A model for universal spatial variations of temperature fluctuations in turbulent Rayleigh-Bénard convection

We propose a theoretical model for spatial variations of the temperature variance σ~2( z, r)( z is the distance from the sample bottom and r the radial coordinate) in turbulent Rayleigh-Bénard convection(RBC).Adapting the "attached-eddy" model of shear flow to the plumes of RBC, we derived an equation for σ~2 which is based on the universal scaling of the normalized RBC temperature spectra. This equation includes both logarithmic and power-law dependences on z/λ_(th), where λ_(th) is the thermal boundary layer thickness. The equation parameters depend on r and the Prandtl number Pr, but have only an extremely weak dependence on the Rayleigh number Ra Thus our model provides a near-universal equation for the temperature variance profile in turbulent RBC.     

Concise Total Synthesis of Enigmazole A

An efficient entry into the phosphorylated marine macrolide enigmazole A is described. Enigmazole A interferes with c‐Kit signaling by an as yet unknown mode of action and is therefore a potential lead in the quest for novel anticancer agents. Key to success is a gold‐catalyzed cascade comprising a [3,3]‐sigmatropic rearrangement of a propargyl acetate along the periphery of a macrocyclic scaffold, followed by a transannular hydroalkoxylation of the resulting transient allenyl acetate. This transformation mandated the use of a chiral gold catalyst to ensure a matching double‐asymmetric setting. Other noteworthy steps are the preparation of the oxazole building block by a palladium‐catalyzed C?H activation, as well as the smooth ring‐closing alkyne metathesis of a diyne substrate bearing a propargylic leaving group, which has only little precedent.     

Starke erleichterung der carbonylolefinierung mit organoschwermetall-reagenzien durch eine diphenylphosphorylgruppe im reagenz

Carbonyl olefination with a lithiomethane, having besides the group P(O)Ph₂ at the methane C-atom an organo-heavy-metal group, occurs at low temperatures to give exclusively diphenylphosphoryl substituted olefins.     

The Hume-Rothery rules and phase stabilities in noble metal alloys

Results from martensitic transformations are used to evaluate stabilities of the equilibrium phases in the Hume-Rothery electron compounds based on Cu, Ag and Au, and to give arguments why the electron concentration plays such an important role in the selection of the crystal structures. It is shown that the vibrational entropy difference observed for the martensitic transformation from ordered bcc to the close packed martensite and its e/a dependence can also account for the entropy difference ΔS ^α/β between the equilibrium α and β at high temperatures, and can be made largely responsible for the composition dependence of the (α+β) two phase field. The enthalpy of mixing can be decomposed into a small term which depends on the average periodic lattice, which is different in α and β but which is nearly the same in all alloys studied, and a contribution which is due to the difference in the properties of the atoms and which can be expressed by pair interchange energies. This contribution depends strongly on the specific alloy system, but is independent of structure, which is compatible with a pair interchange energy depending only on pair distance but not on structure, as suggested by simple pseudopotential theory. The same pair interchange energies account also for long range order and the critical ordering temperature. The evaluation for several alloy systems shows a surprisingly good agreement within this picture, and permits to understand better why the electron concentration plays such an important role also for other structures, although the energy contribution of the conduction electrons is only a small part of the total enthalpy of formation of any of the equilibrium structures.     

Synthesis and Spectroscopic Characterization of Arylated Selenoureas

Abstract

Selenoureas were prepared from isoselenocyanates and structurally characterized by spectroscopic methods and by X-ray crystal structure analyses.     

Electron transfer from excited F centres to F+, F+A and F+AA centres in CaO studied by optically detected magnetic resonance (ODMR)

ODMR measurements via the magnetic circular polarization of the emission (MCPE) of F_A(Mg) centres in CaO at 1.6 K showed besides the known F_A-ODMR lines a non zero MCPE, which vanishes at g ? 2. From the dynamical behaviour of this MCPE and the F_A luminescence is shown that an electron tunneling takes place from the excited T_1u F centre state to F⁺_A centres forming excited F_A centres. The ground state back tunneling rate is 10⁷ times smaller and cannot explain the observed electron transfer cycle. The back tunneling is also going via excited states. The results can be understood assuming a random centre distribution. A new luminescence band at 785 nm could be identified by ODMR as being due to tetragonal F_AA centres. The electron transfer is also observed between F and F⁺ and F⁺_AA centres.