Por?se Bl?cke

Ohne Zusammenfassung     

Chemical Characterization,Antioxidant and Antimicrobial Properties of Different Types of Tissue of Cedrus brevifolia Henry Extracts

This study aimed to determine the chemical composition of different types of tissue of Cedrus brevifolia Henry (Pinaceae) methanolic extracts, namely needles, twigs, branches, and bark. Cedrus brevifolia is a narrow endemic coniferous tree species of Cyprus, growing in a sole population in the mountainous area of Paphos Forest. Chemical analysis of the extracts was performed using liquid chromatography combined with time-of-flight high-resolution mass spectrometry (LC/Q-TOF/HRMS). The majority of the 36 compounds tentatively identified belonged to the flavonoids family. The extract of needles was the richest extract in terms of secondary metabolites. The extracts were studied for their antioxidant activity using the DPPH free radical scavenging assay. Additionally, the antibacterial activity was evaluated by determining both the minimum inhibitory concentration and the minimum bactericidal concentration against Staphylococcus aureus and Escherichia coli. All extracts demonstrated antioxidant property, while bark gave the highest antioxidant capacity (IC₅₀ value of 0.011 mg/mL) compared to the other tissues. Antibacterial activity was observed against both types of bacteria, with the extract of branches presenting the strongest activity against S. aureus (MIC, 0.097 mg/mL and MBC, 0.195 mg/mL). This is the first time that extracts of needles, twigs, branches, and bark of C. brevifolia are compared regarding their chemical composition as well as their antimicrobial and antioxidant properties.     

A parallel multi-configuration self-consistent field algorithm

A scalable multi-configuration self-consistent field (MCSCF) algorithm is described. The method for optimizing the orbital and configurational parameters is based upon the two-step Newton–Raphson approach with an augmented orbital Hessian matrix. A single copy of the two-electron integrals in the molecular orbital basis is distributed over the memory of all processors. Storage of the augmented Hessian is avoided by re-computing its elements as needed. A replicated data approach is used to parallelize the configuration interaction step. Scalability to 1024 processors is demonstrated.     

Adsorption reactions of alkali-halide nanocrystals: Identification of an important surface defect

The adsorption reactions of alkali-halide clusters have been investigated on a size-selected basis using flow-reactor methods. The reactivity of larger sodium-fluoride clusters [Na_nF_n?1]⁺ toward polar molecules NH₃ and H₂O shows a distinctive pattern as a function ofn at ambient temperature. Comparison with computed structures shows that aparticular kind of defect greatly facilitates the initial adsorption process. This defect can be formed by removal of an ion-pair from the face of an otherwise perfect nanocrystal, to create a basket-like opening for the adsorbed molecule. It is shown that this kind of defect occurs as a most stable low-temperature structure only to a certain size, after which a less reactive defect takes its place. The implications of these findings for the adsorption reactivity of the surfaces of ionic solids are briefly discussed.     

Chaos and algorithmic complexity

Our aim is to discover whether the notion of algorithmic orbit-complexity can serve to define chaos in a dynamical system. We begin with a mostly expository discussion of algorithmic complexity and certain results of Brudno, Pesin, and Ruelle (BRP theorems) which relate the degree of exponential instability of a dynamical system to the average algorithmic complexity of its orbits. When one speaks of predicting the behavior of a dynamical system, one usually has in mind one or more variables in the phase space that are of particular interest. To say that the system is unpredictable is, roughly, to say that one cannot feasibly determine future values of these variables from an approximation of the initial conditions of the system. We introduce the notions of restrictedexponential instability and conditionalorbit-complexity, and announce a new and rather general result, similar in spirit to the BRP theorems, establishing average conditional orbit-complexity as a lower bound for the degree of restricted exponential instability in a dynamical system. The BRP theorems require the phase space to be compact and metrizable. We construct a noncompact kicked rotor dynamical system of physical interest, and show that the relationship between orbit-complexity and exponential instability fails to hold for this system. We conclude that orbit-complexity cannot serve as a general definition of chaos.     

