首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   403篇
  免费   17篇
  国内免费   1篇
化学   283篇
晶体学   4篇
力学   3篇
数学   38篇
物理学   93篇
  2023年   3篇
  2021年   5篇
  2020年   10篇
  2019年   8篇
  2018年   8篇
  2017年   5篇
  2016年   9篇
  2015年   8篇
  2014年   4篇
  2013年   18篇
  2012年   14篇
  2011年   22篇
  2010年   10篇
  2009年   9篇
  2008年   23篇
  2007年   23篇
  2006年   26篇
  2005年   21篇
  2004年   14篇
  2003年   7篇
  2002年   11篇
  2001年   8篇
  2000年   6篇
  1999年   4篇
  1998年   8篇
  1997年   5篇
  1996年   9篇
  1995年   10篇
  1994年   7篇
  1993年   7篇
  1992年   6篇
  1991年   4篇
  1990年   6篇
  1987年   4篇
  1986年   5篇
  1984年   5篇
  1981年   6篇
  1980年   4篇
  1979年   3篇
  1977年   4篇
  1976年   5篇
  1975年   3篇
  1973年   2篇
  1971年   2篇
  1969年   4篇
  1968年   3篇
  1966年   4篇
  1932年   4篇
  1923年   2篇
  1920年   2篇
排序方式: 共有421条查询结果,搜索用时 15 毫秒
381.
A series of bicyclo analogues of several 2-phenylmorpholines were synthesized and tested for anti-tetrabenazine activity in mice. Most of the target compounds were prepared by reaction of 2-bromopropio-phenone ( 22 ) with the appropriate amino alcohol to form 2-phenylmorpholinols. Reduction of the 2-phenyl-morpholinols with sodium borohydride gave amino diols, which were cyclized to morpholines with acid. Alternatively, oxazines 17 and 18 were synthesized by alkylation of phenyl-(2-pyrrolo)carbinol (32a) and phenyl-(2-piperidyl)carbinol (32b) with 2-bromoethanol, followed by cyclization of the resulting amino diols with acid. Only the spirocyclic compounds 8 and 9 had i.p. anti-tetrabenazine activity comparable to the non-cyclic compounds 2a-3b , but 8 and 9 were less active by the oral route of administration.  相似文献   
382.
383.
The anti-HIV cyanobacterial protein cyanovirin-N can undergo domain swapping to form an intertwined dimer. The dimeric form is stable at low pH and millimolar concentrations. By deleting an amino acid from the hinge linker about which domain swapping occurs, we have constructed an obligate domain-swapped dimer of cyanovirin-N that represents a new tetravalent carbohydrate binding protein that is stable over a large range of pH values. This obligate dimer displays enhanced anti-HIV activity relative to the wild-type cyanovirin-N monomer with an observed 3.5-fold decrease in IC(50) (9nM for the dimer vs 32 nM for the monomer) for inhibition of HIV-1 envelope-mediated cell fusion and, when expressed in Escherichia coli, can be rapidly obtained in >98% purity in a single chromatographic step.  相似文献   
384.
6-[[(Hydroxyimino)phenyl]methyl]-1-[(1-methylethyl)sulfonyl]-1H-imidazo[4,5-b]pyridin-2-amine ( 1 ), an aza analogue of enviroxime, was synthesized in eight steps from 6-hydroxynicotinic acid ( 2 ). Acid 2 was nitrated, chlorinated with phosphorus pentachloride, and subjected to Friedel-Crafts aroylation to give 6-chloro-5-nitro-3-pyridyl phenyl ketone ( 5 ). Amination of 5 was followed by reduction of the nitro group and condensation with ethoxycarbonylisothiocyanate to give 6-benzyl-2-ethoxycarbonylamino-1H-imidazo[4,5-d]pyridine ( 8 ). The ethoxycarbonyl moiety of 8 was cleaved, N-1 was isopropylsulfonylated, and the carbonyl moiety was condensed with hydroxylamine to give 1 . Compound 1 was inactive against rhinovirus 1B and poliovirus type 1 .  相似文献   
385.
We observe well-defined regions of superexchange and thermally activated hopping in the temperature dependence of charge recombination (CR) in a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Ph(n)), n = 1-4, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI). A fit to the thermally activated CR rates of the n = 3 and n = 4 compounds yields activation barriers of 1290 and 2030 cm(-1), respectively, which match closely with theoretically predicted and experimentally observed barriers for the planarization of terphenyl and quaterphenyl. Negative activation of CR in the temperature regions dominated by superexchange charge transport is the result of a fast conformational equilibrium that increasingly depopulates the reactive state for CR as temperature is increased. The temperature dependence of the effective donor-acceptor superexchange coupling, V(DA), measured using magnetic field effects on the efficiency of the charge recombination process, shows that CR occurs out of the conformation with lower V(DA) via the energetically favored triplet pathway.  相似文献   
386.
