Atomgewichte

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A kinetic study of the air oxidation of pyrogallol and purpurogallin

Pyrogallol (1,2.3-benzentriol, 1 ) reacts with dioxygen in weakly alkaline solutions to form purpurogallin (2,3,4,6-tetrahydroxy-5H-benzocycloheptene-5-one, 2 ) which then reacts to form purpurogalloquinone ( 3 ) which then decays to a product absorbing at 440 nm. The formation of 3 requires 1.5 mol O₂ per mol 1 , and 1.0 mol per mol 2 . No O₂ is consumed during the decay of 3 , and this reaction, being insensitive to O₂ concentrations and the presence of both superoxide dismutase (SOD) and catalase, does not appear to be an oxidation. The first-order rate of decay of 3 decreases with increasing pH. 1 and 3 react at pH 9.0 with a second-order rate constant of ca. 100 M^?1 s^?1. SOD inhibits the oxidation of 1 and 2 . Slight and variable apparent inhibition of the oxidation of 2 and 3 by SOD may be due to trace impurities in 2 , but not contamination by 1 . The peak concentration of 3 is attained more rapidly during the oxidation of 1 than during the oxidation of 2 . A kinetic scheme based on parallel oxidation of 1 by dioxygen and superoxide, and it which the semiquinone of 1 oxidizes 2 , is partially successful in simulating the observed kinetic behavior.     

Hydrogen‐bonded bilayers in ­piperazinium(2+) bis(mandelate) bis(methanol) solvate

In the title compound, C₄H₁₂N₂²⁺·2C₈H₇O₃^?·2CH₄O, the cations lie across centres of inversion and are disordered over two orientations with equal occupancy; there are equal numbers of (R)‐ and (S)‐mandelate anions present (mandelate is α‐hydroxy­benzene­acetate). The anions and the neutral water mol­ecules are linked by O—H?O hydrogen bonds [O?O 2.658 (3) and 2.682 (3) Å, and O—H?O 176 and 166°] into deeply folded zigzag chains. Each orientation of the cation forms two symmetry‐related two‐centre N—H?O hydrogen bonds [N?O 2.588 (4) and 2.678 (4) Å, and N—H?O 177 and 171°] and two asymmetric, but planar, three‐centre N—H?(O)₂ hydrogen bonds [N?O 2.686 (4)–3.137 (4) Å and N—H?O 137–147°], and by means of these the cations link the anion/water chains into bilayers.     

A structural hierarchy in the hydrogen‐bonded adduct N,N′‐di­methyl­piperazine–tartaric acid–water (1/2/2): an N‐component N‐dimensional structure (N = 3) with substructures having N = 1 and 2

In the hydrated adduct N,N′‐di­methyl­piperazine‐1,4‐diium bis(3‐carboxy‐2,3‐di­hydroxy­propanoate) dihydrate, [MeNH(CH₂CH₂)₂NHMe]²⁺·2(C₄H₅O₆)^?·2H₂O or C₆H₁₆N₂²⁺·2C₄H₅O₆^?·2H₂O, formed between racemic tartaric acid and N,N′‐di­methyl­piperazine (triclinic P, Z′ = 0.5), the cations lie across centres of inversion. The anions alone form chains, and anions and water mol­ecules together form sheets; the sheets are linked by the cations to form a pillared‐layer framework. The supramolecular architecture thus takes the form of a family of N‐dimensional N‐component structures having N = 1, 2 or 3.     

Synthesis,thermal reactivity,and kinetics of stabilized phosphorus ylides,part 2: [(Arylcarbamoyl)(cyano)methylene]triphenylphosphoranes and their thiocarbamoyl analogues

A series of five cyano(arylcarbamoyl) phosphorus ylides 2 and five cyano(arylthiocarbamoyl) phosphorus ylides 3 are prepared and fully characterized. Pyrolytic reaction products of a representative example of each type obtained by flash vacuum pyrolysis technique show that they undergo thermal extrusion of Ph₃PO or Ph₃PS. Kinetic study of the gas‐phase pyrolysis of each ylide by static method shows that these reactions are unimolecular and first order with no significant substituent effect, but the thiocarbamoyl ylides 3 react 40–65 times more rapidly than their carbamoyl analogues 2 . © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 496–502, 2006     

Synthesis, thermal reactivity and kinetics of substituted [(benzoyl)(phenylcarbamoyl)methylene]triphenylphosphoranes and their thiocarbamoyl analogues

A range of 16 substituted benzoyl/arylcarbamoyl and benzoyl/arylthiocarbamoyl stabilised ylides have been prepared. They are found under conditions of flash vacuum pyrolysis to fragment giving a benzoyl ylide and aryl isocyanate or isothiocyanate accompanied in some cases by secondary pyrolysis products. Kinetic studies show the thiocarbamoyl ylides to react consistently faster than their carbamoyl analogues and substituent effects suggest a polar cyclic transition state, which involves attack by the benzoyl oxygen on the carbamoyl/thiocarbamoyl NH.     

Sufficient uniqueness conditions for the solution of the time harmonic Maxwell’s equations associated with surface impedance boundary conditions

We consider the time-harmonic Maxwell’s equations for the scattering or radiating problem from a 3-D object that is either a metallic surface coated with material layers (MCS) or a dichroic structure (DS) made up of multiple frequency selective surfaces (FSS) embedded in materials. Low or high order impedance boundary conditions (IBC) are employed to reduce the numerical complexity of the solution of this problem via an integral equation or a finite element formulation. An IBC links the tangential components of the electric field to those of the magnetic field on the outer surface of the MCS, or on the FSSs, and avoids the solution of Maxwell’s equations inside the inhomogeneous domain for a MCS or, for a DS, the meshing of the numerous unit cells in a FSS. Sufficient uniqueness conditions (SUC) are established for the solutions of Maxwell’s equations associated with these IBCs, the performances of which, when constrained by the corresponding SUCs, are numerically evaluated for an infinite or finite planar structure.     

Emerging analytical methods to determine gluten markers in processed foods—method development in support of standard setting

The availability of analytical methods to detect and determine levels of markers of priority allergens in foods is of the utmost importance to support standard setting initiatives, the development of compliance and enforcement activities, as well as to provide guidance to industry on implementation of quality control practices, ensuring the effectiveness of allergen-related sanitation techniques. This paper describes the development and implementation of a mass-spectrometry-based technique to determine markers for individual sources of gluten in beer products. This methodology was shown to answer the requirements of Health Canada’s proposed labeling standard for individual gluten source declaration, in order to achieve its policy objectives (i.e., protection of sensitive consumers, while promoting choice). Minimal sample work-up was required and the results obtained by ELISA were further complemented using the LC-MS/MS method. This paper aims to demonstrate the feasibility of alternative techniques to ELISA-based methodologies to determine allergen and gluten markers in food.