Diastereoselective addition of organozinc reagents to 2-alkyl-3-(arylsulfanyl)propanals

The preparation of compounds incorporating the 3-hydroxy-2-methyl-1-alkyl moiety of high diastereomeric purity is described. Such compounds can serve as potential building blocks for the preparation of several kinds of natural products. Diastereoselective synthesis of two potential pine sawfly pheromone components, one the pure racemic threo-isomer of 3-methylpentadecan-2-ol and the other the racemic erythro-isomer of 3-methyltridecan-2-ol are described. The diastereoselective addition of R₂Zn (R=Me, Et and n-Bu) to several 2-alkyl-3-(arylsulfanyl)propanals in the presence of a Lewis acid and CH₂Cl₂ as solvent was studied. An excellent diastereomeric ratio (95/5 anti-Cram/Cram) was obtained with 2-[(phenylsulfanyl)methyl]pentanal, 2-[(phenylsulfanyl)methyl]decanal and 2-[(phenylsulfanyl)methyl]dodecanal and Me₂Zn in the presence of TiCl₄.     

Monitoring microstructural material changes in paper through microscopic speckle correlation rate measurements

A system for real-time digital speckle correlation rate measurements is developed and applied on printing paper in the micro-scale. The system consists of a continuous laser and a CCD camera connected to a PC. To measure on objects in the micro-scale the system is connected to a stereomicroscope where one of the channels is used for imaging and the other channel is used for illumination of the object surface. The system is evaluated through measurements on printing paper over an area of 1 mm² and with a spatial resolution of 30 μm on the object. The results show that the system is able to monitor the dynamics in paper during both wetting and drying in real-time over a time scale ranging from one-tenth of a second to several hours.     

Bond lengths and reorganization energies in fullerenes and their ions

A very simple tight binding method with bond-length-dependent couplings, similar to the Su-Schrieffer-Heeger model, is described and applied to fullerenes in different charge states. The bond lengths, calculated from the π bond orders, are in good agreement with results obtained by ab initio methods which include correlation, where comparisons can be made. The Jahn-Teller distortions are found to be small for the examples C₂₀, C₂₈-T_d, and C₆₀-I_h. Often many possible distortions are found, for example two different D_5d distortions for the C₂₀ spin triplet state. Bond reorganization energies (λ_b) for reduction and oxidation, which are measures for trapping efficiency and vibronic coupling, are obtained using a parabolic approximation of the energy surface. Received: 3 December 1996 / Accepted: 3 March 1997     

Intense mass-separated beams of halogens and beta-delayed neutron emission from heavy bromine isotopes

Improved production yields of short-lived halogens were obtained from a ThO₂ target, irradiated with 600 MeV protons, in combination with a negative surface ionization source. Mass-separated samples were studied by decay spectroscopy. Production yields of radioactive isotopes of chlorine, bromine, iodine and astatine are presented. Half-lives and relative neutron emission probabilities were measured for the heavy bromine isotopes^89?92Br. Normalizing to earlier publishedP _n values for⁸⁹Br, the results are:⁸⁹Br (4.30±0.14s,P _n =13.6±0.8%),⁹⁰Br (1.92±0.06s,P _n =24.8±1.5%),⁹¹Br (0.53 ±0.03 s,P _n =30.1 ±2.1%), and⁹²Br (0.31 ±0.02 s,P _n =34.6±2.5%). Energy spectra ofβ-delayed neutrons were measured.     

Numerical solution of parabolic integro-differential equations by the discontinuous Galerkin method

The numerical solution of a parabolic equation with memory is considered. The equation is first discretized in time by means of the discontinuous Galerkin method with piecewise constant or piecewise linear approximating functions. The analysis presented allows variable time steps which, as will be shown, can then efficiently be selected to match singularities in the solution induced by singularities in the kernel of the memory term or by nonsmooth initial data. The combination with finite element discretization in space is also studied.

     

Back donation in Ni(CO)4: Comments on papers by K. H. Johnson and by K. H. Johnson and U. Wahlgren

It is shown that multiple scattering Xα calculations on Ni(CO)₄ predict a strong back-donation effect contrary to what is stated in previous papers on the subject.     

Calorimetry of dual limitations in yeast cultures

Chemostat cultures of one aerobic fermenting yeast (Saccharomyces cerevisiae) and one aerobic respiring yeast (Kluyveromyces fragilis) have been grown under dual C + N limitation in an isothermal reaction calorimeter. The dual limitations resulted in uncoupled oxidation of part of the glucose, which enabled the culture to adapt the ratio of nitrogen to carbon consumption rates precisely to the N/C ratio that was fed into the calorimeter. This conclusion has been reached based on the calorimetric measurements, which reflect uncoupled respiration conspicuously as abnormal heat yields or ratios of heat release per biomass grown.     

A surface state on TiN(100); calculated photoemission results

The photoemission spectrum of TiN(100) at normal electron emission has been calculated for two different surface layer potentials. The results do, when compared to experimental data, give strong evidence of a Tamm surface state on TiN(100) induced by a change in the electrostatic potential at the surface.     

Superheavy elements and variations in the nuclear skin thickness

Although the interpretation of the data is uncertain, the recently claimed empirical evidence of element Z = 126 and some of its neighbours, provides the impetus for a reexamination of the nuclear one-body potentials, on the basis of which extrapolations have been made up to now. In particular the skin thickness parameters of the neutrons and protons are of interest and the authors presently consider the possibility that the nucleus may adjust its skin thickness to achieve a maximal shell energy.