Synthesis of Ag2S quantum dots in water-in-CO2 microemulsions

Ag2S nanocrystals with a mean diameter of 5.9 nm (sigma= 1.65 nm) and characteristic surface plasmon resonance absorption at 330 nm have been synthesized in water-in-supercritical CO2 reverse microemulsion using the commonly used AOT surfactant with 2,2,3,3,4,4,5,5-octafluoro-1-pentanol (F-pentanol) as cosurfactant.     

Alstonia scholaris: Struktur des Indolalkaloides Narelin

Alstonia scholaris: The structure of the indole alkaloid nareline Besides the known akuammidine, picralinal, picrinine and pseudoakuammigine a new indole alkaloid called nareline (M=352) was isolated from Alstonia scholaris R. BR. , which belongs to the plant family of Apocynaceae. Its structure 2 was deduced by single crystal X-ray diffraction. 2 represents the absolute configuration. The spectroscopic data of 2 and its derivatives (Scheme 1) as well as their chemical behavior support this structure. In biogenetic sense nareline is related to the bases akuammiline ( 4 ) and picraline ( 5 ) (Scheme 2). In contrast to those the C-atom 5 is exocyclic and represents an aldehyde group which forms together with the oxygen atom of the N (4)-hydroxylamine group a cyclic half acetale. - By oxidation (CrO₃/CH₃COOH) of 2 the oxindol derivative 19 (oxonareline) is formed which contains a cyclic acetal as a partial structure element (Scheme 4).     

Cu(I)-catalyzed direct enantioselective cross aldol-type reaction of acetonitrile

Direct catalytic enantioselective cross aldol-type reaction of an acetate surrogate was developed using Cu alkoxide-chiral phosphine complexes as catalysts. Chemoselective activation and deprotonation of the donor substrate (acetonitrile) by the soft metal alkoxide in a strongly donating solvent (HMPA) are key to success in this reaction. Useful chemical yields and promising enantioselectivities are produced using either DTBM-SEGPHOS or a tuned BIPHEP as a chiral ligand. [reaction: see text]     

Polymerizability of unsaturated monomers capable of forming stable radicals

Some unsaturated monomers bearing hindered phenol and arylamine groups capable of forming stable radicals were prepared. Radical polymerizations of vinyl monomers having such groups were investigated with the use of azobisisobutyronitrile, benzoyl peroxide, cumene hydroperoxide, and tetraethylthiuram disulfide as initiator. Polymerizations of these monomers went normally only when azobisisobutyronitrile was used as initiator. The other initiators inhibited polymerizations remarkably or completely. The results suggest that radicals resulting from benzoyl peroxide and cumene hydroperoxide or tetraethylthiuram disulfide abstract hydrogen of the phenol or the amine to produce the stable radicals, thereby inhibiting the polymerization. Meanwhile, carbon radicals resulting from azobisisobutyronitrile add selectively to the vinyl double bonds of the monomers to initiate the polymerizations. The vinyl derivatives as well as allyl derivatives and cinnamic acid derivatives copolymerize easily with conventional monomers such as styrene, maleic anhydride, and butadiene, again, only when azobisisobutyronitrile was used as initiator. Antioxidative properties for styrene copolymers and butadiene-styrene copolymers incorporating the hindered phenol monomers were investigated.     

Rhenium complex-catalyzed allylation of aldehydes with allyltributylstannane

It was confirmed that the rhenium complex, ReBr(CO)₅, catalyzed the allylation of aldehydes with allyltributylstannane to give the corresponding homoallylic alcohols in moderate to good yields. Similarly, the reaction of aldehydes with enol silyl ether was efficiently promoted by the rhenium catalyst to afford the corresponding β-hydroxy carbonyl compounds.     

Mössbauer and molecular orbital study of the myoglobin-CO complex

Experimental Mössbauer spectra of the Fe⁵⁷-enriched CO complex of sperm whale myoglobin (MbCO) at T= 4.2 K with and without applied magnetic field (H⊥γ) were measured to derive the sign of the electric field gradient (EFG), the quadrupole splitting ΔE _Q, and the isomer shift δ of the heme iron. We find a positive EFG, δE _Q = 0.363 mm/sec, and δ + 0.266 mm/sec. Molecular orbital calculations were carried out to obtain theoretical estimates of EFG and ΔE _Q for several steric arrangements of the CO ligand relative to the heme group. Our results are most consistent with the conclusion that the iron is situated in the heme plane, and that a bent geometry with a Fe-C-O angle of about 135 ° is more favorable than a more symmetric structure with a linear Fe-C-O geometry.     

