Showing papers in "Zeitschrift für Naturforschung A in 1988"

Onset of Turbulence in a Pipe

Abstract: Calcul de la turbulence dans une conduite a partir de l'equation de Navier-Stokes. L'analyse des simulations numeriques revele que de petites perturbations appelees «meres» induisent des perturbations plus importantes appelees «filles». Les «filles» determinent l'aspect de la turbulence tandis que les «meres» controlent le transfert d'energie de l'ecoulement de base au mouvement turbulent

The Viscosity Coefficients of Oriented Nematic and Nematic Discotic Liquid Crystals; Affine Transformation Model

Abstract: Abstract General expressions are derived for the anisotropy of the viscosity and for the Leslie coefficients governing the flow alignment subject to the assumption that the equipotential surfaces for the interaction of oriented nonspherical molecules in the nematic or nematic discotic phase of a liquid crystal are related to a spherical interaction potential by an affine transformation. In particular, for uniaxial particles, the seven independent viscosity coefficients of a nematic are given in terms of the two viscosity coefficients of a reference fluid of spherical particles and the axes ratio of the nonspherical molecules. The theory is compared with results from nonequilibrium molecular dynamics computer simulations and with experimental data.

Spin-Lattice Relaxation of Water in Cement Gels

Abstract: The time evolution of the magnetization relaxation recovery of exchangeable water in ordinary Portland cement and white cement have been studied as functions of the hydration time together with the temperature and Larmor frequency dependence of the proton spin-lattice relaxation rates. The water self-diffusion coefficient was also studied as a function of both the hydration time and the diffusion time. The results show that the exchangeable water relaxes via cross relaxation to the gel protons, which are in turn relaxed via spin diffusion to paramagnetic impurities. The roughness of the gel-water interface and the wide distribution of pore sizes result in a stretched exponential nuclear magnetization relaxation recovery with the exponent reflecting the fractal geometry of the hydrating cement gel.

Molecular Dynamics Study of a Lithium Ion in Ammonia

Abstract: Abstract A Molecular Dynamics simulation of a solution of one Li+ in 215 NH3 molecules has been performed at an average temperature of 235 K. A newly developed flexible model for NH3 is employed and the Li +−NH3 interactions are derived from ab initio calculations. The structure of the solution is described by radial distribution functions and the orientation of the molecules. A solvation number of six is found for Li+ and a strong preference of the solvation shell molecules exists for an orientation where the Li + −N vector and the dipole moment direction of NH3 are parallel. The self-diffusion coefficient, the hindered translational motions and librations are calculated separately for the ammonia molecules in the solvation shell and in the bulk. The effect of Li + on intramolecular geometry and vibrations is reported.

Rotational Tunnelling of CD3Groups in Molecular Crystals as Studied by NMR Spectra

Abstract: Abstract Tunnelling frequencies of rotating CD3 groups in solids between about 20 kHz and 2 MHz may be obtained from the 2H NMR spectra. The theory of the spectral response is developed where quadrupole and dipole-dipole interactions as well as rotational tunnelling are taken into account. Features characteristic of tunnelling, which distinguish the spectra from those of rapidly reorienting deuterated methyl groups, are found from analytically calculated spectra even for the case of very large tunnel splittings. Numerical calculations have been performed for various conditions to deter mine the tunnel frequency. Experimental spectra measured at 45 MHz and low temperatures have revealed the appearance of rotational tunnelling in CD3I, CD3COONa, and (CD3COO)2Cu • H2O. In the latter case, a tunnelling frequency of 608 kHz has been extracted from the spectrum at 27 K.

On Symbolic Dynamics of One-Humped Maps of the Interval

Abstract: Abstract We present an explicit construction of minimal deterministic automata which accept the languages of L-R symbolic sequences of unimodal maps resp. arbitrarily close approximations thereof. They are used to study a recently introduced complexity measure of this language which we conjecture to be a new invariant under diffeomorphisms. On each graph corresponding to such an automaton, the evolution is a topological Markov chain which does not seem to correspond to a partition of the interval into a countable number of intervals.

