Statistics of numerically generated turbulence

This paper considers numerically generated turbulence obtained by integrating the complete time-dependent three-dimensional Navier-Stokes equations. The simulated unidirectional turbulent flow, bounded by two parallel planes, is strongly inhomogeneous in the direction normal to the planes but homogeneous in the parallel directions. The resulting flow field, which is considered a numerical realization of fully developed turbulent channel flow, contains detailed information on spatial coherent flow structures as well as on the time-dependency and statistics of the three-dimensional velocity and pressure fields. Focussing here on the statistics of the numerically generated turbulence, second-moments and higher-moments are presented and compared with the most recent PTV and LDV laboratory measurements. It is concluded that direct numerical simulations are an invaluable approach to turbulence which complements field studies and laboratory investigations. Numerical experiments are now becoming a principal source of detailed and reliable information, which play a key role in the deepening of our understanding of turbulent flow phenomena.     

Synthesis of a new amino acid based cross-linker for preparation of substrate selective acrylic polymers

The synthesis of N,O-bisacryloyl-L-phenylalaninol is described as well as its application in the preparation of a L-phenylalanine ethyl ester selective acrylic polymer.     

Study of some silver-thiol complexes and polymers: Stoichiometry and optical effects

The silver complexes of various thiols of low molecular weight have been studied by amperometric and spectrophotometric titrations. Cysteine and cysteamine form complexes with silver containing up to 1.5 Ag⁺/—SH. The ligands participating in the binding of silver are the —SH and amino groups. Under some conditions it was found that 1:1 thiol-silver polymers were formed. These polymers probably have the general structure ? Ag? S(R)? Ag? S(R)? . Some of them were prepared in pure form and subjected to various studies. Molecular weight determinations of the cysteine-silver and glutathione-silver polymers yielded values of 4000–7000 daltons. Studies on the UV absorption characteristics revealed that all of the polymers showed some type of regular band pattern. The cysteamine-silver polymer is able to sensitize photographic emulsions.     

Low energy electron diffraction from Na(110) and Na2O(111) surfaces

Surfaces ofNa(110) are grown, investigated and oxidised to give Na₂O(111) surfaces. LEED spectra are taken for these surfaces and compared with theory to determine the surface composition of sodium oxide: the surface terminates the crystal in an integral number of electrically neutral NaONa sandwiches, with a bulk-like inter layer spacing. The effective Debye temperature for the Na(110) surface was found to be 107 K.     

Study of the oxidative half‐reaction catalyzed by a non‐heme ferrous catalytic center by means of structural and computational methodologies

Deacetoxycephalosporin C synthase (DAOCS) is a mononuclear ferrous enzyme that catalyzes the expansion of the five‐membered thiazolidine ring of the penicillin nucleus into the six‐membered dihydrothiazine ring of the cephalosporins. In the first half‐reaction with dioxygen and 2‐oxoglutarate, a reactive iron–oxygen species is produced that can subsequently react with the penicillin substrate to yield the cephalosporin. We describe quantum mechanical calculations of the first part of the reaction based on the high‐resolution structures of the active site of DAOCS and its complexes with ligands. These studies are aimed at understanding how the reactive species can be produced and contained in the active site of the enzyme. The results demonstrate the priming of the active site by the co‐substrate for oxygen binding and hint to the presence of a stable iron–peroxo intermediate in equilibrium with a more reactive ferryl species and the formation of CO₂ as a leaving group by decarboxylation of 2‐oxoglutarate. A conclusion from these studies is that substitution of CO₂ by the penicillin substrate triggers the oxidation reaction in a booby‐trap‐like mechanism. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Size-dependent carbon monoxide adsorption on neutral gold clusters