On nets of orderq 2 and degreeq+1 admitting GL(2,q)

In this paper we consider finite nets of orderq ² and degreeq + 1 which admit GL(2,q). Our main result says that if a net of orderq ² and degreeq + 1 admits a collineation group with a point-regular normal subgroupT such that /T GL(2,q), then is isomorphic to a regulus net, a twisted regulus net, a Hering net, or . Except in the last one, each of them corresponds to a surface in PG(3,q) obtained from a homogeneous polynomial in two variables.     

Diffusion-controlled stress-relaxation in polymers,I

Summary The “chemical” relaxation of stress either in Amilan (6-Nylon) or polyvinyl alcohol caused by penetration of water vapor was studied at 40% R. H. and temperatures 15, 30, and 50° C. A theory was presented of the chemical relaxation in a polymer which comprises penetrant-sensitive bonds as the crosslinkages between the molecular chains. This theory assumes the diffusion coefficient of penetrant in the polymer to he independent of concentration and any other factors. It was found that the system Amilan-water behaves in exact conformity with the theory over the ranges of elongation and temperature studied. The diffusion coefficient,D, of water in Amilan could thus be evaluated from the chemical relaxation data, using the theory presented, as functions of elongation and temperature. An ordinary sorption experiment was carried out for this system at 40% R. H. and 16.8° C and demonstrated that the diffusion of water in Amilan was Fickian with a constant diffusion coefficient. Both diffusion coefficients from the chemical relaxation data and the sorption. data were found to agree quite well when the former was extrapolated to zero-strain. The chemical relaxation behavior in the system polyvinyl alcohol-water was markedly different from that expected from the theory, suggesting primarily that the diffusion coefficient of water in this polymer was not constant. A parallel evidence for this fact was obtained from sorption measurements on this system, which demonstrated the diffusion in this system to be dependent both on concentration and time. From these results it may be concluded that the chemical relaxation technique, when combined properly with an adequate theory, can be used not only to evaluate the diffusion coefficient of penetrant in a given polymer solid but also to investigate, at least qualitatively, the type of sorption behavior characteristic of the system under given experimental conditions.

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Zusammenfassung Die chemische Spannungsrelaxation in Amilan (6-Nylon) und Polyvinylalkohol, verursacht durch Eindringen von Wasserdampf, wurde bei 40% r. F. und Temperaturen von 15, 30 und 50° C untersucht. Es wird eine Theorie der chemischen Relaxation von Polymeren vorgelegt, die als empfindliche Bindungen auf das eindringende Niedermolekulare die Vernetzungen zwischen den molekularen Ketten betrachtet. Diese Theorie setzt voraus, da? der Diffusionskoeffizient im Polymeren für die eindringende Substanz unabh?ngig von Konzentration und anderen Faktoren ist. Das System Amilan-Wasser benimmt sich vollkommen entsprechend dieser Theorie in den studierten Verstreckungs- und Temperaturbereiche. Der Diffusionskoeffizient von Wasser in Amilan konnte daher aus der chemischen Relaxation als Funktion von Dehnung und Temperatur ausgewertet werden. Ein normales Quellungsexperiment wurde für das System bei 40% r. F. und 16,8° C ausgeführt und zeigte, da? die Diffusion von Wasser in Amilan dem Fickschen Gesetz mit konstantem Diffusionskoeffizienten folgt. Beide Diffusionskoeffizienten, der aus chemischer Relaxation und der aus Quellungsdaten gewonnene, stimmten sehr gut überein, falls der erstere auf die Dehnung 0 extrapoliert wurde. Die chemische Relaxation des Systems Polyvinylalkohol-Wasser war merklich anders als nach der Theorie erwartet und lie? in erster Linie vermuten, da? der Diffusionskoeffizient von Wasser in diesem Polymeren nicht konstant ist. Eine entsprechende Andeutung wurde aus Quellungsmessungen an diesem System erhalten. Die Diffusion h?ngt von Konzentration und Zeit ab. Aus diesen Resultaten kann gefolgert werden, da? die Technik der Untersuchungen der chemischen Relaxation unter Verwendung einer geeigneten Theorie nicht nur dazu angewendet werden kann, Diffusionskoeffizienten von kleinen Molekülen in einem gegebenen festen Polymeren zu messen, sondern zumindest auch qualitativ den Typ des Quellungsverhaltens festzustellen, der das System unter den gegebenen experimentellen Bedingungen charakterisiert.

     

Enantioselective ester synthesis in the presence of optically active polymers

Enantiomer-differentiating ester syntheses from acid salts, chlorides, and an anhydride were studied in the presence of optically active polyamines. Enantiomer selection occurred in reactions of a racemic salt with an achiral alkyl bromide and vice versa, which is good evidence of the importance of activation of the carboxylate group by complexation of the counterion and activation of alkyl bromide by the formation of a quaternary salt with polyamine under solid–liquid, phase-transfer reaction conditions. Only small optical activity was observed in the products of acid anhydride or chloride with alcohol.