Elektrometrische Bestimmung von Vanadium und Uran allein, gemeinsam und neben Eisen

Ohne Zusammenfassung     

Nanotome cluster bombardment to recover spatial chemistry after preparation of biological samples for SIMS imaging

A C₆₀⁺ cluster ion projectile is employed for sputter cleaning biological surfaces to reveal spatio-chemical information obscured by contamination overlayers. This protocol is used as a supplemental sample preparation method for time of flight secondary ion mass spectrometry (ToF-SIMS) imaging of frozen and freeze-dried biological materials. Following the removal of nanometers of material from the surface using sputter cleaning, a frozen-patterned cholesterol film and a freeze-dried tissue sample were analyzed using ToF-SIMS imaging. In both experiments, the chemical information was maintained after the sputter dose, due to the minimal chemical damage caused by C₆₀⁺ bombardment. The damage to the surface produced by freeze-drying the tissue sample was found to have a greater effect on the loss of cholesterol signal than the sputter-induced damage. In addition to maintaining the chemical information, sputtering is not found to alter the spatial distribution of molecules on the surface. This approach removes artifacts that might obscure the surface chemistry of the sample and are common to many biological sample preparation schemes for ToF-SIMS imaging.     

Local refinement techniques for elliptic problems on cell-centered grids; II. Optimal order two-grid iterative methods

Two preconditioning techniques for solving difference equations arising in finite difference approximation of elliptic problems on cell-centered grids are studied. It is proven that the BEPS and the FAC preconditioners are spectrally equivalent to the corresponding finite difference schemes, including a nonsymmetric one, which is of higher-order accuracy. Numerical experiments that demonstrate the fast convergence of the preconditioned iterative methods (CG and GCG-LS in the nonsymmetric case) are presented.     

Reductive Insertion of Elemental Chalcogens into Boron–Boron Multiple Bonds

The syntheses of sulfur‐ and selenium‐bridged cyclic compounds containing boron stabilized by N‐heterocyclic carbenes (NHCs) have been achieved by the reductive insertion of elemental chalcogens into boron–boron multiple bonds. The three pairs of bonding electrons between the boron atoms in the triply bonded diboryne enabled six‐electron reduction reactions, resulting in the formation of [2.2.1]‐bicyclic systems wherein bridgehead boron atoms are spanned by three chalcogen bridges. A similar reaction using a diborene (boron–boron double bond) resulted in the reductive transfer of both pairs of bonding electrons to three sulfur atoms, yielding a NHC‐stabilized trisulfidodiborolane. The demonstration of these six‐ and four‐electron reductions lends support to the presence of three and two pairs of bonding electrons between the boron atoms of the diboryne and diborene, respectively, a fact that may be useful in future discussions on bond order.     

Ultraviolet Absorption Induces Hydrogen‐Atom Transfer in G⋅C Watson–Crick DNA Base Pairs in Solution

Ultrafast deactivation pathways bestow photostability on nucleobases and hence preserve the structural integrity of DNA following absorption of ultraviolet (UV) radiation. One controversial recovery mechanism proposed to account for this photostability involves electron‐driven proton transfer (EDPT) in Watson–Crick base pairs. The first direct observation is reported of the EDPT process after UV excitation of individual guanine–cytosine (G?C) Watson–Crick base pairs by ultrafast time‐resolved UV/visible and mid‐infrared spectroscopy. The formation of an intermediate biradical species (G[?H]?C[+H]) with a lifetime of 2.9 ps was tracked. The majority of these biradicals return to the original G?C Watson–Crick pairs, but up to 10 % of the initially excited molecules instead form a stable photoproduct G*?C* that has undergone double hydrogen‐atom transfer. The observation of these sequential EDPT mechanisms across intermolecular hydrogen bonds confirms an important and long debated pathway for the deactivation of photoexcited base pairs, with possible implications for the UV photochemistry of DNA.     

The Synthesis of B2(SIDip)2 and its Reactivity Between the Diboracumulenic and Diborynic Extremes

A new compound with the formula L‐B₂‐L wherein the stabilizing ligand (L) is 1,3‐bis[diisopropylphenyl]‐4,5‐dihydroimidazol‐2‐ylidene (SIDip) has been synthesized, isolated, and characterized. The π‐acidity of the SIDip ligand, intermediate between the relatively non‐acidic IDip (1,3‐bis[diisopropylphenyl]imidazol‐2‐ylidene) ligand and the much more highly acidic CAAC (1‐[2,6‐diisopropylphenyl]‐3,3,5,5‐tetramethylpyrrolidin‐2‐ylidene) ligand, gives rise to a compound with spectroscopic, electrochemical, and structural properties between those of L‐B₂‐L compounds stabilized by CAAC and IDip. Reactions of all three L‐B₂‐L compounds with CO demonstrate the differences caused by their respective ligands, as the π‐acidities of the CAAC and SIDip carbenes enabled the isolation of bis(boraketene) compounds (L(OC)B‐B(CO)L), which could not be isolated from reactions with B₂(IDip)₂. However, only B₂(IDip)₂ and B₂(SIDip)₂ could be converted into bicyclic bis(boralactone) compounds.     

A synthesis of bredinin (Mizoribine®) from an acyclic precursor

Bredinin (4-carbamoyl-1-β-d-ribofuranosylimidazolium-5-olate, 1) was synthesised by the formation of a malonamate from 2,3-isopropylidene-d-ribofuranosylamine and ethyl malonyl chloride, followed by a sequence involving amination, via reduction of an oxime, heterocycle formation and then deprotection.