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51.
2,4-Disubstituted butyrolactones and 2,4,6-trisubstituted-5,6 dihydro-4H-1,3-oxazines show similar features in their 1H and 13C- NMR spectra. Two geminal ring hydrogens of cis isomers give rise to a complex ABXY spectra when the substituent is alkyl or aryl. In spectra of trans isomers these patterns are degenerated. When R is OMe(in 4) or OCOMe (in 6) the difference in chemical shifts of geminal protons and vicinal coupling constants cannot be used for diagnosis. In 13C spectra ring carbons C-2 and C-3 in lactones and C-4 and C-5 in oxazine of trans isomers show a small but consistent shift to higher fields. 相似文献
52.
Eduardo Dfaz Hector Barrios Jose Luis Nava Raul G. Enriquez Angel Guzmn Len G. Leticia Jose Fernando Fuentes Fuentes B. Aidee Angelina Quintero Jose Dolores Solano 《Journal of heterocyclic chemistry》1997,34(3):1037-1041
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. 相似文献
53.
Blends of a linear unsaturated polyester (LUP, commercially named Al100) with poly(?-caprolactone) (PCL) of different molecular weights have been studied. The miscibility and crystallinity have been analyzed through FT-IR spectroscopy, differential scanning calorimetry (DSC) and environmental scanning electronic microscopy (ESEM). All the blends were subjected to the same heat treatment consisting of crystallizing during 45 min at constant temperature (10, 20, 30 or 40 °C). The glass transition temperature, Tg, and fusion temperature, Tfus, have been determined in the whole composition range for each blend. The Tg-composition dependence and the high degree of crystallinity detected at intermediate blend compositions denote an anomalous behaviour that could indicate the lack of homogeneity (phase separation) in the different blends studied. The ESEM measurements confirm the lack of homogeneity of the amorphous region in blends with high content of LUP. The results have been discussed as a function of the crystallization temperature and the molecular weight of PCL. 相似文献
54.
R. Ferrini G. Guizzetti M. Patrini F. Nava P. Vanni C. Lanzieri 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(2):213-216
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 相似文献
55.
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. 相似文献
56.
Anat Fischer Itzhak Gilboa Moshe Shpitalni 《Journal of Algorithms in Cognition, Informatics and Logic》1992,13(4)
The following problem arises in the context of object representation: given two endpoints of an interval in a Gray code table, find a Boolean function in DNF that represents this interval, with as few prime implicants as possible. This paper shows that there is a unique minimal representation and presents a polynomial algorithm that finds it. 相似文献
57.
58.
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. 相似文献
59.
60.
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. 相似文献