The Weak Chebyshev X-Greedy Algorithm in the unweighted Bergman space

We study a greedy algorithm called the Weak Chebyshev X-Greedy Algorithm (WCXGA) and investigate its application to unweighted Bergman spaces. We first show that the WCXGA converges for a wide class of real and complex Banach spaces and dictionaries. We then prove that certain Bergman spaces and their holomorphic monomial dictionaries belong to the class of Banach spaces for which the WCXGA converges.     

Complex formation thermodynamics of some metal(II) phenyl-2-pyridylketoximates

The formation of phenyl-2-pyridylketoximates of nickel(II), zinc(II), cadmium-(II), mercury(II) and lead(II) was studied potentiometrically in a medium of 40% (w/w) acetone and 60% (w/w) water. The values of step formation constants, free energy, entropy and enthalpy of complex formation were calculated. The formation of the mercury chelate was found to be entropy driven. The formation of phenyl-2-pyridylketoximates of other metals of the first transitional series could not be studied owing to experimental difficulties.     

Magnetic properties of nickel clusters in nanoporous carbon

The magnetic properties of nanoporous carbon samples whose pores were loaded by nickel are described. It is shown that a sample becomes superparamagnetic for temperatures T<T _C (Ni) only in the case where a noticeable fraction of Ni is contained in the nanopores. The nanopore size estimated from magnetic measurements coincides with the estimates derived earlier from small-angle x-ray scattering studies.     

Cyclodextrin as a capturing agent for redundant surfactants on Ag nanoparticle surface in phase transfer process

High-yield phase transfer of hydrophilic mercaptosuccinic acid (MSA)-modified Ag nanoparticles into chloroform is readily attained using cetyltrimethylammonium bromide (CTAB) through electrostatic interaction. Increasing CTAB amount to a certain degree has achieved nearly complete phase transfer due to the sufficient formation of stoichiometric ion-pairs on particle surface. However, at high CTAB concentration, some unbonded CTA⁺ cations will be physically adsorbed on particle surface and enter chloroform layer simultaneously, which cannot be removed by simple water washing or centrifugation. By using β-cyclodextrin (CD) as a capturing agent, this portion of CTA⁺ cations can be adequately removed due to the possible inclusion function of CD. Upon removal of the unbonded CTAB, the monolayer formation of phase-transferred Ag nanoparticles at air–water interface presents improved two-dimensional (2D) orderliness owing to the more effective interdigitation among adjacent particles.     

Syntheses of 6,8-disubstituted-9-β-D-ribofuranosylpurine 3′,5′-cyclic phosphates

A series of 6,8-disubstituted-9-β-D-ribofuranosylpurine 3′,5′-cyclic phosphates were prepared employing preformed 9-β-D-ribofuranosylpurine 3′,5′-cyclic phosphate precursors. Three synthetic approaches were utilized to accomplish the syntheses. The first approach involved a study of the order of nucleophilic substitution, 6 vs 8, of the intermediate 6,8-dichloro-9-β-D-ribofuranosyipurine 3′,5′-cyclic phosphates ( 2 ) with various nucleophilic agents to yield 8-amino-6-chloro-, 8-chloro-6-(diethylamino)-, 6-chloro-8-(diethylamino)-, 6,8-bis-(diethylamino)- and 8-(benzylthio)-6-chloro-9-β-D-ribofuranosylpurine 3′,5′-cyclic phosphate (4, 9, 10, 11, 13) respectively and 6-chloro-9-β-D-ribofuranosylpurin-8-one 3′,5′-cyclic phosphate ( 5 ) and 8-amino-9-β-D-ribofuranosylpurine-6-thione 3′,5′-cyclic phosphate ( 6 ). The order of substitution was compared to similar substitutions on 6,8-dichloropurines and 6,8-dichloropurine nucleosides. The second scheme utilized nucleophilic substitution of 6-chloro-8-substituted-9-β-D-ribofuranosylpurine 3′,5′-cyclic, phosphates obtained from the corresponding 8-subslituted inosine 3′,5′-cyclic phosphates by phosphoryl chloride, 6,8-bis-(benzylthio)-, 6-(diethylamino)-8-(benzylthio),8-(p-chlorophenylthio(-6-(diethylamino)- and 6,8-bis-(methyl-thio)-9-β-D-ribofuranosylpurine 3′,5′-cyclic phosphates ( 14, 12, 20 , and 21 ) respectively, were prepared in this manner. The final scheme involved N¹-alkylation of an 8-substituted adenosine 3′,5′-cyclic phosphate followed by a Dimroth rearrangement to give 6-(benzylamino)-8-(methylthio)- and 6-(benzylamino)-8-bromo-9-β-D-ribofuranosylpurine 3′,5′-cyclic phosphate ( 24 and 25 ).     

Chirality without Stereoisomers: Insight from the Helical Response of Bond Electrons

The association between molecular chirality and helical characteristics known as the chirality-helicity equivalence is determined for the first time by quantifying a chirality-helicity measure consistent with photoexcitation circular dichroism experiments. This is demonstrated using a formally achiral S_N2 reaction and a chiral S_N2 reaction. Both the achiral and chiral S_N2 reactions possess significant values of the chirality-helicity measure and display a Walden inversion, i. e. an inversion of the chirality between the reactant and product. We also track the chirality-helicity measure along the reaction path and discover the presence of chirality at the transition state and two intermediate structures for both reactions. We demonstrate the need for the chirality-helicity measure to differentiate between steric effects due to eclipsed conformations and chiral behaviors in formally achiral species. We explain the significance of this work for asymmetric synthetic reactions including the intermediate structures where the Cahn-Ingold-Prelog (CIP) rules cannot be used.     

Clusters of sulfur and selenium in nanoporous carbon

The results of x-ray diffraction, x-ray photoelectron spectroscopy, electrical, and galvanomagnetic studies of nanoporous carbon with sulfur and selenium nanoclusters introduced into pores are analyzed. Conclusions are drawn concerning the short-range order in the introduced clusters and their interaction with carbon.     

Photoluminescence of SiO<Subscript>2</Subscript> layers prepared on β-SiC films and an analysis of their elemental composition

The spectra of pulsed time-resolved photoluminescence of oxidized films prepared through electrochemical oxidation on SiC/Si films are investigated. The elemental composition of the films is analyzed using Rutherford backscattering and nuclear reactions. It is revealed that a certain part of the carbon atoms remain in the oxide. The specific features of the parameters of the spectral bands and their kinetics with time are explained by the presence of carbon uniformly distributed throughout the oxide. The structure of oxides at different oxidation times is examined with the use of electron microscopy. Conclusions are drawn regarding possible photoluminescence centers.     

Many-body ionization in a frozen Rydberg gas

In a dense gas of 300 microK 85Rb atoms of n approximately 50 ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 K blackbody radiation. Rapid ionization is accompanied by spectral broadening, with the spectrum becoming continuous at n=88 at a density of 5x10(10)cm(-3). The atomic transitions broaden both smoothly and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms.