Increased structural complexity leads to higher activity: peptides as efficient and versatile catalysts for asymmetric aldol reactions

[reaction: see text] Peptides containing a secondary amine and a carboxylic acid in a specific orientation to each other are presented as highly efficient catalysts for asymmetric aldol reactions: (1) their activity is considerably higher compared to that of proline, and (2) the enantioselectivity of the peptidic catalysts can be changed from (R)- to (S)-selectivity by simple modifications of the secondary structure.     

Redundancy in string cone inequalities and multiplicities in potential functions on cluster varieties

Journal of Algebraic Combinatorics - We study defining inequalities of string cones via a potential function on a reduced double Bruhat cell. We give a necessary criterion for the potential...     

The reactions of phenylacetylene with naphthalene complexes of europium and ytterbium. Molecular structure of [IYbC≡CPh(DME)2]2

Reactions of naphthaleneeuropium and naphthaleneytterbium, C₁₀H₈Ln(DME) (Ln = Eu or Yb), with phenylacetylene are accompanied by the formation of the C-C bond and yield the complexes of composition Ph₂C₄H₂Ln(DME)₂. Hydrolysis of the Ph₂C₄H₂Ln(DME)₂ complexes affords a mixture of isomers of 1,4-diphenyl-1,3-butadiene. Reactions of C₁₀H₈[LnI(DME)₂]₂ with PhCCH yield mixed iodine-ethynyl complexes [ILn(-CCPh)(DME)₂]₂. According to the data of X-ray diffraction analysis, the ytterbium complex consists of two YbI(DME)₂ units bonded through two bridging CCPh groups. The crystals of this complex belong to the space groupP2₁/c. The central cyclic Yb-C-Yb-C fragment is planar; the C(I)-Yb(I)-C(I) angle is 86.4(3)°. The Yb-C bond lengths are 2.584(8) and 2.603(9) Å.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2101–2104, August, 1996.     

A possible explanation of countergradient fluxes in homogeneous turbulence

This study addresses the phenomenon of persistent countergradient (PCG) fluxes of momentum and heat (density) as observed in homogeneous turbulence forced by shear and stratification. Countergradient fluxes may occur at large scales when stratification is strong. However, they always occur at small scales, independently of stratification. A conceptional model is introduced to explain PCG fluxes at small scales as the result of the collision of large-scale fluid parcels. The large parcels collide under the driving force of inclined vortex structures (in a shear-dominated flow) or of buoyancy (in a strongly stratified shear flow). This collision model also explains the PCG heat flux in an unsheared stratified flow with zero average momentum flux. It is found that the energy of the small-scale PCG motions is provided (i) by quick transport of kinetic energy from the scales of production to relatively slowly dissipating scales if the flow is shear-driven and (ii) by conversion of available potential energy to kinetic energy at small scales when the flow is stratified. The collision mechanism is an inherent property of the turbulence dynamics. Therefore, the PCG fluxes at small scales reflect a universal character of homogeneous turbulence, and are found over a large range of Reynolds numbers. The Prandtl (or Schmidt) number influences the rate of dissipation of temperature (or density) variance but not the dissipation rate of the velocity variance. In stratified flows, therefore, the number directly affects the strength of the PCG heat flux at small scales. It is found, however, that the PCG momentum flux is also altered slightly when the Prandtl number is large enough to sustain small buoyantly moving parcels after collision.     

Quantitative depth profiling in glow discharge spectroscopies – A new deconvolution technique to separate effects of an uneven erosion crater shape

An algorithm is presented as a concept for the quantification in direct current and radiofrequency glow discharge (GD) modes for GD optical emission spectroscopy. The algorithm is divided into excitation and sputtering part and thus it is possible to distinguish between the different excitation processes and to consider equivalent sputtering crater formations in both modes. Intensity-time profiles are affected corresponding to the method by several effects. One important effect is that sputtering occurs at a single time in different depths because of curved crater bottoms, this is usually called crater effect. The main purpose is to introduce an iterative deconvolution technique which for the quantification numerically takes into account the curved sputtering crater bottom. Input data for the deconvolution technique are the calibrated mass-time profile, the partial densities of the sample constituents and the measured final shape of the sputtering crater. Using a relatively simple model for ion sputtering the deconvolution technique improves iteratively the calculated layer structure by means of information on crater formation. The mathematical handling is illustrated for the quantification of a depth profile of a multilayer sample of ten 100 nm layers. The resulting concentration-depth profile reflects excellently the real elemental distribution of the multilayer system.     

Raman- und infrorofspektroskopische Untersuchungen on Alkylderivaten der Arsensäure. V1 Die Schwingungsspektren der Dimethyl- und Diäthylarsinsäure sowie deren Reaktionsprodukte mit Chlorwasserstoff

RAMAN and IR Spectroscopic Investigation on Alkyl Derivatives of Arsenic-Acid. V. Vibrational Spectra of Dimethyl and Diethyl Arsinic Acid and their Reaction Products with HCl The RAMAN and IR spectra of (CH₃)₂AsO₂H–partially deuterated–and (C₂H₅)₂AsO₂H and of the reaction products of these acids with HCl (solid and in concentrated aqueous solution) are discussed. The symmetry of the R₂AsO₂H skeleton is C_s. of the [R₂As(OH)₂]+ ion very probably C_2v. Whereas (CH₃)₂AsO₂H gives with HCl only a compound (CH₃)₂ACO₂H · HCl (connected by H bonds), the weaker (C₂H₅)₂AsO₂H is able to form a salt [(C₂H₅)₂As(OH)₂]Cl. The H bonds in the substances are discussed.