H-spectroscopic imaging using a modified Dixon method

Inhomogeneities of the static magnetic field and the different susceptibilities of the various types of tissue are a serious problem for all imaging methods of spectral separation of fat and water. In the Dixon method this problem is solved by using the absolute values of the image signals for the separation. In image regions where the fat signal is greater than the water signal, however, this results in an incorrect assignment of the computed solutions. A modified Dixon method was developed to easily carry out the spectral separation completely over the entire image by interactively building up a phase correction matrix after the data acquisition. The spectral delineation of the fat tissue finds an interesting application in the treatment planning with fast neutrons in accounting for the increase in dose.     

Synthesis of Enantiomerically Pure Pheromones of South-Pacific Brown Algae: Hormosirene and Dictyopterene A

Hormosirene ((?)- 1 ; (1R,2R)-1-((1E,3Z)-1,3-hexadienyl)-2-vinylcyclopropane) is the specific sex attractant of several brown algae of the Australian shelf, while dictyopterene A ((+)- 2 ; (1R, 2R)-1-((1E)-1-hexenyl)-2-vinylcyclopropane) is found as a minor constituent of the pheromone bouquets. The asymmetric synthesis of the two hydrocarbons is performed by resolution of the amides (?)- 5 and (+)- 5a obtained from (?)-(R)-2-phenylglycinol and racemic trans-vinylcyclopropanecarboxylic acid (rac- 4 ) on silica gel. Both diastereoisomers are obtained optically pure. They are converted by stereoselective Wittig olefination into the title compounds. Compound (?)- 1 is the active mating pheromone of the reproductive system of the seaweed Xiphophora chondrophylla as established by biological-activity assays.     

Ab initio studies of structural features not easily amenable to experiment: Part 6. Quantitative estimate of the effect of bond delocalization on structure and hyperconjugative interaction of the amide group

The structural changes, which occur in the amide unit when the NH₂-group is twisted out of plane by rotation about the NC bond, have been determined by comparing the completely relaxed ab initio geometries of planar and perpendicular formamide and acetamide. In the perpendicular conformation, in which the π-electron amide resonance is uncoupled, the NC bond distance is 0.080.09 Å longer than in the planar form; the CO bond distance is about 0.01 Å shorter; NH distances are about 0.01 Å longer; and HNC angles are 510° smaller, whereas the CNO angle is relatively constant. Because of the apparent invariance of CH₃-hyperconjugation effects in planar and perpendicular acetamide, it is tentatively postulated that anomeric orbital interactive effects (involving the lone pair on NH, the CO π-electron pair and antibonding π*-group-orbitals on C(α) in NHC(HR)C(O)), which should be an important factor in determining peptide chain conformation, do not vary significantly with small deviations from amide group planarity.     

Natural frequencies and axial response of a spinning liquid layer in a micro-gravity field

The natural frequencies of a solidly rotating liquid layer around a center-core, which under the action of an axial gravity changes to an amphora-type geometry, are determined. The liquid is frictionless and incompressible. According to the rotational Webernumber and the axial Bondnumber we have to distinguish natural frequencies for different modes. In additional the response of the free liquid surface to axial excitation has been obtained.

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Zusammenfassung Es werden Eigenfrequenzen einer rotierenden Flüssigkeitsschicht bestimmt, die sich unter dem Einfluß eines axialen Gravitationsfeldes zu einer Amphora-Geometrie verändert. Die Flüssigkeit ist reibungsfrei und inkompressibel. In Abhängigkeit von Weber- und Bondzahl zeigen sich unterschiedliche Eigenfrequenzen für verschiedene Moden. Zusätzlich wird das Antwortverhalten der freien Flüssigkeitsoberfläche auf axiale Anregung untersucht.

     

The 16-electron electrocyclisation of vinylogous heptafulvalene

The vinylogous heptafulvalene (“heptafulvadiene”) (5) cleanly undergoes the antarafacial 16-electron electrocyclisation to (11) upon thermal activation (benzene, E_a = 22.1 kcal/mol; log A = 10.8).