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241.
Rate studies on five azoalkanes (4a-e) have been studied to assess important radical stabilizing effects. MO calculations of spin densities for three radicals with pi systems are correlated with kinetic rates of radical formation. 相似文献
242.
A novel version of the Biginelli reaction using an unprotected aldose as a biorenewable aldehyde component and 2-methyl-2-phenyl-1,3-oxathiolan-5-one as a mercaptoacetylating active methylene building block with urea/thiourea is reported. The reaction is nanoclay-catalysed, expeditious and effected under solvent-free microwave irradiation conditions in a one-pot procedure to yield diastereoselectively, thiosugar-annulated multifunctionalized dihydropyrimidines via intramolecular domino cyclocondensation reactions of an isolable intermediate. 相似文献
243.
M. Dube R. Amin Sanayei A. Penlidis K. F. O'Driscoll P. M. Reilly 《Journal of polymer science. Part A, Polymer chemistry》1991,29(5):703-708
A user-friendly program has been developed to estimate copolymerization reactivity ratios based on a nonlinear minimization algorithm. The use of an optimal experimental design for copolymerization when the Mayo–Lewis model applies is presented. The applicability of the program is demonstrated using actual and simulated experimental data. 相似文献
244.
Electron Impact Fragmentation of Substituted Dimethylalkoxysilanes The mass spectra of substitued dimethylalkoxysilanes (H3C)2SiOCH3R (R ? ? F, ? Cl, ? H, ? OCH3, ? C6H5, ? CH3, ? C2H5, ? n-C3H7), and (H3C)2SiOC2H5R (R ? ? Cl, ? C6H5, ? CH3, ? C2H5) have been recorded and the fragmentation patterns are presented. The yield of the electron impact induced reaction (M-15)+→(M-45)++ H2CO occuring upon fragmentation of substituted dimethylmethoxysilanes depends on the substituent R. A quantum chemical calculation was carried out by CNDO/2 method to determine the electron density distribution in the ion at mass number (M-15). It is shown that a correlation exists between the Si? O? π bond order in this ion and the yield as well as the activation energy of this reaction. 相似文献
245.
The mass spectra of trimethylalkoxy silanes (CH3)3Si? O? (CH2)n? CH3 (n = 0-8) and (CH3)3Si? O? (CH2)n? CH(CH3)2 (n = 0-3) have been investigated and most of the fragmentations, induced by electron-impact, have been identified by metastable ions. The fragmentation mechanisms of trimethylpentoxysilane, established by Diekman, Thomson and Djerassi, are also observed in the mass spectra of these trimethylalkoxy silanes. Characteristic features of these reactions in the mass spectra of compounds with smaller and longer alkoxy groups, together with some additional fragmentations, are discussed. 相似文献
246.
Emission properties of manganese-doped ZnS nanocrystals 总被引:1,自引:0,他引:1
Sapra S Prakash A Ghangrekar A Periasamy N Sarma DD 《The journal of physical chemistry. B》2005,109(5):1663-1668
We have performed steady-state and time-resolved fluorescence studies on undoped and Mn-doped ZnS nanocrystals with approximately 16 A diameter. While there is no band-edge emission, the intensity of the steady-state blue fluorescence from ZnS surface states decreases upon Mn incorporation, which gives rise to an orange emission. These results show that Mn incorporation competes very effectively with the donor-acceptor surface states for the energy transfer from the electron-hole pair excited across the band gap. In both undoped and doped samples, the time-resolved fluorescence studies establish the presence of a distribution of decay lifetimes possibly due to a number of emission centers in the nanocrystals. A faster short-time decay of the blue emission in the Mn-doped samples compared to that in the undoped sample suggests an additional decay channel for the surface states via an energy transfer from these states to the dopant levels. 相似文献
247.
Nanocrystalline nickel-zinc ferrite thin films with the general formula Ni1−xZnxFe2O4, where x=0.0, 0.2, 0.4 and 0.6 were fabricated via a chemical route known as the citrate precursor route. These films were spin-deposited on indium-tin oxide coated glass, fused quartz and amorphous Si-wafer substrates, and annealed at various temperatures up to 650 °C. The films annealed below 400 °C were found to be X-ray amorphous, while the films annealed at and above 400 °C were polycrystalline exhibiting a single-phase spinel structure. The average grain size of the films evaluated by transmission electron microscopy, is found to be in the range 4-8.5 nm. The room temperature DC resistivity of the films is in the range 103-107 Ω m. Dielectric constant and dielectric loss were measured in the frequency range 100 Hz-1 MHz. Dielectric constant of the films is found to lie between 25 and 44, while the loss factor is if the order of 10−2. The higher values of the dielectric constant for films having higher zinc concentration are attributable to the enhanced hopping between Fe2+ and Fe3+ ions in these samples. The M-H hysteresis measurement of the nickel ferrite thin films annealed at 650 °C showed narrow hysteresis loop—a characteristic of soft ferromagnetic material. 相似文献
248.
Conformational changes of amide cavitands A – C were investigated at varied temperatures and in several solvents. While cavitands A and B , with comparatively smaller substituents such as Et and iPr, were always in vase conformation in non‐polar solvents such as CDCl3, CD2Cl2, (D8)THF, and C6D6, their thermoswitching (vase to kite) was observed in polar solvents such as (D7)DMF and (D6)DMSO or in the presence of acid (TFA) and H‐bonding inhibitor (TFE). Intra‐ and interannular H‐bonds of A and B were clearly observed by low‐temperature 1H‐NMR spectra in CDCl3. No conformational change of cavitand C with bigger substituent (tBu) was observed under any tested temperature range and in polar or non‐polar solvents; C was always in the kite conformation. 相似文献
249.
Vishnu J. Ram Vidyohama Dube A. J. Vlietinck 《Journal of heterocyclic chemistry》1987,24(5):1435-1437
Several new 1,2,4-triazolo[4,3-b]triazino[7,6-b]indoles were prepared in anticipation that they would have better chemotherapeutic activities, but in the Screening test none of the Compounds was found to be active. 相似文献
250.
Acta physica Academiae Scientiarum Hungaricae - The unsteady flow of a dusty viscous liquid in a parallel plate channel rotating with an angular velocity Ω is analysed. Analytical expressions... 相似文献