The chromatographic behavior of 8 lanostanoid triterpenes including 4 pairs of C-3 epimers and 2 pairs of C-3/C-15 positional isomers was studied by reversed phase HPLC. A consistent elution sequence of these paired isomers in all combination of three distinct solvent systems was observed. The mobile phase selectivity to separate specific, paired isomers was mainly dependent on the polarity difference contributed by C-3/C-15 hydroxyl and acetoxyl groups. The stereochemistry at C-3 played more profound role in guiding the hydrophobic interaction with C18 stationary phase in the separation of C-3/C-15 positional isomers. 相似文献
A convenient method for the synthesis of condensed tricyclic pyridines using intramolecular cyclization of 4-arylpyridines is reported. The tricyclic compounds, 2-azafluorenone (9), 2-azafluorenone (10), 2-azafluorenol (11), lactones 12 and 13, and anhydride 14 have been prepared. 相似文献
A silicon nitride (SiNx) membrane diffractive optical element (DOE) designed to exhibit beam-splitting and focusing behavior at visible wavelengths has been fabricated and tested. Since the fabrication process is based on silicon micromachining technology, the DOE is easily integrated with a laser diode chip and a photodiode chip on a silicon substrate to function as the hologram-laser-photodiode unit for use in the pickup head of a CD or DVD system. The SiNx film is deposited with low-pressure chemical-vapor deposition and the free-standing membrane is formed by KOH etching. The transmissive DOE showed a high diffraction efficiency (>20% for a binary-phase-level element). The experimental evaluation was in good agreement with the designed and modeled predictions. 相似文献
Zusammenfassung Die periodische laminare Strömung einer zähen inkompressiblen Flüssigkeit in der Eintrittspartie eines zylindrischen Rohres wird auf Grund der linearisierten Gleichungen vonNavier undStokes berechnet. Es wird dabei angenommen, dass im Eintrittsquerschnitt in jedem Zeitpunkt eine über den Querschnitt konstante Geschwindigkeit herrsche, die jedoch eine Funktion der Zeit ist. Formeln für den Druckgradienten und die Geschwindigkeitskomponenten in Richtung längs der Rohrachse werden erhalten. Ebenso wird die Einlaufstrecke berechnet.
Sommaire On étudie l'écoulement laminaire oscillatoire d'un fluide incompressible visqueux près de l'entrée d'un tube circulaire sur la base des équations linéairisées Navier-Stokes. On suppose que la vitesse du fluide soit uniforme à l'entrée et qu'elle soit une fonction de temps connue périodique arbitraire. On obtient des formules pour les composantes longitudinales de la vitesse et le gradient de pression et on calcule la longueur d'entrée.
A unified approach to interacting vacuum excitations and quark confinement is formulated in quantum field theories with symmetry breakdown. Vacuum excitations are shown to be coherent clouds of Goldstone bosons or gauge bosons and are interpreted as new asymptotic extended particle states. They correspond to all dynamically possible space-time dependent Bose condensations of the Goldstone bosons in a given theory. Different configurations of vacuum excitations are connected to one another by a family of invariant boson transformations. As an example, the Nambu theory of interacting vortex strings is derived from a Nambu-Heisenberg quark-gluon field theory. The quarks can be completely confined to the strings while the gluons cluster in quantized magnetic flux bundles of penetration width mv?1 and provide a short range interaction force. 相似文献
Summary: A series of methacrylate copolymers containing polyhedral oligomeric silsesquioxane (POSS) was synthesized from the free radical copolymerization of methacrylic acid, methyl methacrylate, and isobutyl propylmethacryl polyhedral oligosilsesquioxanes, and then were modified with glycidyl methacrylate to serve as negative‐type photoresists. The UV/Vis spectroscopy reveals that the incorporation of POSS moiety into the copolymer results in a slight decrease in transparency from 99 to 92.5% (at wavelength = 365 nm). The photosensitivity in terms of resist sensitivity (D), contrast (γ), and photopolymerization rate are significantly increased with increase in the POSS content. In addition, the induction time is reduced from 0.520 to 0.515 min after incorporating the POSS unit based on photo‐DSC analyses. These observed results can be rationalized as due to hydrogen bonding interactions between siloxane and hydroxyl groups in copolymers which tend to attract the methacrylate double bonds surrounding POSS units to crosslink, thereby enhancing the photopolymerization rate and sensitivity. We further evaluate the lithographic property of a photoresist under a collimated exposure.
Compact double bonds zone induced via hydrogen bonding. 相似文献