Utilisation des mélanges H2O+H2SO4+SO3 en tant que solvants: I. Étude du ferrocène et de quelques hydrocarbures polynucléaires dans les mélanges H2O+H2SO4. Comparaison des domaines d'électroactivité

The variation of the electroactivity range with composition of water+ sulphuric acid mixtures has been studied. It is shown that ferrocene provides a reference potential independent of solvent at low H₂SO₄ contents while polynuclear hydrocarbons fulfill the same function at high H₂SO₄ contents. Water+H₂SO₄ mixtures become more oxidizing (reduction limit) and less reducing (oxidation limit) when the H₂SO₄ concentration increases.     

Eine einfache Theorie des mechanischen Relaxationsverhaltens der amorphen Polymeren

Zusammenfassung In der Arbeit wird eine einfache Theorie der mechanischen Relaxation der amorphen Polymeren entwickelt. Als kinetische Grundeinheit des Relaxationsvorgangs tritt ein ausN Monomereinheiten bestehendes Segment auf. Unter Einwirkung mechanischer Kräfte erfolgt eine Orientierung des Segments. Au\erdem werden durch die mechanischen Kräfte Umlagerungen der Monomereinheiten hervorgerufen, die eine Längenänderung des Segments bedingen. Davon ist die Segmentorientierung im wesentlichen nur vom freien Volumen abhängig, während die Umlagerungen als ein Diffusionsproze\ in einem äu\eren Potentialfeld betrachtet werden, das durch die benachbarten Monomereinheiten der gleichen Kette und der anderen Ketten gebildet wird.Die Theorie liefert für dieYoungsche Nachgiebigkeit; zwei Relaxationsgebiete, wobei im einzelnen Aussagen über die Temperaturabhängigkeit der Relaxations-frequenz, der Intensität und des Breitenparameters der Relaxationsgebiete erfolgen.     

Zur Theorie der dielektrischen Relaxation der amorphen Polymeren

Zusammenfassung In der Arbeit wird im Rahmen der theoretischen Deutung der dielektrischen Eigenschaften der amorphen Polymeren ein einfaches Modell diskutiert, das erstmalig auf zwei Relaxationsgebiete führt. Die berechneten Relaxationsspektren sind zwar gegenüber den experimentellen Gr?\en noch zu schmal, in bezug auf das Frequenz- und Intensit?tsverh?ltnis der beiden Relaxationsgebiete ergeben sich jedoch sinnvolle Werte. Das Modell gehorcht nicht dem Reduktionsverfahren vonTobolsky undFerry, es erm?glicht deshalb prinzipiell eine Deutung der Temperaturabh?ngigkeit des Frequenzverh?ltnisses.     

Ein neues statistisches Segmentmodell zur Deutung der dielektrischen Eigenschaften hochpolymerer Lösungen

Zusammenfassung Es wird ein neues statistisches Segmentmodell angegeben, das zur Deutung der dielektrischen Eigenschaften der Lösungen von Kettenmolekülen mit seitständigen Dipolgruppen herangezogen werden soll.Es wird zunächst die exakte Lösung für das Relaxationsspektrum angegeben. Anschlie\end wird eine Näherungslösung abgeleitet, die einen Vergleich mit den Me\ergebnissen vonBrouckère und Mitarb. gestattet. Dabei zeigt sich eine gute übereinstimmung zwischen Experiment und Theorie.     

Selective, controllable, and reversible aggregation of polystyrene latex microspheres via DNA hybridization

The directed three-dimensional self-assembly of microstructures and nanostructures through the selective hybridization of DNA is the focus of great interest toward the fabrication of new materials. Single-stranded DNA is covalently attached to polystyrene latex microspheres. Single-stranded DNA can function as a sequence-selective Velcro by only bonding to another strand of DNA that has a complementary sequence. The attachment of the DNA increases the charge stabilization of the microspheres and allows controllable aggregation of microspheres by hybridization of complementary DNA sequences. In a mixture of microspheres derivatized with different sequences of DNA, microspheres with complementary DNA form aggregates, while microspheres with noncomplementary sequences remain suspended. The process is reversible by heating, with a characteristic "aggregate dissociation temperature" that is predictably dependent on salt concentration, and the evolution of aggregate dissociation with temperature is observed with optical microscopy.     