A new network model for the integer quantum Hall effect

A Landauer–Büttiker-type formulation of backscattering between pairs of opposite directed channels is used to describe the coupling at the nodes of a network. Physically, these nodes correspond to saddle points of a slowly varying lateral potential modulation in a 2D electron system in the high magnetic field regime. We show that the network can be solved without needing a transfer matrix as used by Chalker and Coddington. We use an exponential dependence of the coupling on the filling factor of the associated Landau level. We demonstrate that our network representation allows a quantitative modeling of almost every realistic sample geometry in the quantum Hall regime, including the effect of gate electrodes across a Hall bar.     

Light-emitting diodes based on poly(2,3-diphenyl-1,4-phenylene vinylene)

In this paper we report polymer light-emitting diodes based on (2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV), a novel π-conjugated polymer made by using the chlorine precursor route (CPR). Thin films of the precursor polymer were formed by spin-casting on indium-tin oxide (ITO) coated glass substrates, followed by thermal conversion to give DP-PPV thin films. Single layer DP-PPV LEDs were completed by thermally evaporating magnesium (Mg) electrodes. The electroluminescent characteristics of ITO/DP-PPV/Mg devices as well as variations between precursor polymer batches are presented. Bilayer LEDs were also made, for which tris(8-hydroxyquinoline)aluminum (Alq₃) was thermally sublimed on the fully converted DP-PPV films in vacuum, followed by Mg deposition. Both significant improvement in the quantum efficiency (up to 0.7% ph/el) and a reduction in the turn-on voltage of the device were found upon incorporation of the Alq₃ layer. These observations suggest that Alq₃ enhances the injection of electrons and also participates in the recombination process. © 1997 John Wiley & Sons, Ltd.     

SINNMR studies of acoustically induced rotation of suspended particles

Experiments and data analyses are reported on the 20 kHz acoustic manipulation of samples of trisodium phosphate dodecahydrate, of varying particle size, suspended in suitable support media. Data are obtained to expand the understanding and optimisation of the SINNMR (sonically induced narrowing of the nuclear magnetic resonance spectra of solids) technique, and use this to determine acoustically induced particle rotational correlation times. It is concluded that the average particle rotational correlation time decreases with increasing particle size and decreasing viscosity/density of the support media. Acoustic cavitation and accompanying interparticle collisions are shown to be important in the observation of SINNMR spectra.     

Free‐Radical Frontal Copolymerization: The Dependence of the Front Velocity on the Monomer Feed Composition and Reactivity Ratios

Frontal copolymerization is a process in which a spatially localized reaction zone propagates into a mixture of two monomers, converting them into a copolymer. In the simplest case of free‐radical copolymerization, a mixture of monomers and initiator is placed into a test tube. Reaction is initiated at one end of the tube, and a self‐sustained thermal wave, in which chemical conversion occurs, develops and propagates through the tube. We develop a mathematical model of the frontal copolymerization process and analytically determine the structure of the polymerization wave, the propagation velocity, maximum temperature, and degree of conversion of the monomers. Specifically, we examine their dependence on reactivity ratios as well as other kinetic parameters, monomer feed composition, and exothermicity of the reactions. Our analytic results are in good quantitative agreement with both direct numerical simulations of the model and experimental data, which are also presented in the paper.

Dependence of front velocity on monomer feed composition for different heat release parameters.     

Experimental results on J/ψ production by π±, K±, p and p incident on hydrogen at 39.5 GeV/c

J/ψ production at 40 GeV/c by π^±, K^±, p and $\text{p}$ incident on hydrogen has been studied and results compared with those obtained on tungsten in the same experiment. On hydrogen, J/ψ cross-section ratios relative to π^? have been measured to be (for $\text{x}{}_{\mathrm{<mtext>F</mtext>}}\text{> 0) σ(π}{}^{\mathrm{?}}\text{) : σ(π}{}^{\mathrm{+}}\text{) : σ(}\text{p}\text{) : σ(}\text{p}\text{) = 1 : (0.78 ± 0.09) : (0.83 ± 0.35) : (0.07 ± 0.04)}$. The suppression of the proton induced cross sections shows the importance of calence quark-antiquark fusiin J/ψ production at this energy (i.e. M_J²/_ψ/s=0.13).