Stereoselective Michael addition of 6-amino-1,3-dimethyl-2,4-pyrimidinedione to the exocyclic methylene of three sesquiterpene lactones. 1H and 13C NMR evidence of a new C-C bond and lactam formation

The Stereoselective addition of the pyrimidine derivative 1 to the exocyclic methylene of the α,β unsaturated dehydrocostus lactone 2, Ivalin acetate 3 and Zaluzanin A diacetate 4, was achieved resulting in a new C-C bond formation. In the cases of compounds 3 and 4, after the addition, the lactone was cleaved followed by reclosure into a lactam ring system.     

Influence of acceptor impurities on semi-insulating GaAs particle detectors

GaAs Schottky diodes, made on semi-insulating liquid encapsulated Czochralski grown material with concentrations of acceptor dopants varying from 10¹⁴ to 10¹⁷ cm^-3, were investigated as alpha particle detectors. The charge collection efficiency (CCE) was found to decrease dramatically with increasing . Optical spectra in transmittance and reflectance were accurately measured to determine the concentrations of both neutral and ionised EL2 defects as a function of . The concentration of ionised EL2⁺ centres was shown to increase with , and to be quasi inversely proportional to the CCE values. This behaviour strongly supports the hypothesis that the EL2 defects play the main role in the compensation of the material and in limitation of the detection properties. Received 21 March 2000     

Isotope effects on resonance cross sections in predissociation/preassociation

Isotope effects on the resonance cross sections of preassociative scattering are explicitly studied for the O₂, OH and OD systems, based on a semiclassical theory similar to Stückelberg's theory of inelastic scattering. It is found that there are significant isotopic differentiations for the systems studied, which may be exploited in the laboratory for selective photodissociations of the molecules.     

Direct transfer of aliphatic and aromatic substituents from organosilatranes to mercury(II) species

The relative reaction rates of several silatranes (derivatives of 2,8.9-trioxa-5-aza- 1-silatricyclo[3.3.3.0^1,5]undecane) and HgCl₂ in acetone-d₆to yield the corresponding organomercury compound are of the order of e.g., 5 × 10^?1 1 mol^?1 sec^?1 or slightly less, a rate that is unexpectedly high compared to the essentially inert parent organotrialkoxysilanes. Thus, the apical Si&.zsbnd;C bond of the silatrane is extraordinarily susceptible to direct electrophilic attack by mercury(II). The rates decrease in the order CH₂CH, C₆H₅, p-ClC₆H₄ > CH₃ > CH₃CH₂, CH₃CH₂CH₂ > C₆H₁₁, ClCH₂, Cl₂CH, CH₃CH₂O. The effects of varying the solvent and the counterions are noted, and the probable mechanism is discussed.     

Dielectric dispersion in KCl: OH

Dielectric measurements at 9.4 GHz in KCl doped with a concentration of 0.2 at. % OH are reported. The dielectric constant exhibits a relaxational anomaly at temperatures far above that observed in the kilohertz frequency range for the same OH concentration. The results indicate that electric dipole freezing in KCl:OH is governed by a relaxational mechanism.     

Theoretical investigation of clusters of phosphorus and arsenic: fascination and temptation of high symmetries

We present a theoretical study of the energetic and thermodynamic stability of selected phosphorus and arsenic clusters containing 18 to 168 atoms. For this purpose we employ MP2 as well as DFT functionals BP86 and B3LYP with extended basis sets. All procedures predict the family of one-dimensional polymers X18+12n, each with 2n-1 isomers of virtually identical energy, to be more stable than other structures investigated so far. Furthermore, islands of stability result for ring-shaped clusters X24n with Dnd symmetry for n=4 (only for arsenic), 5, 6, and 7. Phosphorus and arsenic show otherwise a very similar behavior. An investigation of basis set effects shows that a doubly polarized triple zeta valence basis (TZVPP) is both necessary and sufficient. In comparison to the reliable spin component scaled MP2 (SCS-MP2) procedure, DFT methods underestimate and MP2 overestimates the stability of larger clusters; the discrepancy increases with the number of atoms. The addition of a long-range dispersion correction to B3LYP energies does not rectify the shortcomings of DFT in comparison with SCS-MP2.