Silver deposition from silver rubidium iodide

The halogenide (Cl^?, Br^?, I^?) complexes of indium(III) were investigated polarographically. The potential obtained in the presence of iodide ion (0.0004 M) was taken as the half-wave potential of “free” indium ion. The half-wave potentials at low halogenide concentrations were corrected for the kinetic effect. The approximate values of the stability constants read off from the curve (half-wave potential versus logarithm of ligand concentration) at points corresponding to the mean ligand numbers n=0.5, 1.5, 2.5, and 3.5 were then refined by the trial and error method. Four stability constants were found for each of the ligands investigated. The logarithms of β_1–4 are 2.70, 3.20, 4.20 and 3.30 for chloride ions, 2.10, 2.40, 2.50 and 0.60 for bromide ions and 1.35, 1.40, 1.30 and 0.50 for iodide ions. It was also assumed that the too high values of β₁ found by Bond resulted from neglect of the kinetic character of the waves.     

Enzyme electrokinetics: electrochemical studies of the anaerobic interconversions between active and inactive states of Allochromatium vinosum [NiFe]-hydrogenase

The cycling between active and inactive states of the catalytic center of [NiFe]-hydrogenase from Allochromatium vinosum has been investigated by dynamic electrochemical techniques. Adsorbed on a rotating disk pyrolytic graphite "edge" electrode, the enzyme is highly electroactive: this allows precise manipulations of the complex redox chemistry and facilitates quantitative measurements of the interconversions between active catalytic states and the inactive oxidized form Ni(r) (also called Ni-B or "ready") as functions of pH, H(2) partial pressure, temperature, and electrode potential. Cyclic voltammograms for catalytic H(2) oxidation (current is directly related to turnover rate) are highly asymmetric (except at pH > 8 and high temperature) due to inactivation being much slower than activation. Controlled potential-step experiments show that the rate of oxidative inactivation increases at high pH but is independent of potential, whereas the rate of reductive activation increases as the potential becomes more negative. Indeed, at 45 degrees C, activation takes just a few seconds at -288 mV. The cyclic asymmetry arises because interconversion is a two-stage reaction, as expected if the reduced inactive Ni(r)-S state is an intermediate. The rate of inactivation depends on a chemical process (rearrangement and uptake of a ligand) that is independent of potential, but sensitive to pH, while activation is driven by an electron-transfer process, Ni(III) to Ni(II), that responds directly to the driving force. The potentials at which fast activation occurs under different conditions have been analyzed to yield the potential-pH dependence and the corresponding entropies and enthalpies. The reduced (active) enzyme shows a pK of 7.6; thus, when a one-electron process is assumed, reductive activation at pH < 7 involves a net uptake of one proton (or release of one hydroxide), whereas, at pH > 8, there is no net exchange of protons with solvent. Activation is favored by a large positive entropy, consistent with the release of a ligand and/or relaxation of the structure around the active site.     

Non-empirical LCAO-MO-SCF calculations with gaussian type functions on the electrocyclic transformation of cyclopropyl to allyl

Non-empirical molecular quantum chemical calculations have been performed on the electrocyclic transformation of planar and non planar cyclopropyl anions, to allyl anions in the LCAO-MO-SCF framework using gaussian type functions as atomic orbitals. Employing a total of 37 GTF 3 disrotatory and 2 conrotatory modes of transformation have been considered. A total of 34 calculations have been carried out. In the transformation of planar and non planar cyclopropyl anions to allyl anions, both the disrotatory and conrotatory modes possess activations barriers, those for the conrotatory modes being the lower by substantial margins. A detailed analysis of the energy terms involved show that is little theoretical justification for the postulate that the mode of ring opening depends only on the symmetries of the highest occupied orbitals.

