Hierarchical assembly of helicate-type dinuclear titanium(IV) complexes

The ligands 4-7-H(2) were used in coordination studies with titanium(IV) and gallium(III) ions to obtain dimeric complexes Li(4)[(4-7)(6)Ti(2)] and Li(6)[(4/5a)(6)Ga(2)]. The X-ray crystal structures of Li(4)[(4)(6)Ti(2)], Li(4)[(5b)(6)Ti(2)], and Li(4)[(7a)(6)Ti(2)] could be obtained. While these complexes are triply lithium-bridged dimers in the solid state, a monomer/dimer equilibrium is observed in solution by NMR spectroscopy and ESI FT-ICR MS. The stability of the dimer is enhanced by high negative charges (Ti(IV) versus Ga(III)) of the monomers, when the carbonyl units are good donors (aldehydes versus ketones and esters), when the solvent does not efficiently solvate the bridging lithium ions (DMSO versus acetone), and when sterical hindrance is minimized (methyl versus primary and secondary carbon substituents). The dimer is thermodynamically favored by enthalpy as well as entropy. ESI FT-ICR mass spectrometry provides detailed insight into the mechanisms with which monomeric triscatecholate complexes as well as single catechol ligands exchange in the dimers. Tandem mass spectrometric experiments in the gas phase show the dimers to decompose either in a symmetric (Ti) or in an unsymmetric (Ga) fashion when collisionally activated. The differences between the Ti and Ga complexes can be attributed to different electronic properties and a charge-controlled reactivity of the ions in the gas phase. The complexes represent an excellent example for hierarchical self-assembly, in which two different noncovalent interactions of well balanced strengths bring together eleven individual components into one well-defined aggregate.     

Structural insights into aluminum chlorofluoride (ACF)

The structure of the very strong solid Lewis acid aluminum chlorofluoride (ACF, AlCl(x)F(3-x), x = 0.05-0.3) was studied by IR, ESR, Cl K XANES, (19)F MAS NMR, and (27)Al SATRAS NMR spectroscopic methods and compared with amorphous aluminum fluoride conventionally prepared by dehydration of alpha-AlF(3) x 3H(2)O. The thermal behavior of both compounds was investigated by DTA and XRD. In comparison to ACF, amorphous AlF(3) prepared in a conventional way is not catalytically active for the isomerization reaction of 1,2-dibromohexafluoropropane, which requires a very strong Lewis acid. Both compounds are mainly built up of corner-sharing AlF(6) octahedra forming a random network. The degree of disorder in ACF is higher than in amorphous AlF(3). Terminal fluorine atoms were detected in ACF by (19)F NMR. The chlorine in ACF does not exist as a separate, crystalline AlCl(3) phase. Additionally, chlorine-containing radicals, remaining from the synthesis, are trapped in cavities of ACF. These radicals are stable at room temperature but do not take part in the catalytic reaction.     

On the cavitation energy of water

Free-energy-perturbation theory from molecular dynamics calculations has been used to obtain the DeltaG of adjoining cavities' formation in water. The DeltaGs for systems with three, five and seven cavities are compared with that of a single cavity of the same volume, and found to be in good agreement. The conditions under which the analytical formulation of the energy of cavity formation proposed by Pierotti holds are discussed. The data for a single cavity have been tabulated and can lend themselves to a simple numerical implementation in standard quantum chemical packages, which can be used when high accuracy for DeltaG(cav) is required.     

Thermoanalytical methods in the study of inorganic thin films

Thermoanalytical (TA) methods are relatively seldom applied for assessing the physical and chemical proeprties of thin films, but they can be used in studies of composition, phase transitions and film—substrate interactions. In the present paper the possibilities of TA methods in thin film studies are reviewed. The thermoanalytical methods considered are the classical TG and DTA/DSC methods but some complementary methods will also be briefly mentioned. The main emphasis is given to true thin films. Details of sample preparation are also given. An important application of TA methods is characterization of precursors for the CVD growth of thin films, and this is also discussed.     

Monitoring of optical isomers of chiral alcohols and derivatives by chiral gas chromatography. Effect of derivatization on the enantio-differentiation

Summary Gas chromatographic chiral separation of several chiral 2-and 3-alkanols and diols was studied both in their free hydroxyl and in their trimethylsilyl ether forms. First, the derivatization procedure was verified through the identification of the trimethylsilyl ethers formed on the basis of their mass spectra and optimized to obtain quantitative reaction. The optimized procedure was applied to the trimethylsilylation of racemic mixtures of various hydroxyl compounds. The silylation was found to be highly effective in the improvement of the separation of the individual enantiomers. The major advantages of the derivatization process can be summarized as: (i) excellent baseline separation of the enantiomers of the silyl ethers was achieved in contrast to the parent OH-containing compounds, (ii) the sensitivity of detection highly increased, (iii) the separations do not show any significant concentration dependence and finally (iv) the analysis time needed decreased significantly. Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997     

Intercalation materials for lithium rechargeable batteries

An overview is given of intercalation materials for both the negative and the positive electrodes of lithium batteries, including the results of our own research. As well as lithium metal as a negative electrode, we consider insertion materials based on aluminium alloys. In the case of the positive electrode metal-oxides based on manganese, nickel and cobalt are discussed. Received: 27 May 1997 / Accepted: 30 July 1997     

Induced Absorption of C60 and a Water-Soluble C60-Derivative in SiO2 Sol-Gel Matrices

