Effect of the platinum content on the microstructure and micropore size distribution of Pt/alumina-pillared clays

The aim of this work is to study the effect of the platinum content (0-1.8 wt % Pt) on the microstructure of an alumina-pillared clay. For this purpose, the nitrogen physisorption data at -196 degrees C, the micropore size distributions of the supported platinum catalysts, and the hydrogen chemisorption results at 30 degrees C have been analyzed and compared. The preparation of the catalysts has modified the textural properties of the Al-pillared clay support, giving rise to a loss of surface area and micropore volume. After reduction at 420 degrees C, the presence of dispersed metallic platinum with mean crystallite size in the 22-55 A range has been found by hydrogen adsorption. Comparison of all results reveals that the platinum species block the micropore entrances by steric hindrance to nitrogen access as the platinum content increases.     

Polarity effects on ZnO films grown along the nonpolar [1120] direction

The surface electrical properties of ZnO thin films grown along the nonpolar [1120] direction have been investigated by Kelvin probe microscopy on a nanometer scale. Two different charge domains, with a 75 meV work function difference, coexist within the ZnO surface, which is covered by rhombohedral pyramids whose sidewalls are shown to be {1011}-type planes. The presence and relative orientation of the two kinds of charge domains are explained in terms of the atomic arrangement at the {1011} polar surfaces.     

Electron correlation in the second Landau level: a competition between many nearly degenerate quantum phases

At a very low-temperature of 9 mK, electrons in the second Landau level of an extremely high-mobility two-dimensional electron system exhibit a very complex electronic behavior. With a varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed nu=2+2/5 state separated from the even-denominator nu=2+1/2 state by an insulating phase and a nu=2+2/7 and nu=2+1/5 state surrounded by such phases. A developing plateau at nu=2+3/8 points to the existence of other even-denominator states.     

Hierarchical porphyrin self-assembly in aqueous solution

In aqueous solution meso-tetrakis(4-phosphonatophenyl)porphyrin shows self-aggregation processes controlled by the "sergeant-soldier rule". After partial protonation of the external phosphonic groups, it is possible (i) to further protonate the inner nitrogen atoms of the molecules or (ii) to allow, over time, the system to aggregate. Therefore, the two procedure lead to a different system evolution, producing species with different chemico-physical properties.     

Anhydrides as acylating agents in the enzymatic resolution of an intermediate of (-)-Paroxetine

A new chemoenzymatic method for the preparation of an intermediate of (-)-Paroxetine is reported. Cyclic anhydrides are used as acylating agents in the lipase-catalyzed esterification of trans-4-(4'-fluorophenyl)-3-hydroxymethyl-N-phenyloxycarbonylpiperidine in organic solvents. The best enantioselectivities are obtained with two different lipases from Candida antarctica. These two lipases show opposite stereochemical preference in these processes, so that both enantiomers can be obtained in their optically pure forms. The (3S,4R) isomer is an intermediate for the synthesis of (-)-Paroxetine.     

Localization and photodynamic efficacy of two cationic porphyrins varying in charge distributions

Two meso-tetraphenylporphyrin derivatives bearing adjacent: 5,10-di[4-(N-trimethylaminophenyl)-15,20-diphenylporphyrin (DADP-a) or opposite: 5,15-di[4-(N-trimethylaminophenyl)-10,20-diphenylporphyrin (DADP-o) cationic-N-(CH3)3+ groups on two of the para-phenyl positions were examined with regard to photodynamic properties as a function of charge distribution. The two adjacent positive charges in the DADP-a structure result in a molecular distortion (asymmetry), likely from electrostatic repulsion. This could be responsible for the unusual interaction of this compound with some solvents and detergent micelles. In contrast, DADP-o is a much more symmetric molecule. In a cellular environment, fluorescence spectra of the two agents were essentially identical. Subcellular localization played a major role in photodynamic efficacy. DADP-a localized in mitochondria, and irradiation of photosensitized cells (640-650 nm) resulted in a rapid loss of the mitochondrial membrane potential (delta(psi)m), usually a prelude to apoptotic cell death. In contrast, DADP-o localized in lysosomes, and extensive lysosomal photodamage was observed after irradiation. Both steady-state accumulation levels and absorbance spectra favored DADP-o, but the light dose required for a 90% cell kill was two-fold greater for DADP-o than for DADP-a, at a constant extracellular sensitizer concentration. These data indicate that, on a photons/cell basis, DADP-a was five-fold more efficacious. Fluorescence emission spectra in different solvents and detergents demonstrated a tendency for DADP-a association. We interpret these results to indicate partition of both drugs to membrane loci, with mitochondriabeing the more lethal site for photodamage.     

Benzoylbiliverdins from chemical oxidation of dodeca-substituted porphyrins

Oxidation of dodeca-substituted porphyrin 1 in the presence of NaNO2/TFA and air afforded benzoylbiliverdin 3 in 77% yield; under similar conditions the Ni(II)-porphyrin 2 was mainly nitrated at a cyclohexenyl ring; key X-ray structures are presented.     

Influence of low intensity noise on assemblies of diffusively coupled chaotic cells

The effect of time-correlated and white Gaussian noises of low intensity on one-dimensional arrays consisting of diffusively coupled chaotic cells is analyzed. An improvement or worsening of the synchronization between cells of the array driven by low-intensity colored noise is observed for a resonant interval of time correlation values. A comparison between colored and white noise and additive and multiplicative contribution has been carried out investigating the nonlinear cooperative effects of noise strength, correlation time, and coupling strength to control spatiotemporal chaos in coupled arrays of chaotic cells. The possibility to distinguish highly correlated areas of a diffusively coupled network of cells by using low-intensity time correlated noise is discussed. (c) 2001 American Institute of Physics.     

Intrinsic Definitions of “Relative Velocity” in General Relativity

Given two observers, we define the “relative velocity” of one observer with respect to the other in four different ways. All four definitions are given intrinsically, i.e. independently of any coordinate system. Two of them are given in the framework of spacelike simultaneity and, analogously, the other two are given in the framework of observed (lightlike) simultaneity. Properties and physical interpretations are discussed. Finally, we study relations between them in special relativity, and we give some examples in Schwarzschild and Robertson-Walker spacetimes.     

The xraylib library for X-ray–matter interactions. Recent developments

This work presents the recent developments of xraylib, an ANSI C library that provides convenient access to a large number of X-ray related databases, with a focus on quantitative X-ray fluorescence applications. The enhancements include improved X-ray fluorescence production cross sections that take into account cascade effects and M-lines, as well as revised line energies, atomic level widths, Compton broadening profiles etc. A full overview of the complete application programming interface is presented.