Vibration suppression of a string through cyclic application and removal of constraints

A new method for the control of string vibration is presented. The method relies on application and removal of a constraint at one point on the string close to one of its boundaries. Application of the constraint temporarily results in two vibrating strings, one of which is significantly shorter in length than the other. The vibration of the shorter length string decays rapidly due to high damping and results in reduction in the overall energy of the system. Removal of the constraint does not change the energy of the system but allows the constraint to be applied repeatedly for vibration suppression. A mathematical model of the cycle of constraint application and removal is presented and multiple cycles of constraint application and removal are simulated. Experiments were performed with a coiled extension spring which behaves like a lightly damped string. Simulation and experimental results match well with each other and demonstrate the efficacy of the simple control strategy.     

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

Cationic d⁰ group 6 olefin metathesis catalysts have been recently shown to display in most instances superior activity in comparison to their neutral congeners. Furthermore, their catalytic performance is greatly improved upon immobilization on silica. In this context, we have developed the new family of molecular cationic molybdenum oxo alkylidene complexes stabilized by N-heterocyclic carbenes of the general formula [Mo(O)(CHCMe₃)(IMes)(OR)[X⁻]] (IMes = 1,3-dimesitylimidazol-2-ylidene; R = 1,3-dimesityl-C₆H₃, C₆F₅; X⁻ = B(3,5-(CF₃)₂C₆H₃)₄⁻, B(Ar^F)₄, tetrakis(perfluoro-t-butoxy)aluminate (PFTA)). Immobilization of [Mo(O)(CHCMe₃)(IMes)(O-1,3-dimesityl-C₆H₃)⁺B(Ar^F)₄⁻] on silica via surface organometallic chemistry yields an active alkene metathesis catalyst that shows the highest productivity towards terminal olefins amongst all existing molybdenum oxo alkylidene catalysts.

The first cationic molybdenum oxo complexes were synthesized and immobilized on partially dehydroxylated silica. Vastly enhanced catalytic activity for terminal olefins was found compared to their neutral congeners.     

De Novo Designed α-Helical Coiled-Coil Peptides as Scaffolds for Chemical Reactions

Coiled coils (CCs) are well-understood protein-folding motifs. They appear in a variety of oligomer states and as homo- and heteromeric assemblies. This versatility and the general accessibility by de novo design makes them ideal building blocks for synthetic biology. This Minireview highlights the efforts being made in designing small peptide catalysts or reaction templates based on the CC scaffold. The first reports described autocatalysis or mediation of peptide ligation based on CC recognition. Over the years, the designs became more advanced, catalyzing ester hydrolysis, acyl transfer and redox reactions with partial enzyme-like reactivity. Due to the ability to control CC assembly, and, in heterodimeric systems, the association and dissociation, the CC motif has become a common peptide tag in chemical biology.     

Three-stage melting of an annelide-type copper complex. A new type of organized phase: Tegma crystals

The solid—solid phase transition of an annlide type copper complex are described. As the temperature is raised, three different constituents of the complex successively melt. In the temperature range 77–95°C, the polyethylene oxide chains and the paraffinic tail are both in a very mobile state while the copper complex sub-units still form a two-dimensional crystalline array.     

Anionic polymerization with alkaline earth counterion. III. Aging of oligo- and polystyryl-barium and -strontium in THF

The self-deactivation of polystyryl-barium and polystyryl-strontium in tetrahydrofuran (THF) results essentially from protonation by the solvent. The deactivation constant k_d of this reaction is independent of carbanion concentration, length, and functionality of chains. Relations of k_d with temperature are: polystyryl-barium: k_d = 6.25 × 10⁷ exp(-18,900/RT) sec^?1; polystyryl-strontium: k_d = 4.1 × 10⁶ exp(?16,000/RT) sec^?1. The self-deactivation of α,ω-dicarbanionic oligostyryl-barium is a nonrandomlike reaction. Residual living oligomers are left dicarbanionic even if an important deactivation occurs.