Experimental Investigation of Spatial Pattern Formation in Physical Systems of Activator Inhibitor Type

Abstract: Abstract We investigate experimentally stationary stable states of activator (w) inhibitor (υ) type systems corresponding to the reaction diffusion equation δ · υ̇ = Δυ + w - υ; ẇ = σ Δw + f(w) - υ; δ, σ = const > 0 with f(w) monotonically increasing for small and decreasing for large |w|. We first describe some general mathematical properties and present qualitative results obtained from numerical calculations. We then investigate experimentally electrical networks described by the spatially discretized version of the above equation. Calculation and experiment are in good agreement. We also interprete a two dimensional-network as an equivalent circuit for a composite material consisting of a linear and a nonlinear layer with an s-shaped current density electric field characteristic. This model is used for a phenomenological description of spatial structures and global current voltage characteristics observed experimentally in pin-diode like and gas discharge devices. The model accounts very well for the experimental results obtained so far. It is concluded that the above equation and the corresponding experimental setup are of great interest for fundamental investigations of self controlled processes in nature.

The Vibrational State Dependence of the 14N Quadrupole Coupling Tensor in Aniline . A Microwave Fourier-Transform Study Combined with Semirigid Bender Calculations

Abstract: Abstract Experimental values for the diagonal elements of the 14N quadrupole coupling tensor derived from analysis of the hyperfine structure of low-J rotational transitions are reported for the two lowest states of the amino wagging mode. They show the expected difference due to averaging over the large amplitude amino wag. A semirigid bender analysis of previously published infrared data provides the inversion angle expectation values for use in a quantum-chemical calculation of the corresponding effective Coulomb field gradients at the nitrogen nucleus. The results are compared to those obtained earlier for the related molecules vinylamine, cyanamide and ammonia.

Molekulares Verdrillungsvermögen von Kohlenhydrat-Derivaten

Abstract: For the first time carbohydrate derivatives were systematically studied as doping material for a liquid crystalline phase. These compounds can exhibit a large twisting power. It may be controlled by the configurations of single chiral centers. A series of phemomena are realized in the cholesteric phase such as helix inversion, large or small temperature dependencies of the pich, as well as broad blue phases

Distortions in Power Spectra

Abstract: Abstract Power spectra are commonly used in microwave Fourier transform spectroscopy. We point out distortions, which may lead to erroneous frequency measurements.

Notizen: Fast Pulsed Laser Induced Electron Generation for Electron Impact Mass Spectrometry

Abstract: A new ultra fast electron impact (EI) ion source is pre­ sented that produces a very short, high intensity electron beam, allowing medium resolution mass spectra to be re­ corded without pulsing the ion accelerating voltages in a time-of-flight mass spectrometer (TOF-MS). The ion source requires minimum modification of any TOF-MS equipped with an electrostatic ion reflector and UV-laser. El-spectra are presented for comparison with literature spectra.

Fluoreszenz-Untersuchungen an Distyrylbenzolen

Abstract: Abstract The absorption and fluorescence of three isomeric distyrylbenzenes are investigated as function of temperature. From the fluorescence decay times and fluorescence quantum yields two classes of oligostyrylarenes can be distinguished. A decisive criterion for this classification is, whether the first excited singlet state S1 belongs to an allowed or forbidden transition S0→S1.

Structural and Dynamical Properties of an LiCl · 3H2O Solution

Abstract: An MD simulation of an 18,5 model LiCl aqueous solution was performed with the flexible Bopp-Jancso-Heinzinger model for water, ion-water pair potentials derived from ab initio calculations and the ion-ion interactions described by a potential of Born-Mayer-Huggins (BMH) type. The comparison with a simulation of the same system, where the ion-ion interactions were described by a (12-6) Lennard-Jones + Coulomb a potential, demonstrates that such a change affects not only the ion-ion but also the ion-water radial distribution functions significantly, and that the results with the BMH potential conform better to X-ray results. The self-diffusion coefficients for water and the ions are found to be lower by almost one order of magnitute compared with dilute solutions

General Relations between Molecular Sensitivities and Their Physical Content

Abstract: Abstract The molecular Legendre transformed representations are reexamined. They correspond to alternative sets of parameters defining the constraints imposed on the system. In the global description the molecular (equilibrium) states are defined by the global state parameter G = (N or μ) and the state function f (r) = [υ(r) or n(r)]; N is the system number of electrons, μ its chemical potential, υ stands for the external potential due to the nuclei, and n is the electron density. In the local description, capable of defining non-equilibrium molecular density distributions, the state is defined by two state functions, g(r) = [n(r) or μ (r)] and f (r); here μ (r) is the local chemical potential of the electron distribution in small volume element around r. The Maxwell cross differentiation identities in both approaches, relating various molecular sensitivities, are physically interpreted. A qualitative discussion of general relations between molecular sensitivities is given and their physical content and possible applications are commented upon.

