Finite element error bounds for a curve shrinking with prescribed normal contact to a fixed boundary

We approximate the evolution of a curve subject to motion bycurvature by linear finite elements. The curve evolves insidea given domain and meets orthogonally. We derive optimal boundsfor the error with respect to the L²- and H¹-norms and presentsome computed examples.     

Ultrafast photodissociation of Kn=3...9 clusters

By means of femtosecond pump and probe technique we investigated the temporal evolution of the photodissociation of excited potassium clusters (3 ≤ n ≤ 9). For three different excitation energies E = 1.47 eV, 2.00 eV, and 2.94 eV we were able to measure the transient multiphoton ionization spectra. In the frame of a simple energy level model the photodissociation process could be analyzed. Fragmentation time constants are estimated to be cluster size- and energydependent in the range of 200 fs up to 10 ps. The results are compared with data received from similar experiments on Nan clusters.     

Single ion activity coefficients in concentrated aqueous hydrogen chloride solutions

The adsorption behaviour of the steroids: ouabagenin, ouabain, proscillaridin, digoxin, and k-strophanthine was studied by a.c. polarography and capacitance—time curves at the dropping mercury electrode. The steroids behave like strongly surface-active substances: the relative capacitance decrease ΔC/C₀ depends linearly both on the concentration of the solution and on the root of the drop age. The surface area determined for ouabagenin to be 125 Ų on the basis of adsorption kinetics corresponds roughly to the maximum crosssection of the molecule. With glycosides, the surface area increases with the number of monosaccharide molecules being attached in position 3 of the steroid. It was inferred, therefore, that the adsorbed molecule rests with its steroid and sugar moiety flat on the electrode. A capacitance minimum with sharply defined edges (pit) occurs in the a.c. polarograms of ouabain. In this potential area, the dependences Δ/₀=f(c) and ΔC/C₀=f(t) show a step-like course with two plateaus, which is discussed to be due to association of the adsorbed molecules.     

Mechanism for the reactions of sulfides and sulfoxides with hypochlorites: racemization and oxygen exchange of oxysulfonium salts and sulfoxides

Density functional theory computations have been performed on the oxidations of sulfides and sulfoxides with hypochlorite ion (OCl^?), hypochlorous acid, and alkyl hypochlorites to study the mechanism of the reactions. The OCl^? anion transforms sulfides to sulfoxides and sulfoxides to sulfones in oxygen transfers. The oxygen atom of QOCl hypochlorites (Q = H, Me, t‐Bu) attacks at the sulfur atom of the substrates, and oxysulfonium cation intermediates are formed; the departure of the leaving Cl^? is catalyzed by soft Lewis acids. The structures of the early transition states are determined by highest occupied molecular orbital–lowest unoccupied molecular orbital interactions. The sulfur compounds are the electron acceptors in the reaction with OCl^?, but they are the electron donors in the reactions with QOCl. The attack of Cl^? at the oxygen atom of oxysulfonium cation intermediates leads to the sulfide and QOCl precursors and can result in racemization, oxygen exchange, and reduction of oxysulfonium salts in reversible reactions. The attack of Cl^? at the sulfur atom of oxysulfonium salts produces λ⁴‐sulfane intermediates. Oxysulfonium cations can be transformed into sulfoxide products with the attack of Cl^? or water at the α‐carbon atom of the O‐alkyl group. The attack of water at the sulfur atom of oxysulfonium cation leads to hydrolysis or oxygen exchange reactions. Racemization and oxygen exchange of sulfoxides proceeds in similar reactions, through the formation of hydroxysulfonium cation intermediates in acidic media in the presence of Cl^?. Chlorosulfonium cations are of very high energy; their intermediacy can be ruled out in the reactions of sulfides with hypochlorites. Copyright © 2012 John Wiley & Sons, Ltd.     

Biocatalytic production of tetrahydroisoquinolines

The promiscuity of the enzyme norcoclaurine synthase is described. This biocatalyst yielded a diverse array of substituted tetrahydroisoquinolines by cyclizing dopamine with various acetaldehydes in a Pictet-Spengler reaction. This enzymatic reaction may provide a biocatalytic route to a range of tetrahydroisoquinoline alkaloids.     

2H/1H Ratio Analysis of Flavor Compounds by On‐Line Gas Chromatography Pyrolysis Isotope Ratio Mass Spectrometry (HRGC‐P‐IRMS): Benzaldehyde

Isotope ratio monitoring gas chromatography‐mass spectrometry of the ²H/¹H ratio by pyrolysis isotope ratio mass spectrometry (P‐IRMS) was used to analyze benzaldehyde originating from various sources. Based on the δ²H_SMOW value of an authentic reference sample determined with an elemental analyzer (EA), the range of reproducibility and linearity was checked. Correct (EA related) and reproducible data were obtained for sample amounts >0.6 μg benzaldehyde (on column). In another series of experiments, the influence of sample preparation, i. e. simultaneous distillation‐extraction (SDE) was found to be negligible. The following ranges of δ²H_SMOW values were determined for benzaldehyde using five types of samples, i. e. (i) synthetic (δ²H_SMOW –78 to –85‰, ex benzal chloride; +420 to +668‰, ex toluene) and ‘natural’ (including ‘ex‐cassia’) references (δ²H_SMOW –83 to –144‰); (ii) bitter almond oils (δ²H_SMOW –113 to –148‰); (iii) fruits (δ²H_SMOW –111 to –146‰); (iv) kernels (δ²H_SMOW –115 to –188‰); and (v) leaves (δ²H_SMOW –165 to –189‰).     

Design of an Os Complex‐Modified Hydrogel with Optimized Redox Potential for Biosensors and Biofuel Cells

Multistep synthesis and electrochemical characterization of an Os complex‐modified redox hydrogel exhibiting a redox potential ≈+30 mV (vs. Ag/AgCl 3 m KCl) is demonstrated. The careful selection of bipyridine‐based ligands bearing N,N‐dimethylamino moieties and an amino‐linker for the covalent attachment to the polymer backbone ensures the formation of a stable redox polymer with an envisaged redox potential close to 0 V. Most importantly, the formation of an octahedral N6‐coordination sphere around the Os central atoms provides improved stability concomitantly with the low formal potential, a low reorganization energy during the Os^3+/2+ redox conversion and a negligible impact on oxygen reduction. By wiring a variety of enzymes such as pyrroloquinoline quinone (PQQ)‐dependent glucose dehydrogenase, flavin adenine dinucleotide (FAD)‐dependent glucose dehydrogenase and the FAD‐dependent dehydrogenase domain of cellobiose dehydrogenase, low‐potential glucose biosensors could be obtained with negligible co‐oxidation of common interfering compounds such as uric acid or ascorbic acid. In combination with a bilirubin oxidase‐based biocathode, enzymatic biofuel cells with open‐circuit voltages of up to 0.54 V were obtained.