Porous sol-gel glasses, either impregnated with pure C₆₀ or doped with a methanofullerene derivative, have been studied and induced absorption or reverse saturable absorption (RSA) has been observed in both types of solid materials. The samples impregnated by pure C₆₀ mainly contain well-dispersed fullerene molecules. Unlike crystalline films of C₆₀, their absorption dynamics can be well described by a 5-level model, developed for non-interacting C₆₀-molecules in solutions. Methanofullerene samples, on the other hand, show signs of micellar aggregation and therefore RSA dynamics that are influenced by solid state effects. We observe an important decrease of transmission at high fluences for both kinds of samples, a shortened singlet-state lifetime to that observed in solution, but nonetheless, a triplet yield, that cannot be considered as negligible. In the case of pure C₆₀ in a sol-gel matrix, we can explain the faster de-excitation dynamics, relative to behavior in solution, mainly by the absence of stabilizing aromatic solvents and also by the interaction of the amorphous environment with the molecules. Concerning the methanofullerene samples, the acceleration of the de-excitation dynamics can be principally attributed to solid-state effects due to the micellar aggregation.     

Intramolecular homolytic substitution at selenium: synthesis of novel selenium-containing vitamin E analogues

Treatment of 1-(benzylselenenyl)-5-butyl-5-nonanol (10) with oxalyl chloride followed by the sodium salt of N-hydroxypyridine-2-thione afforded the corresponding pyridine-2-thione-N-oxycarbonyl (PTOC) oxalate ester which was not isolated but immediately heated to provide 2,2-dibutylselenane (7). This transformation presumably involves a tertiary alkyl radical that undergoes intramolecular homolytic substitution at selenium with loss of the benzyl radical to provide the selenium-containing ring system (7). A similar protocol, when applied to 1-(2-benzylselenenyl-5-methoxyphenyl)-3-methyl-3-heptanol (18) and 1-(2-benzylselenenyl-5-methoxyphenyl)-3,7,11,15-tetramethyl-3-hexadecanol (19), followed by deprotection, afforded the selenium-containing alpha-tocopherol analogues 4 and 1f, respectively, in moderate yields. To the best of our knowledge, these transformations represent the first examples of tertiary radicals involved in homolytic substitution chemistry at selenium.     

Thermal Analysis for plastics and rubber engineering

A survey of the possibilities of Thermal Analysis (TA) in the plastics and rubber industry is presented. As well as giving examples from research (liquid crystalline main and side chain polymers) and construction (laminates of carbon fibre reinforced epoxy resin prepregs), the field of quality assurance and material characterization is specially discussed. The correlation between TA-characteristics such as the degree of crystallinity and mechanical properties (e.g. hardness or green strength) is demonstrated. The application of thermogravimetry for a precise compositional analysis of compounded rubbers is shown. In practice, Thermal Analysis is most important for failure analysis and the characterization of processing parameters during the production of moulded parts as TA-methods are usually much faster than traditional standard methods. Moreover, the increasing importance of combining TA-methods with other techniques of instrumental analysis is demonstrated. An example of this combination is the TG-DSC-MS coupling which is invaluable in the area of environmental protection especially in the production, application and recycling of moulded parts from polymeric materials.

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Zusammenfassung Es wird ein überblick über die Möglichkeiten der Thermischen Analyse für die Kunststoff- und Kautschukindustrie gegeben. Neben Beispielen aus dem Bereich der Forschung (Haupt- und Seitenketten-Flüssigkristallpolymere) und Konstruktion (Laminate aus kohlefaserverstÄrkten Epoxyharz-Prepregs) wird besonders der Bereich der QualitÄtssicherung und Werkstoff-Charakterisierung behandelt. Hier werden ZusammenhÄnge zwischen TA-Kenngrössen wie z. B. KristallinitÄtsgrad und mechanischen Eigenschaften wie z. B. HÄrte oder Rohfestigkeit dargestellt sowie die Möglichkeiten der Thermogravimetrie zur quantitativen Gummianalyse gezeigt. Für den Praktiker besonders wichtig sind die Fehleranalyse und die Charakterisierung von Verarbeitungseinflüssen bei der Herstellung von Formteilen, da die Thermische Analyse oft wesentlich schneller Ergebnisse liefert als klassische Methoden. Ausserdem wird die zunehmende Bedeutung der Kopplung thermoanalytischer Methoden mit anderen Methoden der instrumentellen Analytik am Beispiel der TG-DSC-MS Kopplung für Probleme aus dem Bereich des Umweltschutzes im Zusammenhang mit der Herstellung und Anwendung von Formteilen aus polymeren Werkstoffen gezeigt.

     

On the extraction with long-chain amines

The extraction of tracer mercury(II) from aqueous HCl-solutions by TLA (trilaurylamine) dissolved in benzene can simply be described by the reaction (1) $$TLAHCl \cdot H_2 O(org) + HgCl_2 (aq) \rightleftharpoons TLAHCl \cdot HgCl_2 (org) + H_2 O(aq)$$ Using the chloride ion activity coefficient function recently introduced by HÖGFELDT (2) $$\lg y_{Cl^ - } = - 0.5115 \sqrt I /(1 + 1.176 \sqrt I ) - \lg C_{corr} $$ together with the model for excess acid extraction developed by AGUILAR and HÖGFELDT, the only unknown quantity is the equilibrium constant for the reaction above since the stability constants for the formation of the Hg(II)-CT-complexes are also well known. Using distribution data for HgCl₂ (tracer) between HCl-solutions and benzene Eq. (2) gets further support as being a useful expression for the activity coefficient of Cl^? in solutions more concentrated than ≈0.6M. Some scouting experiments about the extraction of HgCl₂ from NaCl brines containing 270 g NaCl/l showed that in the pH-range 1–13 the best extraction was obtained at about pH 1–2 irrespective if the diluent was benzene, o-xylene or a mixture of kerosene and benzene.