Accurate calibration of TXRF using microdroplet samples

TXRF has been applied in combination with VPD to the analysis of trace impurities in the native oxide layer of Si wafer surfaces down to the range of 10(8) atoms. cm(-2). Proper quantification of VPD/TXRF data requires calibration with microdroplet standard reference wafers. The precision of calibration function has been evaluated and found to allow quantification at a high level of 3 sigma confidence with microdroplet standard reference.     

New tools: Expert systems for uncertainty budgets

An important part of quality assurance in any analytical laboratory is the production of comprehensive results integrating uncertainty measurements. Many testing laboratories face the problem that the expenditure required to evaluate small uncertainties (high precision and high accuracy) is often uneconomic. In most cases an uncertainty of high reliability has to be calculated from only a few data (one calibration, few replications, etc.). This problem can be solved by an expert system. To achieve this the analytical procedure has to be structured into a dialouge and divided into parts. The uncertainty has to be calculated for each part of the procedure. Addition of the individual uncertainties results in the combined and expanded uncertainty. During the dialouge the system should advise the analyst how to get an efficient and effective calculation of uncertainty. All calculations, mathematical and statistical procedures have to be surveyable but running the system should not be too time consuming for economic reasons. Within the scope of the EURECA-project initiated by the Eidgenössische Materialprüfungs- und Forschungsanstalt (EMPA), St. Gallen, Switzerland, expert system software is being developed in cooperation with other research institutes and manufacturers of analytical instruments. Using this software it will be possible to calculate the uncertainty for analytical procedures such as titration, atomic emission spectrometry (ICP-OES), atomic absorption spectrometry (AAS) and gas- and liquid chromatography (GC, HPLC).     

Structural investigation of high-valent manganese-salen complexes by UV/Vis,Raman, XANES,and EXAFS spectroscopy

XANES and EXAFS spectroscopic studies at the Mn-K- and Br-K-edge of reaction products of (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride ([(salen)Mn(III)Cl], 1) and (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) bromide ([(salen)Mn(III)Br], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxybenzoic acid (MCPBA) are reported. The reaction of the Mn(III) complexes with two equivalents of 4-PPNO leads to a hexacoordinated compound, in which the manganese atom is octahedrally coordinated by four oxygen/nitrogen atoms of the salen ligand at an average distance of approximately 1.90 A and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 A. The oxidation state of this complex was determined as approximately +IV by a comparative study of Mn(III) and Mn(V) reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated with XANES, EXAFS, UV/Vis, and Raman spectroscopy, and an increase of the coordination number of the manganese atoms from 4 to 5 and the complete abstraction of the halide was observed. A formal oxidation state of IV was deduced from the relative position of the pre-edge 1s-->3d feature of the X-ray absorption spectrum of the complex. The broad UV/Vis band of this complex in acetonitrile with lambda(max)=648 nm was consistent with a radical cation structure, in which a MCPBA molecule was bound to the Mn(IV) central atom. An oxomanganese(V) or a dimeric manganese(IV) species was not detected.     

Synthesis, structure, and dynamics of six-membered metallacoronands and metallodendrimers of iron and indium

In the reaction of the N-substituted diethanolamines (H(2)L(1-3)) (1-3) with calcium hydride followed by addition of iron(III) or indium(III) chloride, the iron wheels [Fe(6)Cl(6)(L(1))(6)] (4) and [Fe(6)Cl(6)(L(2))(6)] (6) or indium wheels [In(6)Cl(6)(L(1))(6)] (5), [In(6)Cl(6)(L(2))(6)] (8) and [In(6)Cl(6)(L(3))(6)] (9) were formed in excellent yields. Exchange of the chloride ions of 6 by thiocyanate ions afforded [Fe(6)(SCN)(6)(L(2))(6)] (7). Whereas the structures of 4, 5 and 7 were determined unequivocally by single-crystal X-ray analyses, complexes 8 and 9 were characterised by NMR spectroscopy. Contrary to what is normally presumed, the scaffolds of six-membered metallic wheels are not generally rigid, but rather undergo nondissociative topomerisation processes. This was shown by variable temperature (VT) (1)H NMR spectroscopy for the indium wheel [In(6)Cl(6)(L(1))(6)] (5) and is highlighted for the enantiotopomerisation of one indium centre [ 1/6[S(6)-5]<==>[1/6[S(6)-5']]. The self-assembly of metallic wheels, starting from diethanolamine dendrons, is an efficient strategy for the convergent synthesis of metallodendrimers.