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   106102篇
  免费   1558篇
  国内免费   388篇
化学   44145篇
晶体学   994篇
力学   7215篇
数学   35350篇
物理学   20344篇
  2023年   253篇
  2022年   201篇
  2021年   359篇
  2020年   526篇
  2019年   474篇
  2018年   10837篇
  2017年   10653篇
  2016年   7089篇
  2015年   1752篇
  2014年   1230篇
  2013年   2277篇
  2012年   5870篇
  2011年   12527篇
  2010年   6780篇
  2009年   6951篇
  2008年   8388篇
  2007年   10476篇
  2006年   1895篇
  2005年   2785篇
  2004年   2720篇
  2003年   2872篇
  2002年   1832篇
  2001年   881篇
  2000年   834篇
  1999年   558篇
  1998年   480篇
  1997年   354篇
  1996年   526篇
  1995年   331篇
  1994年   284篇
  1993年   354篇
  1992年   284篇
  1991年   214篇
  1990年   188篇
  1989年   178篇
  1988年   178篇
  1987年   175篇
  1986年   173篇
  1985年   225篇
  1984年   183篇
  1983年   154篇
  1982年   145篇
  1981年   138篇
  1980年   121篇
  1979年   136篇
  1978年   114篇
  1977年   91篇
  1976年   91篇
  1975年   86篇
  1973年   105篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
The microwave-mediated reaction of 4-aryl-4-oxobutanoic acids with benzylamines furnished 1-arylmethyl-3-[(E)-1-arylmethylidene]-5-phenyl-2,3-dihydro-1H-pyrrolones. This result is in contract to the earlier report on this reaction conducted under neat conditions. Structures for the products were assigned on the basis of spectral data and confirmed by independent synthesis. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 368–373, March, 2007.  相似文献   
992.
A flow-injection configuration is proposed for the individual determination of cysteine and cystine and for the mixtures of both analytes. The procedure is based on the inhibitory effect of cysteine on the oxidation of thiamine to thiochrome by mercury(II). Linear calibration graphs were obtained between 1.0 x 10(-5) and 1.0 x 10(-4)M, with a sampling rate of 22 samples/hr and relative standard deviation of 1.14%. The inclusion of a selecting valve in the configuration, to pump water or hydroxylamine, allows the successive determination of these two analytes. The applicability of the method to the determination of cysteine and cystine in pharmaceutical preparations was demonstrated by investigating the effect of potential interferences and by the analysis of commercial preparations.  相似文献   
993.
A new class of pi-extended TTF-type electron donors (11 a-c) has been synthesized by Wittig-Horner olefination of bianthrone (9) with 1,3-dithiole phosphonate esters (10 a-c). In cyclic voltammetry experiments, donors 11 a-c reveal a single, electrochemically irreversible oxidation-yielding the corresponding dicationic products-at relatively low oxidation potentials (approximately 0.7-0.8 V). Theoretical calculations, performed at the DFT level (B3 P86/6-31 G*), predict a highly-folded C(2h) structure for 11 a. In the ground state, the molecule adopts a double saddle-like conformation to compensate the steric hindrance. The calculations suggest that the intramolecular charge transfer associated with the HOMO-->LUMO transition is responsible for an absorption band observed above 400 nm. While the radical cation 11 a*+ retains the folded C(2h) structure predicted for the neutral molecule as the most stable conformation, the dication 11 a(2+) has a fully aromatic D(2) structure, formed by an orthogonal 9,9'-bianthryl central unit to which two singly-charged dithiole rings are attached. The drastic conformational changes that compounds 11 undergo upon oxidation account for their electrochemical properties. By means of pulse radiolysis measurements, radical-induced one-electron oxidation of 11 a-c was shown to lead to the radical cation species (11 a-c*+), which were found to disproportionate with generation of the respective dication species (11 a-c(2+)) and the neutral molecules (11 a-c).  相似文献   
994.
The structure elucidation and 1H and 13C assignments of the new triterpenes 3beta-palmitoxy-7beta-hydroxyolean-12-ene (1) and its hydrolysis product 3beta,7beta-dihydroxyolean-12-ene (2) and 3beta-sn-glyceroyl-(1'-palmitoxy)urs-12-ene (3), isolated from the aerial parts of Cladocolea grahami (Loranthaceae), are reported.  相似文献   
995.