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Zusammenfassung Nichtempirische SCF-MO-Rechnungen mit Gaußfunktionen (GF) für den Übergang des planaren und des unplanaren Cyclopropylanions zum Allyanion wurden vorgenommen. Mit einem Basissatz von 37 GF wurden 34 Rechnungen für 3 disrotatorische und 2 conrotatorische Übergänge durchgeführt. Sowohl die disrotatorischen als auch die conrotatorischen Übergänge besitzen Aktivierungsbarrieren, von denen diejenigen für die conrotatorischen Übergänge stets kleiner sind. Die Analyse der Energieterme gibt wenig Anlaß zu der Annahme, daß die Art der Ringöffnung nur von der Symmetrie des obersten besetzten Orbitals abhängt.

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Résumé Des calculs ab-initio en orbitales gaussiennes ont été effetués sur la transformation de cations cyclopropyliques en cations allyliques. Les barrières d'activation sont toujours plus basses pour la mode conrotatoire que pour la mode disrotatoire. Il est peu évident que l'ouverture du cycle dépende que de la Symmetrie de la plus haute orbitale occupée.

     

Enantioselective epoxidation of alkenes catalyzed by 2-fluoro-N-carbethoxytropinone and related tropinone derivatives

Several alpha-substituted N-carbethoxytropinones have been evaluated as catalysts for asymmetric epoxidation of alkenes with Oxone, via a dioxirane intermediate. alpha-Fluoro-N-carbethoxytropinone (2) has been studied in detail and is an efficient catalyst which does not suffer from Baeyer-Villiger decomposition and can be used in relatively low loadings. This ketone was prepared in enantiomerically pure form using chiral base desymmetrization of N-carbethoxytropinone. Asymmetric epoxidation catalyzed by 2 affords epoxides with up to 83% ee. Among other derivatives tested, the alpha-acetoxy derivative 7 affords the highest enantioselectivities.     

Analysis of derivatized and underivatized theanine enantiomers by high-performance liquid chromatography/atmospheric pressure ionization-mass spectrometry

Theanine, a naturally occurring non-proteinic amino acid found in tea leaves, has demonstrated wide-ranging physiological activity, from lowering blood pressure to enhancing the anti-tumor activity of chemotherapeutic drugs. The chiral nature of theanine suggests that enantiospecificity plays a significant role in its various pharmacological functions. Using the Chirobiotic T (teicoplanin) chiral stationary phase, native and derivatized theanine enantiomers were separated and detected via high-performance liquid chromatography (HPLC) coupled to atmospheric pressure ionization mass spectrometry (API-MS). With the use of flow rates compatible with each ionization source, native theanine standards achieved excellent sensitivity and detection limits (10 ng/mL) for both atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). Optimum sensitivity and detection limits for derivatized theanine standards were achieved using ESI-MS. The enantiomeric composition of six commercially available L-theanine samples was evaluated using the high-flow APCI-MS method and confirmed with photodiode array detection. Five of the six products contained significant amounts of D-theanine. Only one product, SunTheanine, appeared to contain only the L-theanine enantiomer.     

The thermal decomposition of aluminium sulfate

The mechanism of thermal decomposition of aluminum sulfate has been investigated in the 500–700°C temperature range using a flow reactor system with the emitted gaseous sulfur oxides collected in a Goksøyr—Ross coil and a hydrogen peroxide impinger. Sulfur trioxide (SO₃) was found to be the primary sulfur oxide released during thermal decomposition (1).

Less than 3% of the released sulfur oxides were sulfur dioxide (SO₂), indicating that the SO₃ dissociation reaction (2) is slow relative to the residence time of the SO₃ in the reactor (～ 1 sec).

The experimental technique should be readily adaptable to the study of the thermal decomposition of other metal sulfates.     

A "traceless" Staudinger ligation for the chemoselective synthesis of amide bonds

[reaction: see text] Here we report a novel modification of our previously reported "Staudinger ligation" that generates an amide bond from an azide and a specifically functionalized phosphine. This method for the selective formation of an amide bond, which does not require the orthogonal protection of distal functional groups, should find general utility in synthetic and biological chemistry.