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   73949篇
  免费   12401篇
  国内免费   7294篇
化学   50895篇
晶体学   657篇
力学   4743篇
综合类   310篇
数学   8200篇
物理学   28839篇
  2024年   279篇
  2023年   1601篇
  2022年   2716篇
  2021年   2862篇
  2020年   3105篇
  2019年   2908篇
  2018年   2563篇
  2017年   2335篇
  2016年   3573篇
  2015年   3472篇
  2014年   4135篇
  2013年   5306篇
  2012年   6736篇
  2011年   6728篇
  2010年   4469篇
  2009年   4254篇
  2008年   4595篇
  2007年   4111篇
  2006年   3756篇
  2005年   3133篇
  2004年   2429篇
  2003年   1920篇
  2002年   1751篇
  2001年   1471篇
  2000年   1355篇
  1999年   1535篇
  1998年   1305篇
  1997年   1291篇
  1996年   1289篇
  1995年   1051篇
  1994年   922篇
  1993年   763篇
  1992年   675篇
  1991年   580篇
  1990年   501篇
  1989年   384篇
  1988年   335篇
  1987年   274篇
  1986年   249篇
  1985年   204篇
  1984年   143篇
  1983年   102篇
  1982年   80篇
  1981年   67篇
  1980年   65篇
  1979年   33篇
  1978年   30篇
  1977年   40篇
  1976年   30篇
  1973年   28篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
脂肪族酰胺二元醇的合成与表征   总被引:3,自引:1,他引:3  
刘孝波  张军华 《合成化学》1998,6(4):339-341
以DL-乳酸、ε己内酯及乙二胺和己二胺为原料合成了四种结构的脂肪族酰胺二元醇,并用IR,^1HNMR,DSC对酰胺二元醇的结构进行了表征。  相似文献   
942.
Treatment of tetracopper(I)-phosphonitocavitand [1·Cu4(μ-Cl)44-Cl)] (2) (1 = tetraphosphonitocavitand [rccc-2,8,14,20-tetrakis-(iso-butyl)-phosphonitocavitand (C44H48O8P4Ph4)]) with PhSeSiMe3 in THF at low temperature afforded a novel polyanionic cluster [pyH]6[(CuCl)93-SePh)54-SePh)] (4) as a major product along with a new tetracopper(I)-phosphonitocavitand (3) with a centered μ3-Cl. Molecular structure of anionic cluster in 4 consists of six PhSe bridging ligands containing five μ3-SePh and one exceptional μ4-SePh bridging nine copper atoms, of which eight copper atoms have trigonal coordination geometry and the other has distorted tetrahedral geometry. Dedicated to Professor Han-Qin Liu on the occasion of his 70th birthday.  相似文献   
943.
Direct trimethylsilylation of naphthalene under certain condition has been found to afford substitution as well as addition products: 1-and 2-trimethylsilylnaphtalene (I, II), 1-trimethylsilyl-1,4-dihydronaphthalene (III), trans-1,2-bis(trimethylsilyl)-1,2-dihydronaphthalene (IV-a) and its isomer (IV-b), and 1,2,4-tris(trimethylsilyl)-1,2-dihydronaphthalene (V). The configuration has been determined by nmr spectroscopy, and the possible reaction path was proposed.  相似文献   
944.
扩张床吸附技术   总被引:7,自引:0,他引:7  
扩张床是流化床的一种特例。它具有流化床的特点,能处理含悬浮颗粒的液体。又具有固定床的优点,流动成活塞流;返混程度低,分离效率高。作为蛋白质的初步分离方法,它能取代固液分离、浓缩和初步纯化等三步操作。具有提高收率、降低投资费用、缩短操作时间等优点,成为生物工程下游过程的研究热点。本文综述了近年来扩张床吸附技术的发展。包括:1、原理:床层的分层稳定性、吸附剂和吸附柱;2、操作:吸附、洗涤、洗脱和再生4个步骤;3、流动动力学特性:床层扩展特征和停留时间分布;4.在蛋白质纯化中的应用。  相似文献   
945.