A new isoprenyl phenyl ether, 3-hydroxy-4-(3-methylbut-2-enyloxy)benzoic acid methyl ester (1), together with 4-hydroxybenzoic acid (2), 2-hydroxy-6-methylbenzoic acid (3), and 4-hydroxy-3-methoxybenzoic acid (4) were isolated from Mangrove fungus (No. B60) from the South China Sea. The structures of the compounds were established on the basis of NMR spectroscopic and mass spectrometric data. In the preliminary bioassay, compound 1 exhibited antibacterial and antifungal activities. Compound 1 also inhibited cytotoxicity to the hepG2 cell line with an IC50 value of 10.0 μg/mL. Published in Khimiya Prirodnykh Soedinenii, No. 4, pp. 313–314, July–August, 2007.  相似文献   
996.
A global forcing set in a simple connected graph G with a perfect matching is any subset S of E(G) such that the restriction of the characteristic function of perfect matchings of G on S is an injection. The number of edges in a global forcing set of the smallest cardinality is called the global forcing number of G. In this paper we prove several results concerning global forcing sets and numbers of benzenoid graphs. In particular, we prove that all catacondensed benzenoids and catafused coronoids with n hexagons have the global forcing number equal to n, and that for pericondensed benzenoids the global forcing number is always strictly smaller than the number of hexagons.  相似文献   
997.
Compatibility studies between active drugs and excipients are substantial in the pharmaceutical technology. The objective of the present work was to develop pre-formulated mixtures of metronidazole (MT) obtained by spray drying (SPDR) and their thermoanalytical characterization. Dynamic and isothermal TG, conventional DSC and DSC coupled to a photovisual system were used. DSC experiments with both techniques confirmed the homogeneity of the conventional and pre-formulated mixtures. The TG data made possible the comparison the thermal stability of the different mixtures. Similar thermal stabilities were found of the conventional and pre-formulated mixtures, with slower particles sizes of MT.  相似文献   
998.
Non steroidal anti-inflammatory drugs are the most widely used medicines for relief of pain. These drugs have some side effects, particularly toxicity in the gastrointestinal tract and kidneys. Various approaches have been used for obtaining safer anti-inflammatory drugs. In this review we have summarized the recent developments in the following areas; (i) mode of action of NSAIDs (ii) Role of COX-1 & COX-2 in inflammation, (iii) Different approaches used to improve gastric tolerance i.e. chemical manipulation, formulation & co-administration, development of non specific (COX-1 & COX-2 inhibitors) and specific (COX-2 inhibitors) inflammation inhibitors, and development of inflammation inhibitors having a mode of action other than COX-1 & COX-2 inhibition. We have also focused on the safety of COX-2 inhibitors and the synthesis of heterocyclic compounds and their role as inflammation inhibitors.  相似文献   
999.
Four diterpenes and a nor-diterpenoid, all of which possess unusual carbocyclic skeletons, were isolated from the hexane solubles of the West Indian gorgonian Pseudopterogorgia elisabethae. The structures and relative configurations of novel metabolites elisabethin D (2), elisabethin D acetate (3), 3-epi-elisabanolide (5), elisapterosin A (6), and elisapterosin B (7) were elucidated by interpretation of overall spectral data, which included 2D NMR correlation methods, IR, UV, and accurate mass measurements (HREI-MS and HRFAB-MS), chemical reactions, and X-ray diffraction analyses. The tetracyclic carbon skeleton of the elisapterosins is undescribed and constitutes a new class of C(20) rearranged diterpenes. Elisapterosin B displays strong in vitro anti-tuberculosis activity.  相似文献   
1000.
The arginine repressor (ArgR) from Escherichia coli regulates genes for L-arginine metabolism and is a required recombination factor for colE1 plasmid replication. Both functions require binding of L-arginine to the protein. In this work, nano-electrospray ionization time-of-flight mass spectrometry (nano-ESI-TOFMS) is used to study conformational and oligomeric states of intact ArgR and its isolated structural domains. In agreement with X-ray diffraction studies, it is shown that ArgR oligomerizes to form hexamers in both the presence and absence of L-arginine, and the basic unit of oligomerization appears to be the trimer. Higher-order assembly into dodecamers is also detected. The isolated C-terminal domain is found to associate into trimers and hexamers whereas the N-terminal domain is detected in its monomeric form. The observed species distributions suggest a role for the N-terminal domain in hexamer stabilization.  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