Preparation and evaluation of N‐sulfinyl dienophiles for asymmetric hetero‐diels‐alder reactions

A series of N‐sulfinyl dienophiles 1c‐i has been screened in asymmetric hetero‐Diels‐Alder reactions using chiral bis(oxazoline)copper(II) and ‐zinc(II) triflates. The survey pointed out N‐sulfine 1c (R ? P(?O)(OPh)₂) as the most promising N‐sulfine regarding yield and stereoselectivity (up to 97% ee). The relative configurations, and in one case the absolute configuration, of the HDA adducts were established by X‐ray analysis.     

Characterization of ammonia two-photon laser-induced fluorescence for gas-phase diagnostics

Two-photon laser-induced fluorescence (LIF) of ammonia (NH₃) with excitation of the C′-X transition at 304.8 nm and fluorescence detection in the 565 nm C′-A band has been investigated, targeting combustion diagnostics. The impact of laser irradiance, temperature, and pressure has been studied, and simulation of NH₃-spectra, fitted to experimental data, facilitated interpretation of the results. The LIF-signal showed quadratic dependence on laser irradiance up to 2 GW/cm². Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm², i.e., above irradiances attainable for LIF imaging. Maximum LIF-signal was obtained for excitation at the 304.8 nm bandhead; however, lower temperature sensitivity over the range 400–700 K can be obtained probing lines around 304.9 nm. A decrease in fluorescence signal was observed with pressure up to 5 bar absolute and attributed to collisional quenching. A detection limit of 800 ppm, at signal-to-noise ratio 1.5, was identified for single-shot LIF imaging over an area of centimeter scale, whereas for single-point measurements, the technique shows potential for sub-ppm detection. Moreover, high-quality NH₃-imaging has been achieved in laminar and turbulent premixed flames. Altogether, two-photon fluorescence provides a useful tool for imaging NH₃-detection in combustion diagnostics.     

Chiral hetero- and carbocyclic compounds from the asymmetric hydrogenation of cyclic alkenes

Several types of chiral hetero- and carbocyclic compounds have been synthesized by using the asymmetric hydrogenation of cyclic alkenes. N,P-Ligated iridium catalysts reduced six-membered cyclic alkenes with various substituents and heterofunctionality in good to excellent enantioselectivity, whereas the reduction of five-membered cyclic alkenes was generally less selective, giving modest enantiomeric excesses. The stereoselectivity of the hydrogenation depended more strongly on the substrate structure for the five- rather than the six-membered cyclic alkenes. The major enantiomer formed in the reduction of six-membered alkenes could be predicted from a selectivity model and isomeric alkenes had complementary enantioselectivity, giving opposite optical isomers upon hydrogenation. The utility of the reaction was demonstrated by using it as a key step in the preparation of chiral 1,3-cis-cyclohexane carboxylates.     

A kinetic study of the high temperature rearrangement of 4‐ethyl‐3,5‐diphenyl‐4H‐1,2,4‐triazole

The kinetics of the thermal rearrangement 4‐ethyl‐3,5‐diphenyl‐4H‐1,2,4‐triazoles, 1 , to the corresponding 1‐ethyl‐3,5‐diphenyl‐1‐alkyl‐1H‐1,2,4‐triazoles, 2 , was studied in 15‐Crown‐5 and octadecane at 330 °C. The reaction was very slow in octadecane but proceed well in 15‐Crown‐5. The reaction order for the reaction was not constant but changed from an initial second order rate law towards a first order rate law as the reaction progressed. This was confirmed by the concentration dependent reaction order, n_c, which was larger than the time dependent rate law, n_t. The rationale for the observation was, that at high substrate concentrations the reaction order was second order while at lower concentrations a competing solvent assisted reaction plays an increasing important role. The data were in agreement with a mechanism in which the neutral 4‐alkyl‐triazoles in an intermolecular nucleophilic displacement reaction form a triazolium triazolate, which in a subsequent nucleophilic reaction gives the observed product.     

A determinantal approach to spin-orbit configuration interaction

A general configuration interaction (CI) algorithm incorporating one- and two-electron spin-orbit operators is presented. The algorithm is determinant based and enables the use of highly vectorized non-relativistic algorithms in the most operation-intensive part. Excitations between α and β spin orbitals are avoided in the time consuming parts by performing separate S ₊ and S ₋ operations. The relativistic CI expansions are often very large, so the algorithms require only the presence of segments of vectors in memory. Double-group symmetry is fully accounted for and time-reversal symmetry is exploited for both even and odd numbers of electrons. Received: 12 October 1996 / Accepted: 24 January 1997     

Weak convergence of stochastic integrals related to counting processes

Probability Theory and Related Fields -     

Interaction between shed vorticity, free surface waves and forced roll motion of a two-dimensional floating body

A vortex tracking method is used to study forced harmonic roll motion of a two-dimensional floating body with sharp corners. The effect of free surface waves is incorporated. Errors due to numerical and physical problems are discussed. The presented test cases show that free surface waves have an effect on the vortex generation, whereas the free shear layers do not have a noticeable influence on the free surface waves. The results show that roll damping due to eddymaking and wave generation cannot be separated.     

A local directional ghost cell approach for incompressible viscous flow problems with irregular boundaries

An immersed boundary method for the incompressible Navier–Stokes equations in irregular domains is developed using a local ghost cell approach. This method extends the solution smoothly across the boundary in the same direction as the discretization it will be used for. The ghost cell value is determined locally for each irregular grid cell, making it possible to treat both sharp corners and thin plates accurately. The time stepping is done explicitly using a second order Runge–Kutta method. The spatial derivatives are approximated by finite difference methods on a staggered, Cartesian grid with local grid refinements near the immersed boundary. The WENO scheme is used to treat the convective terms, while all other terms are discretized with central schemes. It is demonstrated that the spatial accuracy of the present numerical method is second order. Further, the method is tested and validated for a number of problems including uniform flow past a circular cylinder, impulsively started flow past a circular cylinder and a flat plate, and planar oscillatory flow past a circular cylinder and objects with sharp corners, such as a facing square and a chamfered plate.     

An advanced decision support system for European disaster management: the feature of the skills taxonomy

Central European Journal of Operations Research - Mankind has faced a huge increase in severe natural and man-made disasters worldwide in the last few years. Emergency responders on a strategic,...     

Some infinite series related to Feynman diagrams

Results are presented for some infinite series appearing in Feynman diagram calculations, many of which are similar to the Euler series. These include both one-, two- and three-dimensional series. The sums of these series can be evaluated with the help of various integral representations for hypergeometric functions, and expressed in terms of ζ(2),ζ(3), the Catalan constant G and Cl₂(π/3) where Cl₂(θ) is Clausen's function.