Swimming against the Stream? A Discussion of the Bonding in d6 and d8 Fluoro Complexes and Its Consequences for Catalytic Applications

Fluoride has recently found increasing application as coligand both in complexes of late transition metals and as a reagent in asymmetric catalysis. In recent papers, this topic was reviewed, with emphasis on the role played by so‐called ‘push‐pull interactions' between the fluoro ligand and a π acid in the stabilization of fluoro complexes with a d⁶ or d⁸ electron configuration. This picture has led to the concept that fluoride is the strongest π‐donor in the halide series. Herein, the latter concept is discussed and criticized. In particular, the effect of the ionicity of the metal‐fluoride bond is proposed as an alternative explanation to most of the observations that show a ‘reversed' halide order. The ionic character of the M F bond also accounts for its intrinsic reactivity and suggests some strategies for the stabilizatioin of fluoro complexes, including the use of coordinative unsaturation. The scope and limitations of the application of d⁶ and d⁸ fluoro complexes in catalysis are also discussed. The most promising application is the use of 16‐electron fluoro complexes in the metal‐promoted formation of the C F bond.     

Justus Liebig trifft Georg Büchner: Sturm und Dung — eine fiktive Begegnung

In a fictitious meeting between the chemist and agricultural reformer Justus Liebig and the poet and social revolutionary Georg Büchner alternative paths of historical development in the early 19^th century as well as current developments in food engeneering are discussed.     

In Situ Observation of Adiabatic Shear Band Formation in Aluminum Alloys

We used high-speed X-ray phase contrast imaging and infrared thermal imaging techniques to study the formation processes of adiabatic shear bands in aluminum 7075-T6 and 6061-T6 alloys. A modified compression Kolsky bar setup was used to apply the dynamic loading. A flat hat-shaped specimen design was adopted for generating the shear bands at the designated locations. Experimental results show that 7075-T6 exhibits less ductility and a narrower shear band than 6061-T6. Maximum temperatures of 720 K and 770 K were locally determined within the shear band zones for 7075-T6 and 6061-T6 respectively. This local high temperature zone and the resulting thermal instability were found to relate to the shear band formation in these aluminum alloys.

     

1H‐Benzotriazole as Synthetic Auxiliary in a Facile Route to N6‐(Arylmethyl)‐2′‐deoxyadenosines: DNA Intercalators Inserted into Three‐Way Junctions

The 2′‐deoxy‐N⁶‐(naphthalen‐1‐ylmethyl)‐ ( 5a ) and N⁶‐(pyren‐1‐ylmethyl)adenosine ( 5b ) were synthesized in two steps from 2′‐deoxyadenosine and the adequate arenecarbaldehyde with 1H‐benzotriazole as a synthetic auxiliary (Scheme). When the N⁶‐(arylmethyl)‐2′‐deoxyadenosines were inserted into the junction region of a DNA three‐way junction, its thermal stability increased.     

Canonical transformations and exact invariants for time‐dependent Hamiltonian systems

An exact invariant is derived for n‐degree‐of‐freedom non‐relativistic Hamiltonian systems with general time‐dependent potentials. To work out the invariant, an infinitesimalcanonical transformation is performed in the framework of the extended phase‐space. We apply this approach to derive the invariant for a specific class of Hamiltonian systems. For the considered class of Hamiltonian systems, the invariant is obtained equivalently performing in the extended phase‐space a finitecanonical transformation of the initially time‐dependent Hamiltonian to a time‐independent one. It is furthermore shown that the invariant can be expressed as an integral of an energy balance equation. The invariant itself contains a time‐dependent auxiliary function ξ (t) that represents a solution of a linear third‐order differential equation, referred to as the auxiliary equation. The coefficients of the auxiliary equation depend in general on the explicitly known configuration space trajectory defined by the system's time evolution. This complexity of the auxiliary equation reflects the generally involved phase‐space symmetry associated with the conserved quantity of a time‐dependent non‐linear Hamiltonian system. Our results are applied to three examples of time‐dependent damped and undamped oscillators. The known invariants for time‐dependent and time‐independent harmonic oscillators are shown to follow directly from our generalized formulation.     

Static and dynamic light scattering in turbid suspensions

The application of static and dynamic light scattering to many colloidal systems of practical interest has often been considered too complicated due to strong multiple scattering. There are two new approaches to overcome this problem. One of them aims at suppressing contributions from multiple scattering using novel cross‐correlation schemes. While this relies on the suppression of multiple scattering, the so‐called diffusing wave spectroscopy (DWS) works in the limit of very strong multiple scattering. DWS can be used for the characterization of dynamic and static properties of colloidal systems on a large range of time and length scales ranging from a few Ångstroms to hundreds of nanometers. We demonstrate that a wealth of information can be obtained from these methods on the structure, dynamics, interaction effects, stability, aggregation, and sol‐gel transition in colloidal dispersions.