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主要介绍自由电子激光相干强太赫兹源(FEL-THz)装置上的砷化镓光阴极直流高压注入器的研究进展,并讨论其驱动未来高重复频率短波长自由电子激光器的差距。通过综合砷化镓阴极寿命的三大影响因素,提出了其工作寿命的定性物理模型;通过该模型对阴极和注入器进行优化,在直流高压电子枪上得到了5 mA, 32 min的连续稳定输出;测量了电子束在4.8 mA下归一化发射度约为4.0 mmmrad,阴极热发射度约为0.6 mmmrad,电子束本征横向能量约为92 meV,250 keV电子束在距离阴极90.6 cm处纵向均方根长度约为11.5 ps。这一束流状态已经基本满足FEL-THz需求。  相似文献   
2.
An effective method for cis-stereoselective attachment of functionalized alkenyl appendages to sp3 carbon is reported. This method is based on a free-radical process, involving a sequence of addition-elimination steps, resulting in alkenyl group transposition from divalent sulfur to a prochiral carbon radical. Absolute stereoselectivity is secured since the new carbon-carbon bond is formed in a ring-closure reaction leading to a bridged bicyclic carbon-centered radical intermediate. The latter undergoes beta-scission of the C-S bond, leaving the alkenyl side chain in its predetermined position while releasing a thiyl radical. This thiyl radical is trapped by tri-n-butylstyryltin, affording a (styrylsulfanyl)methyl side chain and a tri-n-butyltin radical that continues the chain. When 2-(alkenylsulfanyl)methyl-4-bromo(or iodo)pyrrolidines were used as starting materials 2,4-cis-disubstituted 4-alkenyl-2-(styrylsulfanyl)methylpyrrolidines were obtained as products (70-90% yield). Tri-n-butylstyryltin was used rather then the more common n-Bu3SnH as tin radicals sources because the latter led predominantly to bridged bicyclic 3-thia-6-azabicyclo[3.2.1]octanes (up to 77% yield). An additional advantage of using tri-n-butylstyryltin derives from the discovery that the resulting styrylsulfide functionality is an excellent synthetic equivalent to the formyl group. Thus, using a Pummerer-type oxidative desulfurization, 4-cis-alkenyl-proline aldehydes were obtained.  相似文献   
3.
A series of anthraquinone (C(14)O(2)H(8)) derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), and AM1 quantum chemical calculations. Mean lifetimes of molecular negative ions M(-.) (MNI) have been measured. The mechanism of long-lived MNI formation in the epithermal energy region of incident electrons has been investigated. A simple model of a molecule (a spherical potential well with the repulsive centrifugal term) has been applied for the analysis of the energy dependence of cross sections at the first stage of the electron capture process. It has been shown that a temporary resonance of MNI at the energy approximately 0.5 eV corresponds to a shape resonance with lifetime 1-2.10(-13) s in the f-partial wave (l = 3) of the incident electron. The next resonant state of MNI at the energy approximately 1.7 eV has been associated with the electron excited Feshbach resonance (whose parent state is a triplet npi* transition). In all cases the initial electron state of the MNI relaxes into the ground state by means of a radiationless transition, and the final state of the MNI is a nuclear excited resonance with a lifetime measurable on the mass spectrometry timescale. Copyright 1999 John Wiley & Sons, Ltd.  相似文献   
4.
New derivatives of 3-hydroxyquinolone (3HQ) with a fused benzene ring (3-hydroxybenzo[g]quinolones) have been synthesized. They display a remarkable red shift of their absorption spectrum in comparison with other 3HQ analogs allowing their excitation by common He/Cd and Ar-ion lasers. As a result of their irreversible excited-state intramolecular proton transfer (ESIPT) reaction, they display a dual fluorescence in a series of solvents of varying polarities, starting from toluene to methanol. The dual emission of these dyes correlates well with solvent H-bond basicity, which is connected with the effect of this solvent property on the kinetics of the ESIPT reaction. In addition to their red-shifted absorption and fluorescence, these new derivatives show a larger separation of their two emission bands and a more appropriate range of their intensity ratio than the previously synthesized 3HQs. These properties allow an improved ratiometric evaluation of the local H-bond basicity of unknown environments, which will favor future applications of the new dyes in polymer and biological sciences.  相似文献   
5.
3-Hydroxyquinolones (3HQs) are a new class of water soluble dual fluorescence probes that can monitor both polarity and basicity (H-bond accepting ability) parameters. Both parameters play an important role in proteins and lipid membranes. Nevertheless, no method exists actually to measure the basicity parameter separately from the polarity. To achieve this aim, we synthesized 2-benzofuryl-3-hydroxy-4(1H)-quinolone (3HQ-Bf) and characterized its photophysical properties by UV, steady-state and time-resolved fluorescence spectroscopy. Due to its extended conjugation and totally planar conformation, 3HQ-Bf is characterized by a high fluorescence quantum yield. In solution, this dye shows an excited state intramolecular proton transfer (ESIPT) reaction resulting in two tautomer bands in the emission spectra. The ESIPT reaction can be considered as irreversible and is governed by rate constants from 0.6 to 8 × 109 s−1, depending on the solvent. The analysis of the spectral properties of 3HQ-Bf in a series of organic solvents revealed a marginal sensitivity to the solvent polarity, but an exquisite sensitivity to solvent basicity, as shown by the linear dependence of the logarithm of the emission bands intensity ratio, log(IN*/IT*), as well as the absorption or emission maxima wavenumbers as a function of the solvent basicity parameter. This probe may find useful applications through coupling to a protein ligand, for characterizing the H-bond acceptor ability at the ligand binding site as well as for studying the basicity changes of lipid membranes during their chemo- and thermotropic conversions.  相似文献   
6.
