases within the superfamily of heme-copper oxidases