Influence of steric parameters on the synthesis of tetramates from α-amino-β-alkoxy-esters and Ph3PCCO

α-Aminoesters react with Ph₃PCCO in a domino addition–Wittig cyclization sequence affording enantiomerically pure tetramates. In the case of β-oxo functionalized α-aminoesters, e.g., esters of serine, threonine or β-hydroxyornithine the yields of this reaction depend heavily on the bulkiness of the β-OR group and on the configuration of β-carbon atom C-3. Smaller residues and 2R/3R-configured aminoesters give better yields. The alkoxycarbonyl group of the ester moiety and the residue on the N-atom are less important. These findings can be accounted for by assuming an early puckered transition state for the intramolecular ring-closing Wittig reaction. The addition of sub-stoichiometric amounts of benzoic acid or N-hydroxysuccinimide (for acid-sensitive compounds) is advantageous in some cases as it accelerates the formation of the intermediate amide ylides.     

LaPt2Ge2 und EuPt2Ge2 – Neubestimmung der Kristallstrukturen

LaPt₂Ge₂ and EuPt₂Ge₂ – Revision of the Crystal Structures LaPt₂Ge₂ was rechecked by single crystal X‐ray methods resulting in space group P2₁/c (in place of P2₁) and the lattice constants a = 9.953(3), b = 4.439(1), c = 8.879Å, β = 90.62(4)°, and Z = 4. In contrast to previous reports the cell volume had to be doubled. The same is true for EuPt₂Ge₂ (a = 9.731(1), b = 4.446(1), c = 8.823(1) Å, β = 91.26(1)°). The crystal structures correspond to a monoclinic variant of the tetragonal CaBe₂Ge₂ type, whereas the distortion can be described as different rotations of the coordination polyhedra around the La and Eu atoms, respectively. It is most likely that the compounds APt₂Ge₂ with A = Ca, Y, La‐Dy undergo phase transitions at higher temperatures forming then the undistorted CaBe₂Ge₂ type, space group P4/nmm. This was confirmed for SmPt₂Ge₂ (a = 4.292(1), c = 9.980(1) Å; Z = 2) and might also be the case for APt₂Ge₂ with A = Ca, Nd, Sm, Eu, and Gd.     

Influence of Substituents and Functional Groups on the Surface Composition of Ionic Liquids

We have performed a systematic study addressing the surface behavior of a variety of functionalized and non‐functionalized ionic liquids (ILs). From angle‐resolved X‐ray photoelectron spectroscopy, detailed conclusions on the surface enrichment of the functional groups and the molecular orientation of the cations and anions is derived. The systems include imidazolium‐based ILs methylated at the C2 position, a phenyl‐functionalized IL, an alkoxysilane‐functionalized IL, halo‐functionalized ILs, thioether‐functionalized ILs, and amine‐functionalized ILs. The results are compared with the results for corresponding non‐functionalized ILs where available. Generally, enrichment of the functional group at the surface is only observed for systems that have very weak interaction between the functional group and the ionic head groups.     

Nanoparticle-mediated electron transfer across ultrathin self-assembled films

The electrochemical behavior of arrays of Au nanoparticles assembled on Au electrodes modified by 11-mercaptoundecanoic acid (MUA) and poly-L-lysine (PLYS) was investigated as a function of the particle number density. The self-assembled MUA and PLYS layers formed compact ultrathin films with a low density of defects as examined by scanning tunneling microscopy. The electrostatic adsorption of Au particles of 19 +/- 3 nm on the PLYS layer resulted in randomly distributed arrays in which the particle number density is controlled by the adsorption time. In the absence of the nanoparticles, the dynamics of electron transfer involving the hexacynoferrate redox couple is strongly hindered by the self-assembled film. This effect is primarily associated with a decrease in the electron tunneling probability as the redox couple cannot permeate through the MUA monolayer at the electrode surface. Adsorption of the Au nanoparticles dramatically affects the electron-transfer dynamics even at low particle number density. Cyclic voltammetry and impedance spectroscopy were interpreted in terms of classical models developed for partially blocked surfaces. The analysis shows that the electron transfer across a single particle exhibits the same phenomenological rate constant of electron transfer as for a clean Au surface. The apparent unhindered electron exchange between the nanoparticles and the electrode surface is discussed in terms of established models for electron tunneling across metal-insulator-metal junctions.     

Über den Einfluß von Temperatur,Druck und Substitution auf die Kristallstruktur von ARh2P2 (A = Ca,Sr, Eu,Ba)

About the Effect of Temperature, Pressure, and Substitution on the Crystal Structure of ARh₂P₂ (A = Ca, Sr, Eu, Ba) Four compounds ARh₂P₂ (A = Ca, Sr, Eu, Ba) were prepared by heating mixtures of the elements and investigated by means of single crystal X-ray methods. They crystallize in the ThCr₂Si₂ type structure (I4/mmm; Z = 2) with P? P distances along [001] reaching from 2.26 Å (CaRh₂P₂) to 3.74 Å (BaRh₂P₂). With increasing temperature (EuRh₂P₂) or increasing pressure (SrRh₂P₂) a first order phase transition occurs with strong changes of the P? P distances. Substitution of the atoms changes the bond lengths of the compounds too.     

Current trends in suspension rheology

The complex rheological behaviour of suspensions is very strongly affected by the details of the microstructure and the interparticle forces by which it is controlled. The difficulties involved in characterizing the microstructure, in particular during flow, caused the development of suspension rheology to lag behind that of polymer rheology. Progress in theory, experiments and simulation is rapidly changing the picture, providing opportunities for further significant developments. Possible evolutions in six areas of suspension rheology are discussed here.     

Structure and dynamics of a polymer solution subject to flow-induced phase separation

Experiments combining mechanical rheometry with polarimetry (birefringence and scattering dichroism) have been conducted on a 6% solution of polystyrene (1.86x10⁶ molecular weight) in dioctyl phthalate. Birefringence is used to measure the extent of segmental orientation, whereas the dichroism is sensitive to orientation and deformation of concentration fluctuations associated with the process of flow-induced phase separation. The results indicate that these fluctuations grow predominately along the neutral (or vorticity axis) of a simple shear flow. At higher rates of shear, orientation in the flow direction is favored. The transition in orientation direction is accompanied by time-dependent behavior in the optical properties of the solution during shear and the onset of shear thickening of the viscosity and the first normal stress difference coefficient.     

Revisiting roaming trajectories in ketene isomerization at higher dimensionality

Roaming dynamics have been observed in a three-dimensional model of the ketene isomerization reaction. The roaming trajectories sample the region between the outer potential barriers closest to the respective ketene isomers and involve turning points along the reaction coordinate in a polar representation. These roaming trajectories avoid the intrinsic reaction coordinate and the intermediates to which it is associated. Thus, one-dimensional transition state theory (TST) is generally insufficient as has been confirmed through an analysis of the reactive flux along the dividing surface (DS). A global representation of the DS, however, leads to accurate TST rate constants. The exact and TST microcanonical rates of isomerization have been obtained for the three-dimensional model and compare well to experiment. The global DS is therefore particularly important for obtaining rates in reactions that exhibit roaming. This work thus confirms the findings of our previous two-dimensional treatment of ketene isomerization (Ulusoy et al. in J. Phys. Chem. A 117:7553–7560, 2013).