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61.
Two possible isomers of the natural product callipeltin E (1, 5) were synthesized by using an Fmoc-based solid-phase strategy in 7 steps, in 20% and 26% overall yields, respectively. The (1)H NMR spectrum of synthetic 5 correlated closely with that of the natural product, whereas that of 1 did not, providing confirmation of the configurational reassignment of the N-terminal residue of callipeltin E as D-allothreonine. This result strongly implies that the corresponding residue in the closely related cyclic depsipeptides callipeltins A and B should also be considered a D-allothreonine residue. 相似文献
62.
1,1-Dimethylallyl (DMA) esters of various N-protected amino acids have been synthesized using prenyldimethylsulfonium tetrafluroborate, a reagent that can be readily made and stored, in conjunction with catalytic CuBr. These reactions were complete within several hours and afforded DMA esters in high yields. As has been previously shown in our group, DMA esters represent a palladium-labile proctecting group for carboxylic acids that resists nucleophilic attack as a tert-butyl ester would. 相似文献
63.
The ESR spectrum of La3Ga5SiO14 single crystals activated by chromium ions is investigated. It is shown that chromium ions are incorporated into the La3Ga5SiO14 crystal lattice in the trivalent state at laoctahedric positions. Orientations of the principal magnetic axes are determined,
and parameters of the spin Hamiltonian are estimated. The effect of disorder of the crystal structure of the matrix on line
widths and shapes of the ESR spectrum of impurity Cr3+ ions is discussed.
I. Franko L’vov State University, 50, Dragomilov St., L’vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii,
Vol. 65, No. 6, pp. 942–946, November–December, 1998. 相似文献
64.
Olaosebikan D Yerci S Gondarenko A Preston K Li R Dal Negro L Lipson M 《Optics letters》2011,36(1):4-6
Stimulated emission from sensitized erbium ions in silicon-rich silicon nitride is demonstrated by pump-probe measurements carried out in waveguides. A decrease in the photoinduced absorption of the probe at the wavelength of erbium emission is observed and is attributed to stimulated emission from erbium excited indirectly via localized states in the silicon nitride matrix. 相似文献
65.
An intrinsic, carbon-rich a-SiC x :H thin film, prepared by the plasma-enhanced chemical vapour deposition (PECVD) technique, has been studied mainly by AC admittance and small-pulse deep-level transient spectroscopy (DLTS) measurements on an Al/a-SiC x :H/p-Si metal–insulator–semiconductor (MIS) structure. The effects of measurement temperature, voltage and small-signal AC modulation frequency on the MIS capacitor are qualitatively and quantitatively described. The kinetics of charge injection from the silicon substrate into the a-SiC x :H film, as a function of temperature and voltage bias stresses, are reported. Nearest-neighbour and variable-range hopping mechanisms are considered. An activation energy of ~?0.09?eV, and a density of states (DOS) of about 1019?cm?3/eV were found. The value of the DOS is in agreement with the effective interface DOS of above 1012?cm?2/eV assessed by both capacitance and DLTS measurements. The frequency (or temperature) dependence of the MIS capacitor over the whole DC voltage range is considered in detail. Single- and double-step carrier exchange mechanisms between the a-SiC x :H film and the silicon substrate, in the accumulation and depletion voltage regimes, respectively, are proposed. 相似文献
66.
The half-metallic properties of novel CuHg2Ti-type Mn2ZnSi full-Heusler compound were examined by density functional theory (DFT) calculations. The electronic band structures and density of states of the Mn2ZnSi compound show that spin-up electrons are metallic, but the spin-down bands are semiconductor with a gap of 0.48 eV, and the spin-flip gap is of 0.28 eV. The Mn2ZnSi Heusler compound has a magnetic moment of 2 μB at the equilibrium lattice constant a = 5.80 Å. The Mn2ZnSi full-Heusler compound is ferrimagnetic and maintains the half-metallic character having 100% polarization for lattice constants ranging between 5.62 and 6.91 Å. 相似文献
67.
A. Yu. Menshikova N. N. Shevchenko I. V. Bugakov A. V. Yakimansky A. V. Sel’kin 《Physics of the Solid State》2011,53(6):1155-1160
Three-dimensional periodic solid-state film structures with a face-centered cubic lattice and a high degree of perfection
have been prepared from monodisperse particles of styrene copolymers with methacrylic acid. It has been shown that these structures
can be successfully used not only as model objects for studying specific features of light propagation in photonic crystals
but also as templates for synthesizing inverse opal-like structures. The influence of the degree of hydrophilization of the
surface layer of polymer particles forming a polymer template and the template synthesis conditions on the quality of an inverse
opal-like TiO2-based structure has been analyzed. 相似文献
68.
Doğan Fatih Özdek Naciye Selçuki Nursel Acar Kaya İsmet 《Journal of Thermal Analysis and Calorimetry》2019,138(1):163-173
Journal of Thermal Analysis and Calorimetry - In this study, the oxidative polymerization of orcinol monohydrate using different oxidants such as NaOCl, H2O2, and air was investigated.... 相似文献
69.
70.
Principles of hierarchical meso- and macropore architectures by liquid crystalline and polymer colloid templating 总被引:1,自引:0,他引:1
Sel O Kuang D Thommes M Smarsly B 《Langmuir : the ACS journal of surfaces and colloids》2006,22(5):2311-2322
The generation of porous silica with hierarchically organized bimodal mesoporosity of adjustable size and well-defined shape was investigated by using surfactant mixtures and the nanocasting procedure (liquid crystalline templating). A systematic study of combinations of various block copolymers (Pluronics F127, KLE (poly(omega-hydroxypoly(ethylene-co-butylene)-co-poly(ethylene oxide))) and SE (PS-co-PEO)) with smaller surfactants (Pluronics P123, C16mimCl, and CTAB) revealed that hierarchical bimodal mesopore architectures could only be obtained by the usage of block copolymers with a strong hydrophilic-hydrophobic contrast, such as KLE and SE, giving rise to pores between 6 and 22 nm. Furthermore, the ionic liquid (IL) C16mimCl appeared to have advantageous templating properties, resulting in 2-3-nm pores being located between the block copolymer mesopores, whereas phase separation was observed for Pluronics and CTAB as small templates. Thereby, the study provided also general insights into the mixing and co-self-assembly behavior of block copolymers and ionic surfactants in water and confirmed the special templating properties of ILs, as recently proposed. In addition to the bimodal mesoporosity, additional tunable macroporosity was created by the presence of poly(styrene) or poly(methyl methacrylate) spheres, leading to well-defined trimodal hierarchical pore architectures with the small pores being located in the walls of the respective larger pores. As a major improvement, due to the pore hierarchy, these large-pore materials showed relatively large surface areas and pore volumes, and the size of densely packed macropores could even be decreased down to 90 nm. The materials were characterized by electron microscopy, small-angle X-ray scattering, and nitrogen sorption using a proper NLDFT (nonlocal density functional theory) approach for calculations of the pore size distribution in the entire range of micro- and mesopores. 相似文献