Sorptionseffekte an Eisen(III)-hydroxid-F?llungen

Zusammenfassung Um Material zur Deutung des Mechanismus der Sorption von Sr²⁺-Ionen an Eisen(III)-hydroxid zu gewinnen, werden Meßreihen mitgeteilt, in denen die pH-Abhängigkeit der Sorption sowie deren Beeinflussung bei Fällung mit verschiedenen Basen (NaOH, Ammoniak, Trimethylamin, Benzylamin) untersucht wird. Aus den charakteristischen Unterschieden der Sorptionskurven muß gefolgert werden, daß die Art der Base für die Sorption eine wichtige Rolle spielt.

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Summary The sorption of Sr²⁺ ions on iron(III) hydroxide was studied as a function of the pH, of the precipitating reagent (NaOH, ammonium hydroxide, benzylamine, trimethylamine), of the temperature and of the total concentration.The curves of the relative sorption vs. pH are found to show characteristic differences which have to be explained in a detailed theory of the mechanism.

     

Synthesis of 3-carbomethoxy-3,4-dialkylcyclohexanones

The uncatalyzed 1,4-addition of phenylmagnesium bromide and furylmagnesium iodide to methyl 5-methoxy-1,5-cyclohexadienylcarboxylate ($\text{1}$), directly followed by alkylation and hydrolysis leads to the corresponding cyclohexanones of type $\text{2}$ (R₁ and R₂ trans) in moderate to high yield.     

Darstellung und Eigenschaften von Chromtricarbonyl-Komplexen strukturisomerer Borazin-Derivate

The new hexaalkylborazine chromium tricarbonyls (n-Pr)₃B₃N₃Me₃Cr(CO)₃ (V), Me₃B₃N₃(n-Pr)₃Cr(CO)₃ (VI), (i-Pr)₃B₃N₃Me₃Cr(CO)₃ (VII) and Me₃B₃N₃(i-Pr)₃Cr(CO)₃ (VIII) have been prepared from fac-Cr(CO)₃(MeCN)₃ and the corresponding borazine in dioxane or without solvent. They are much more labile than the isomeric complex Et₃B₃N₃Et₃Cr(CO)₃ (IV) which can be readily obtained from Et₃B₃N₃Me₃Cr(CO)₃ and Et₃B₃N₃Et₃ by ring ligand exchange. The NMR., IR., UV. and Mass spectroscopic data of the complexes IV–VIII will be briefly discussed. The preparation of the borazine derivatives (n-Pr)₃B₃N₃Me₃ (IX) and Me₃B₃N₃(n-Pr)₃ (X) is also reported.     

Practical asymmetric enzymatic reduction through discovery of a dehydrogenase-compatible biphasic reaction media

[reaction: see text] An enzyme-compatible biphasic reaction media for the asymmetric biocatalytic reduction of ketones with in situ cofactor regeneration has been developed. In this biphasic reaction media, which is advantageous for reactions at higher substrate concentrations, both enzymes (alcohol dehydrogenase and FDH from Candida boidinii) remain stable. The reductions with poorly water-soluble ketones were carried out at substrate concentrations of 10-200 mM, and the optically active (S)-alcohols were formed with moderate to good conversions and with up to >99% ee.     

Historical Aspects of the Chemical Bond. Chemical Relationality versus Physical Objectivity

Chemists have always defined the properties of materials on the basis of the changes observed when they reacted with other substances. Whereas this approach led chemists to relational concepts such as affinity, acid, and oxidant, physicists made measurements of objects they considered unchanged, determining their mass, charge, dipole moment, etc. In the middle of the 18th century, the Jesuit Josip Ruder Bocovi started thinking about the way in which atoms might be present in crystals, introducing a new concept according to which atoms in condensed phases represented punctual centres of attracting and repelling forces. Josef Loschmidt became aware of these ideas through the philosopher J. F. Herbart, and it is to Loschmidt that we owe the first representation of bonds as atomic spheres penetrating one another. The term quantum chemistry was first used by the physicist Arthur Erich Haas, who was born in Brno. However, Hans Hellmann (1903–1938) was the true founder of quantum chemistry. Hellmann, who was shot in Moscow in 1938, was the first person to realize the quantum-physical effect that leads to the chemical bond. In the 1960s K. Ruedenberg and others took theory a stage further when – thanks to the concept of the localized electron pair – they realized that the different approaches to the phenomenon of the chemical bond taken by chemists and physicists were largely comparable. This made it possible to bring the differing standpoints largely into line with one another.     

