Problemi parabolici degeneri

We establish existence theorems for weak solutions of boundary value problems associated with degenerate parabolic operators.     

A journey through quality control

The Euroanalysis VII conference in Vienna included a two-day session: Quality Assurance in Analytical Chemistry. The contributions comprised 15 lectures devoted to: intra-laboratory quality measures, inter-laboratory control, formal aspects and accreditation and implementation. The paper presents an overview of the main items developed by the contributors.A survey on the session on Quality Assurance in Analytical Chemistry of Euroanalysis VIIThe authors thank the organizers of EUROANALYSIS VII and in particular Dr. B. Griepink and Dr. E. Maier of the Community Bureau of Reference (BCR) of the CEC for their support and considerable contributions.     

The decay of neutron deficient97Ag,98Ag,and99g,m Ag

The decays of neutron-deficient 21-sec⁹⁷Ag, 44.5-sec⁹⁸Ag, 15-sec^99mAg, and 124-sec^99gAg nuclides have been investigated with the LISOL facility. Sources were produced by the⁹²Mo(¹⁴N, ypxn) reactions on an enriched⁹²Mo target. Positron-, x-, and γ-ray singles spectra have been performed on mass separated samples. The results are consistent with 9/2⁺ ground state in⁹⁹Ag and a high (6⁺ or 7⁺) spin and parity for⁹⁸Ag.     

Dimers of 3,7-dehydrotropones (bicyclo[3.2.0]hepta-1(7),2,4-trien-6-ones)

2-(tert-Butyl)-3,7-dehydrotropone (7-(tert-butyl)bicyclo[3.2.0]hepta-1(7),2,4-trien-6-one; 1 ) was found to dimerize reversibly to 2A by [2 + 4]-cycloaddition/cycloreversion reaction. The equilibrium lies on the side of the highly strained dimer 2A in the solid state, and on the side of the monomer 1 in solution. The [2 + 4]-reaction is fully perisite-, regio- and stereoselective. Above room temperature, 1 irreversibly formed a decarbonylated dimer 6 , probably via the intermediate 9A or 9B , which resulted either from a dimerisation of 1 by [4 + 6]-cycloaddition or from a sigmatropic rearrangement of the originally formed dimer 2A or 2B . Similary, the 6-bromo derivative 14 afforded the corresponding decarbonylated dimer 15 . Should the formation of 6 and 15 be due to a primary cycloaddition then that reaction is fully peri-, site- and regioselective. Mild LiAlH₄-reduction of 6 and subsequent acetylation yielded the acetate 11 , the structure of which was established by an X-ray analysis. More vigorous LiAlH₄-treatment also reduced the terminal fulvenoid double bond of 6 and acetylation of the crude product led to the acetated 12 and 13 .     

Electric field effects in 13C NMR of some quaternary morphinan derivatives

¹³C NMR chemical shifts are assigned for some quaternary morphinan derivatives, including the ‘minor isomer’ of the thebaine N-oxides. In quaternary morphinan alkaloids having an 8.14-double bond, electric field effects cause unusual ¹³C NMR shifts.     

On the decay of symmetric dynamo fields

The ‘anti-dynamo’ theorems for poloidal magnetic fields with axisymmetry and plane symmetry are generalized to the case of a compressible and time-dependent flow in a fluid with arbitrary conductivity.     

Reaction of polyvinyl alcohol with aluminium isopropoxide

The uncatalyzed and oxalic acid catalyzed modification of polyvinyl alcohol (PVA) with aluminium isopropoxide have been studied for reaction in a suspension of powdered PVA in boiling benzene. Isopropyl alcohol (IPA) formed during the reaction was removed by continuous distillation of an IPA-benzene mixture. The reaction was initially quite fast but slowed up rapidly and practically stopped when only 4–12% of the OH-groups had reacted. Thermogravimetric analysis (TGA) showed that products with 6.8 mole A1/100 mole VA exhibit a significant rise in decomposition temperature from 250 to 285°. The characteristics are attributed to the occurrence of vicinal OH-groups in PVA.     

Mixed self-assembled monolayers of semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)

Single-component and mixed self-assembled monolayers (SAMs) of one- and three-ring semirigid tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes)-that is, thiopyran (1), 4-(4-cyclohexylidene-cyclohexylidene)tetrahydro-4H-thiopyran (2), and 4-(tetrahydro-4H-thiopyran-4-cyclohexylidene-4'-ylidene)tetrahydro-4H-thiopyran (3)--on Au(111) substrates have been prepared and studied by cyclic voltammetry (CV), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). It was found that the shortest adsorbate 1 more readily forms a SAM than 2 or 3. Notwithstanding, the SAMs of 2 or 3 are thermodynamically more stable due to favorable intermolecular attractions. Holes were made with the AFM tip establishing tilt angles of 30-50 degrees with respect to the surface normal for all SAMs. STM imaging showed well-ordered, line-shaped packing patterns with molecular resolution for the SAM of 2. Similar patterned structures were not observed for 1 and 3. Mixed SAMs were prepared by exposing a SAM of 1 to ethanol solutions of either 2 or 3. STM imaging revealed that domains of molecules of 2 or 3 amidst a monolayer of 1 are formed in both cases. Whereas in the mixed SAM of 1 and 2 the domains are irregularly shaped, circular islands of uniform size are found in the mixed SAM of 1 and 3.     

Investigation of the Head-Space of Roasted Meat II. Synthesis of substituted 2, 4, 5-trithia-hexanes

Substituted 2, 4, 5-trithiahexanes 1 , a class of substances recently found in roasted meat, have been synthesized via two different routes.