2H NMR patterns of methylaromatics adsorbed on graphite

Abstract: 2H NMR solid state powder patterns were measured at one monolayer coverage of benzene on the non graphitized carbon black Spheron, and of toluene, p-xylene, mesitylene, and mellitene (hexamethylbenzene) on the graphitized carbon black Graphon. Benzene on Spheron, in contrast to Graphon, exhibits a typical Pake type powder pattern shape demonstrating the minor influence of paramagnetic centers on the pattern shapes as well as confirming the formerly introduced shielding anisotropy interaction in the case of graphite. From the highly asymmetric appearance of the 2H powder patterns for the adsorbed methyl-aromatics on Graphon the flat orientation of these molecules on graphite as well as details of the rotational molecular motions are deduced.

On the Global Quantum Dynamics of Multi- Lattice Systems with Non-linear Classical Effects

Abstract: Abstract The microscopic dynamics for a class of long range interacting multi-lattice quantum systems is constructed in the thermodynamical limit by means of operator algebraic concepts. By direct estimations the existence of the limiting Schrödinger dynamics is proven for a set of states, which comprises also globally non-equilibrium situations. The expectation values of the classical observables in the pure phase states are shown to satisfy a set of coupled non-linear differential equations. The limiting Heisenberg dynamics is derived as a W*-automorphism group in the partially universal von Neumann algebra which corresponds to the selected set of states; it is in general, however, not σ-weakly continuous in the time parameter.

Reversible Energy Transfer and Fluorescence Decay in Solid Solutions

Abstract: Abstract The article deals with the influence of reversible excitation energy transfer on the fluorescence decay in systems with random distribution of molecules. On the basis of a hopping model, we have obtained an expression for the Laplace transform of the decay function and an expression for the average decay time. The case of dipole-dipole interaction is discussed in detail.

Methyl Group Rotation and Tunnellingin Crystals of 5-Membered Ring Molecules

Abstract: Abstract 1H NMR spin-lattice relaxation times T1 and -if accessible -level-crossing peaks and inelastic neutron scattering spectra have been measured for solid 2-and 3-methylfuran, 2-and 3-methylthiophene, 3-and 4-methylpyrazole, 1-methylimidazole, and 5-methylisoxazole. From the tunnel splittings, the torsional excitations and the NMR relaxation rates, the molecular dynamics of the methyl rotators has been evaluated between the limits of quantum tunnelling at low temperatures and thermally activated random reorientation at elevated temperatures.

Homoclinic and Heteroclinic Bifurcations in rf SQUIDs

Abstract: La methode de Melnikov est appliquee a la discussion des bifurcations entre orbites homocliniques et heterocliniques en fonction des parametres du SQUID commande par radiofrequences, dans les cas d'amortissement faible et fort. A cause de la complexite du potentiel, les integrales doivent etre calculees numeriquement. Pour certains ensembles de parametres, les previsions theoriques sont comparees aux solutions numeriques de l'equation du mouvement. L'accord est excellent

Magnetic field mapping and NMR imaging by multi-frequency SENEX

Abstract: Nuclear magnetic resonance imaging (MRI) of selectively excited protons has been used for magnetic field mapping in a 1.5 T whole body imager. The imaging has been performed with a modified two pulse SENEX sequence, adapted to single- or multislice operation. The usual spin-echo image is superimposed by field markers which are created by a periodic non-excitation. Results of the method in the fields of image quality control, influence of the susceptibility of an object, and shim and gradient current field response are presented.

2H-NMR Spectroscopy of tunneling ammonium ion: general site symmetry

Abstract: Abstract 2H-NMR powder spectra of tunneling ammonium-d4 ions are computed. A representation of the tunneling Hamiltonian is worked out in the basis of simple product spin wavefunctions. Secular parts of quadrupole and dipole Hamiltonians are taken into account. Examples of spectra are given for tunneling about one C2 or C3 axis, as well as for overall rotations in potentials of higher symmetry. Ranges of tunneling frequencies measurable from the spectra are given for each case. Characteristic shapes of the spectra allow recognition of various ground torsional level structures. Possible further applications and available data are discussed.