Basische Metalle. XXIV [1]. Ein- und zweikernige Cobaltthiolato-Komplexe aus Disulfiden. Spaltung einer S–S-Bindung durch eine Metall-Base

Basic metals. XXIV. Mono- and dinuclear cobaltthiolato complexes obtained from disulfides. Splitting of a S? S bond by a metal base The dinuclear complex C₅H₅(PMe₃)Co(μ-CO)₂Mn(CO)C₅H₄Me ( 3 ) reacts with the disulfides S₂R₂ (R ? Ph, CH₂Ph) by splitting of the sulfur-sulfur bond to form C₅H₅(PMe₃)Co(SR)₂ ( 4, 5 ). From 3 and S₂Me₂ a mixture of C₅H₅(PMe₃)Co(SMe)₂ ( 6 ) and [C₅H₅Co(μ-SMe)]₂ ( 7 ) is obtained. The synthesis of C₅H₅(PMe₃)Co(SCF₃)₂ ( 8 ) succeeds by treating 3 with N(SCF₃)₃. Whereas the reactions of 4 and 5 with MeI lead to the complex C₅H₅(PMe₃)CoI₂ ( 9 ), the dinuclear complex [C₅H₅(PMe₃)Co(μ-SPh)]₂(BF₄)₂ ( 11 ) is formed from 4 and [OMe₃]BF₄. The reactions of 11 with L = PMe₃ and P(OMe)₃ produce the compounds [C₅H₅Co(PMe₃)(L)SPh]BF₄ ( 12, 13 ), which react with [OMe₃]BF₄ to yield [C₅H₅Co(PMe₃)(L)(MeSPh)](BF₄)₂ ( 14, 15 ).     

Basische Metalle. LXIV. Lewis-basische Bis(trimethylphosphan)cobalt-Komplexe mit Indenyl und Trifluormethylcyclopentadienyl als Liganden

Basic Metals. LXIV. Lewis-basic Bis(trimethylphosphine)cobalt Complexes with Indenyl and Trifluormethylcyclopentadienyl as Ligands The half-sandwich type compounds C₉H₇Co(PMe₃)₂ ( 1 ) and (C₅H₄CF₃)Co(PMe₃)₂ ( 6 ) are prepared from CoCl(PMe₃)₃ and C₉H₇Li or TlC₅H₄CF₃, respectively. They behave like metal bases and react with HBF₄, CH₃I (or CF₃SO₃CH₃), I₂, and CH₃COCl by oxidative addition to give the cationic complexes [C₉H₇CoX(PMe₃)₂]⁺ and [(C₅H₄CF₃)CoX(PMe₃)₂]⁺ (X ? H, CH₃, I, COCH₃) which are isolated as the PF₆ salts ( 2–5 and 7–10 ). The ¹HNMR and the IR spectra of the compounds 1–10 are discussed, also in comparison to those of the corresponding cyclopentadienylcobalt complexes.     

Electrokinetic phenomena at grafted polyelectrolyte layers

During the last decades the electrokinetic theory of Smoluchowski (Z. Phys. Chem. 92 (1918) 129) was extended to be applicable for soft surfaces (grafted polyelectrolyte layers (PL), biological and artificial membranes, etc.) by either using the Debye approximation or numerical solutions. In the theory of Ohshima (Colloids Surf. A 103 (1995) 249) the nonlinearized Poisson-Boltzmann (PB) equation for thick and uniform PL is solved analytically and a general hydrodynamic equation is derived in an integral form. These advantages in the theory of Ohshima provided a base for the further development of a generalized electrokinetic theory for soft surfaces. In his theory the final equation for the electroosmotic (electrophoretic) velocity is specified for the case of the complete dissociation of ionic sites within PL. Accordingly, the equation may be used only if the difference between pK and pH is very large. However, it turned out that an analytical solution of the nonlinearized PB equation for thick PL is possible for any degree of dissociation. This was achieved using the approximation of excluded coions if the absolute value of the reduced Donnan potential is larger than 2 and due to the simplification in the case of weak dissociation, when the absolute value of the reduced Donnan potential is less than 2. Combining this generalized double layer (DL) theory for PL and the theory of Ohshima enables to obtain an analytical equation for electroosmosis for the general case of any degree of dissociation. This equation creates for the first time a theoretical base for the interpretation of electrokinetic fingerprinting (EF) for the characterization of soft surfaces.     

Forward and backward pericyclic photochemical reactions have intermediates in common, yet cyclobutenes break the rules.

Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S(1)), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene-6 and bicyclo[4.2.0]octene-7), the forbidden isomer is formed from ct*. The path branches at the first (S(2)/S(1)) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in C=C twist direction, acquired--such as in other olefins--in the Franck-Condon region of the cyclobutenes.     

Parameter-free structure-property correlation via progressive reaction posets for substituted benzenes

Progressive reaction networks as frequently arise in chemistry are naturally identifiable as "partially ordered sets" (or posets). Here the direction of the reaction identifies the partial ordering of the set of molecular species. The possibility that different properties are similarly ordered is a further natural consideration and is here investigated for a suite of over 30 properties for (methyl and chloro) substituted benzenes. Such a posetic correlation is favorably demonstrated for these substituted benzenes, and it is illustrated how suitable properties may be simply predicted in an interpolative parameter-free (albeit not model-free) fashion through the use of the reaction poset. Some numerical model-quality indicators are identified, and the simple approach is deemed quite reasonable.