Precision medicine has been strongly promoted in recent years. It is used in clinical management for classifying diseases at the molecular level and for selecting the most appropriate drugs or treatments to maximize efficacy and minimize adverse effects. In precision medicine, an in-depth molecular understanding of diseases is of great importance. Therefore, in the last few years, much attention has been given to translating data generated at the molecular level into clinically relevant information. However, current developments in this field lack orderly implementation. For example, high-quality chemical research is not well integrated into clinical practice, especially in the early phase, leading to a lack of understanding in the clinic of the chemistry underlying diseases. In recent years, mass spectrometry (MS) has enabled significant innovations and advances in chemical research. As reported, this technique has shown promise in chemical mapping and profiling for answering “what”, “where”, “how many” and “whose” chemicals underlie the clinical phenotypes, which are assessed by biochemical profiling, MS imaging, molecular targeting and probing, biomarker grading disease classification, etc. These features can potentially enhance the precision of disease diagnosis, monitoring and treatment and thus further transform medicine. For instance, comprehensive MS-based biochemical profiling of ovarian tumors was performed, and the results revealed a number of molecular insights into the pathways and processes that drive ovarian cancer biology and the ways that these pathways are altered in correspondence with clinical phenotypes. Another study demonstrated that quantitative biomarker mapping can be predictive of responses to immunotherapy and of survival in the supposedly homogeneous group of breast cancer patients, allowing for stratification of patients. In this context, our article attempts to provide an overview of MS-based chemical mapping and profiling, and a perspective on their clinical utility to improve the molecular understanding of diseases for advancing precision medicine.

An overview of MS-based chemical mapping and profiling, indicating its contributions to the molecular understanding of diseases in precision medicine by answering "what", "where", "how many" and "whose” chemicals underlying clinical phenotypes.  相似文献   
946.
In spite of great commercial importance of the Phillips CrOx/SiO2 catalyst and long term research efforts, the precise physicochemical nature of active sites and polymerization mechanisms still remains unclear. The difficulties in a clear mechanistic understanding of this catalyst mainly come from the complexity of the surface chemistry of the amorphous silica gel support. In this work, novel silsesquioxane-supported Phillips Cr catalysts are utilized as realistic models of the industrial catalyst for theoretical investigation using the density functional theory (DFT) method in order to elucidate the effects of surface chemistry of silica gel in terms of supporting of chromium compounds and fluorination of the silica surface on the catalytic properties of the Phillips catalyst. Both qualitative and quantitative aspects with respect to various electronic properties and thermodynamic characteristics of the model catalysts were achieved. The future prospects of a state-of-the-art catalyst design and mechanistic approaches for the heterogeneous SiO2-supported Phillips catalyst has been demonstrated. The text was submitted by the authors in English.  相似文献   
947.
beta-cyclodextrin immobilized on Pd nanoparticles was successfully employed as an efficient phase-transfer catalyst in aqueous biphasic hydrogenation reactions.  相似文献   
948.
High-speed counter-current chromatography (HSCCC) technique in semi-preparative scale has been successfully applied to the separation of bioactive flavonoid compounds, liquiritigenin and isoliquiritigenin in one step from the crude extract of Glycyrrhiza uralensis Risch. The HSCCC was performed using a two-phase solvent system composed of n-hexane-ethyl acetate-methanol-acetonitrile-water (2:2:1:0.6:2, v/v). Yields of liquiritigenin (98.9% purity) and isoliquiritigenin (98.3% purity) obtained were 0.52% and 0.32%. Chemical structures of the purified liquiritigenin and isoliquiritigenin were identified by electrospray ionization-MS (ESI-MS) and NMR analysis.  相似文献   
949.
Bromoiodomethane photodissociation in the low-lying excited states has been characterized using unrestricted Hartree-Fock, configuration-interaction-singles, and complete active space self-consistent field calculations with the SDB-aug-cc-pVTZ, aug-cc-pVTZ, and 3-21g** basis sets. According to the results of the vertical excited energies and oscillator strengths of these low-lying excited states, bond selectivity is predicted. Subsequently, the minimum energy paths of the first excited singlet state and the third excited state for the dissociation reactions were calculated using the complete active space self-consistent field method with 3-21g** basis set. Good agreement is found between the calculations and experimental data. The relationships of excitations, the electronic structures at Franck-Condon points, and bond selectivity are discussed.  相似文献   
950.
由于氟氟烃(CFCs)对大气臭氧层有破坏作用,人们拟以另外的化合物来代替它,CF3CX2H(X=H,CI,F)可能是一类合适的取代物.但人们对它及其反应产物对大气的影响还不清楚.CFaC(0)F是CFaCX。H大气光氧化过程的终产物之一[1-3],其在大气中的后继反应行为将直接关系到CF2  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