The synthesis of some symmetrically substituted bis(arylsulfonyl)furoxans from aryl nitromethyl sulfones is reported.  相似文献   
387.
A general framework using density matrices is developed for the analysis of atomic excitation by spin-polarized electrons. This framework is applied to the specific case of the 3S 1/2→3P 3/2 transition in Na, as studied by the time-reversed, superelastic process. The scattering is characterized in terms of physical parameters describing the collisionally excitedp-state, i.e., its angular momentum (L ), linear polarization (P lin), and alignment angle (γ), with these parameters defined separately for singlet and triplet excitation. An expression for the scattering intensity is derived which is valid for arbitrary electron polarization and atomic state preparation. Specific examples are discussed with a view toward complete determination of the relevant scattering amplitudes and phases. Recent experimental results are reevaluated for comparison with theoretical calculations, and suggestions are made for future experiments.  相似文献   
388.
Supramolecular chaperones play an important role in directing the assembly of multiple protein subunits and redox-active metal ions into precise, complex and functional quaternary structures. Here we report that hydroxyl tailed C-alkylpyrogallol[4]arene ligands and redox-active MnII ions, with the assistance of proline chaperone molecules, can assemble into two-dimensional (2D) and/or three-dimensional (3D) networked nanocapsules. Dimensionality is controlled by coordination between the exterior of nanocapsule subunits, and endohedral functionalization within the 2D system is achieved via chaperone guest encapsulation. The tailoring of surface properties of nanocapsules via coordination chemistry is also shown as an effective method for the fine-tuning magnetic properties, and electrochemical and spectroscopic studies support that the nanocapsule is an effective homogeneous water-oxidation electrocatalyst, operating at pH 6.07 with an exceptionally low overpotential of 368 mV.

Molecular chaperones play a critical role in directing the assembly of nanocapsules that assemble into 2D or 3D coordination networks.  相似文献   
389.
The morphology, structure, and transport properties of pentacene thin film transistors (TFTs) are reported showing the influence of the gate dielectric surface roughness. Upon roughening of the amorphous SiO2 gate dielectric prior to pentacene deposition, dramatic reductions in pentacene grain size and crystallinity were observed. The TFT performance of pentacene films deposited on roughened substrates showed reduced free carrier mobility, larger transport activation energies, and larger trap distribution widths. Spin coating roughened dielectrics with polystyrene produced surfaces with 2 A root-mean-square (rms) roughness. The pentacene films deposited on these coated surfaces had grain sizes, crystallinities, mobilities, and trap distributions that were comparable to the range of values observed for pentacene films deposited on thermally grown SiO2 (roughness also approximately 2 A rms).  相似文献   
390.
Deoxy sugars are ubiquitous in nature and contribute to diverse biological activities. Attempts to design systems to control or to mimic their functions are hampered, however, by the lack of biosynthetic knowledge of these unique sugars. To elucidate the mechanism by which the sugar deoxygenation is effected, we have initiated a study to explore the biosynthesis of CDP-ascarylose, a 3,6-dideoxyhexose found in the lipopolysaccharides of Yersinia pseudotuberculosis, and our initial focus centered on C-3 deoxygenation catalyzed by E1 and E3. We have now purified the wild-type enzymes, cloned the corresponding genes (ascC for E1 and ascD for E3), and overexpressed the gene products in Escherichia coli. The purified E3 is a flavoprotein comprising an iron-sulfur center and E1 is an iron-sulfur containing, pyridoxamine 5′-phosphate-de-pendent enzyme. Since these iron-sulfur clusters are well known one-electron carriers, reactions mediated by E1 and E3 must proceed via a radical mechanism. Recently, EPR analysis of E1/E3 catalysis indicated a potential new redox role for pyridoxamine as a cofactor. These findings make this system unique from two perspectives: E1 is the only coenzyme B6-dependent catalyst that interacts with a sugar and not with an amino acid, and it is the first example in which coenzyme B6 may facilitate one-electron redox chemistry. Thus, the unprecedented mechanisms of E1 and E3 distinguish this system as a novel radical deoxygenation with potentially interesting future developments.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号