Fractionation of Hydrogen Isotopes in Aqueous Lithium Chloride Solutions

Abstract: Mesure a 25°C du rapport D/H du gaz H 2 en equilibre avec la vapeur d'eau au-dessus de solutions aqueuses de LiCl, a l'aide d'un catalyseur hydrophobe de Pt

TheInfrared Spectrum of CS2in the ν3Band Region

Abstract: Abstract A total number of 24 bands of the carbon disulfide isotopomers 32S212C, 34S12C32S, 33S12C32S, 34S212C, 32S213C, and 34S13C32S between 1555 and 1455 cm - 1 have been investigated. Vibrational frequencies and rotational constants have been determined for the bands.

Detection of Gas-Phase Hydrogentrisulfide, HSSSH

Abstract: Abstract The rotational spectrum of gas phase HSSSH has been recorded for the first time in absorption with the Cologne free-space-cell millimeterwave spectrometer. In the frequency region between 80 and 300 GHz a prominent series of Q-branches has been observed and assigned. The hitherto identified rotational lines unambigously arise from a perpendicular-type spectrum. In addition, successive J lines of the compact Q-branches show no indication of the easily detectable intensity alternation which arises when the molecule possesses an axis of symmetry due to nuclear spin statistical weights. Therefore the geometrical structure of the molecule does not have a C2 axis of symmetry. The present data are not complete enough for a unique structure determination. However on the basis of the present data we can definitely rule out conformations with C2 or C2v symmetry to be responsible for the strongest observed transitions of the spectrum. The molecule is not floppy as predicted from semi empirical MO calculations. On the contrary the conformation observed is rather rigidly locked into one position with no sign of possible internal rotation. We consider a sulfur triangle with the two hydrogen atoms sticking out symmetrically to the same side of the SSS-plane to be the most likely structure.

Acetonitrile-Water Solutions of Sodium Halides: Viscosity and Self-Diffusion of CH3CN and H2O

Abstract: Mesures des viscosites et des coefficients de diffusion du solvant mixte compose d'eau et d'acetonitrile et du melange d'halogenures de sodium dans ces solvants

Microwave Spectrum of Butyronitrile: Methyl Internal Rotation and 14N Quadrupole Coupling

Abstract: Abstract We determined the internal rotation parameters of butyronitrile, antiperiplanar and synclinal, in the internal rotation ground state. The 14N quadrupole coupling constants were refined.

Determination of Spectral Parameters in Microwave Fouriertransform Spectroscopy by Analysis of Time-Domain Signals

Abstract: Some disadvantages of using the Fouriertransform algorithm to obtain rotational spectra from microwave pulse induced transient signals are demonstrated. To overcome the difficulties, a fitting procedure is suggested which is based on the analysis of time-resolved data.

Dynamic Conductivity of Molten Salts. Part I: 3KNO3 · 2Ca(NO3)2 at Frequencies from 100MHz to 40GHz

Abstract: The dynamic ionic conductivity measured at frequencies from 100 MHz to 40 GHz, at temperatures between 422 K and 478 K, is found to display the dispersion that is characteristic of the so-called «universal dielectric response». The data form arcs in the complex conductivity plane, with centers below the real axis. The results are interpreted in terms of a non-Debye jump-relaxation model involving only reorientational relaxation time

A Numerical and Experimental Study of Reaction Front Propagation in Condensed Phase Systems

Abstract: Certain noncatalytic exothermic chemical reactions of the type solid-solid characterized by high values of activation energy and heat of reaction represent an example of strongly nonlinear chemically reacting systems. In these systems different types of propagating waves can be observed such as constant pattern, planar pulsating, and rotating waves. Numerical simulations in two and three spatial dimensions predict, qualitatively, the same behavior as experimentally observed. For geometrically large systems multihead spinning or erratic waves occur, which bifurcate from a planar pulsating front. In nonadiabatic systems the spinning wave is more resistant to extinction than the one-dimensional planar pulsating front.