富硒蛹虫草试样中硒的形态分析   总被引:6,自引:0,他引:6  
本文采用连续浸提法研究了富硒蛹虫草中硒的赋存形态。结果表明,碱溶态>盐溶态>水溶态>醇溶态>残渣态。可见,碱浸取是获取植物蛋白的有效途径,在富硒蛹虫草试样中硒主要以硒蛋白的形式存在。  相似文献   
7.
Appearance energies of [M-H](-) ions from carbonyl compounds R-CO-R' (R,R' = H, CH(3), NH(2), OH) have been measured by means of negative ion mass spectrometry in resonant electron capture mode. Values of electron affinity of the corresponding radicals, CH(2)&dbond;C(X)O, NH&dbond;C(X)O and O&dbond;C(X)O, have been determined. Copyright 1999 John Wiley & Sons, Ltd.  相似文献   
8.
PPT文件中的动画制作及在教学中的应用   总被引:3,自引:1,他引:2  
介绍了利用PowerPoint软件的自定义动画中的"进入"、"强调"、"退出"、"动作路径"四类效果,制作教学中常用的化学结构式、图表、原理图、选择题等的动画制作的方法,以及常用图表的讲解、重点内容的强调、利用现有矢量性质剪帖画改制成自己所需图画的简易方法.PPT文件制作动画,操作简单,避免了超链接的麻烦,形成的文件小,便于拷贝,可制作出具有Flash效果的漂亮、生动的动画,丰富了教学内容、提升了教学效果.  相似文献   
9.
The synthesis of the C18-C31 subunit of orevactaene (1) in an enantioselective and convergent manner is reported. Four chiral centers in the structure (i.e., carbons 23, 25, 32, and 33) have unknown configuration; thus, a modular approach has been devised to link the two stereocenter-containing ends of the structure together via a trisubstituted olefin template to ultimately produce all possible diastereomers of the target. Keys to the success of this approach include (i) an efficient synthesis of four diastereomeric hydrophobic tails (C22-C29) of the molecule with two stereogenic centers at C23 and C25; (ii) the synthesis of three stereodefined trisubstituted olefins 37, 38, and 43 using palladium(0)-catalyzed hydrometalation and metallometalation; and (iii) the convergent assembly of the aforementioned sections by a 'one-pot' lithium/halogen exchange, boron/lithium exchange, borate ester saponification, and Suzuki cross-coupling followed by oxidative deprotection. The sequence provided the desired aldehydes 49 and 50 as single isomers in good yields. Compiled spectroscopic data from the literature and present work provides evidence that the relative configuration of the methyl groups in the side chain of orevactaene may be 1,3-syn, which will be confirmed when the total synthesis has been completed. These results have paved the way for a parallel synthesis approach to prepare all 16 possible stereoisomers of orevactaene so that the relative and absolute stereochemistry of this compound can be determined.  相似文献   
10.
ABSTRACT

Schiff-base complementary hydrogen-bonded liquid crystals (HBLC), viz., (4)MeOBD(3)AmBA:nOBAs with flexible chain length for n = 3,4,5,6,7,8,9,10,11 and 12 are reported. 1H; 13C-NMR and Infra-Red spectroscopy used to confirm the formation of HBLCs. LC phases and transition temperatures (Tc) determined by polarised optical microscopy (POM) and differential scanning calorimetry (DSC). Tc and enthalpy (?H) determined by DSC also. Odd-Even effect observed at clearing and melting transitions. Influence of Schiff base and Oxygen as bridging atom promote smectic phase abundance. HBLCs exhibit tetra- or penta-phase variance. Maximum (penta) phase variance is exhibited by n = 8 and 12 with long flexible chain. Prevalent abundance of quasi-two-dimensional (2D) LC phases of SmF and SmI is observed. Nematic phase is quenched. Lower (n = 4) and intermediate (n = 6) members exhibited SmBcryst phase. Predominant occurrence of enantiotropic LC phases is noticed. All members exhibited 3D tilted SmG phase. A-C transition exhibited by intermediate homologues (for n = 7, 8, 9 and 10) is found to be either second order or with very small enthalpy. Phase diagram reveals the abundance of multi-critical points with LC phasesinvolving exotic symmetries. Influence of meta-extended rigid core, complementary HB and flexibility is studied for the LC phase abundance with characteristic structural order. POM and DSC results are discussed in the wake of reports in other achiral calamitic LCs.  相似文献   
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