We report on experiments probing the reactivity of neutral Au(n) clusters, n = 9-68, with carbon monoxide. The gold clusters are produced in a pulsed laser vaporization cluster source, operated at room temperature (RT) or at liquid-nitrogen temperature (LNT), pass through a low-pressure reaction cell containing CO gas, and are subsequently laser ionized. The reaction probabilities are determined by recording mass abundance spectra with time-of-flight mass spectrometry. The main observations are a strong temperature dependence and a remarkable size dependence. Upon cooling of the cluster source to LNT, the reactivity increases substantially. At LNT, the reaction probabilities for Au(n) with the first CO molecule are about a factor 10 higher than at RT. Moreover, adsorption of two, three, and even four CO molecules is observed, in contrast to RT clusters which at most adsorb one CO molecule. This temperature dependence is related to the lifetime of the cluster-molecule complexes, being much longer for cold clusters. The observed striking size dependence is similar at both temperatures and is discussed in terms of the electronic structure effects.     

Angle resolved ion scattering spectroscopy reveals the local topography around atoms in a liquid surface

Neutral impact collision ion scattering spectroscopy under normal incidence is known to yield the concentration depth profiles of all elements except hydrogen at the surface of liquids and other amorphous material. In the evaluation of the data one tactically has to assume that the top surface layer and the adjacent layers are laterally homogeneous. In the present paper we establish that the angular resolved mode of this spectroscopy is able to test with high accuracy whether the lateral homogeneity is valid and-if this is not the case-in which way the top layer is structured. In particular, it is possible to map out the local environment of selected atoms. We expect that this so far inaccessible information on the local topography at liquid surfaces will have an impact on the understanding of reactions at the gas/liquid interface.     

Speciation in the aqueous H+/H2VO4-/H2O2/citrate system of biomedical interest

The speciation in the quaternary aqueous H+/H2VO4-/H2O2/citrate (Cit3-) and H+/H2VO4-/Cit3-/L-(+)-lactate (Lac-) systems has been determined at 25 degrees C in the physiological medium of 0.150 M Na(Cl). A combination of 51V NMR integral intensities and chemical shift (Bruker AMX500) as well as potentiometric data (glass electrode) have been collected and evaluated with the computer program LAKE, which is able to treat multimethod data simultaneously. The pKa-values for citric acid have been determined as 2.94, 4.34 and 5.61. Altogether six vanadate-citrate species have been found in the ternary H+/H2VO4-/Cit3- system in the pH region 2-10, only two of which are mononuclear. Reduction of vanadium(V) becomes more pronounced at pH < 2. Solutions, in which reduction occurred to any extent, were excluded from all calculations. In the quaternary H+/H2VO4-/H2O2/Cit3- system, eight complexes have been found in addition to all binary and ternary complexes over the pH region 2-10, including three mononuclear species. Equilibria in general are fast, but the significant and rapid decomposition of peroxide in acidic solutions limited the final model to pH > 4. In the quaternary H+/H2VO4-/Cit3-/Lac- system, two mixed-ligand species have been determined, with the compositions V2CitLac2- and V2CitLac3- (pKa = 5.0). To our knowledge, this is the first time such complexes have been reported for vanadium(V). 51V NMR chemical shifts, compositions and formation constants are given, and equilibrium conditions are illustrated in distribution diagrams as well as the fit of the model to the experimental data. When suitable, structural proposals are given, based on 13C NMR measurements and available literature data of related compounds.     

Kinetic investigation of the sorption of water by barium chloride monohydrate

Using thermogravimetry under controlled water vapour pressure, the kinetics for the rehydration reaction BaCl₂·H₂O(s) + H₂O(g) → BaCl₂ · 2 H₂O(s) was investigated as a function of various parameters of relevance in chemical heat pumps, i.e. the number of cycles, storage time, extent of dehydration, and particle size. The effect of cycling is interpreted in terms of a fragmentation process, as confirmed by scanning electron microscopy studies. Reaction rate isotherms were determined at four different temperatures (34.5, 40.3, 44.5 and 50.5°C), in the pressure range 18–66 torr. An inhibition pressure domain is observed. This effect is assumed to result from the formation of capillary condensed water. The presence of adsorbed water is clearly evidenced by wide-line NMR measurements.