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   3024篇
  免费   85篇
  国内免费   16篇
化学   2127篇
晶体学   33篇
力学   30篇
数学   119篇
物理学   816篇
  2021年   16篇
  2020年   29篇
  2019年   44篇
  2018年   21篇
  2017年   23篇
  2016年   49篇
  2015年   51篇
  2014年   59篇
  2013年   132篇
  2012年   142篇
  2011年   149篇
  2010年   94篇
  2009年   91篇
  2008年   155篇
  2007年   139篇
  2006年   164篇
  2005年   125篇
  2004年   146篇
  2003年   120篇
  2002年   102篇
  2001年   86篇
  2000年   65篇
  1999年   42篇
  1998年   27篇
  1997年   31篇
  1996年   43篇
  1995年   44篇
  1994年   30篇
  1993年   46篇
  1992年   54篇
  1991年   40篇
  1990年   27篇
  1989年   36篇
  1988年   41篇
  1987年   34篇
  1986年   21篇
  1985年   70篇
  1984年   58篇
  1983年   38篇
  1982年   45篇
  1981年   41篇
  1980年   34篇
  1979年   34篇
  1978年   36篇
  1977年   36篇
  1976年   33篇
  1975年   29篇
  1974年   17篇
  1973年   29篇
  1966年   17篇
排序方式: 共有3125条查询结果,搜索用时 31 毫秒
81.
82.
Micrococcus luteus (M. lysodeikticus) labeled with Remazol brilliant blue R (blue ML) was prepared as a novel substrate for the colorimetric assay of lysozyme. The treatment of the labeled substrate with lysozyme resulted in the release of soluble blue products which can be easily measured spectrophotometrically at 600 nm. The blue color was most efficiently released at pH 7 and ionic strength of 0.2 on incubation with hen lysozyme at 40 degrees C. A new colorimetric method for the assay of lysozyme using this substrate was developed. The assay system gave a linear dose-response curve, and as little as 0.1 microgram of human lysozyme (1 microgram/ml, 100 microliters) can be detected. The present method is more convenient and reproducible than the conventional lysozyme assay with bacterial cells. Application of the system to the determination of lysozyme in human serum is described.  相似文献   
83.
Neutral aminyl radicals generated by anodic oxidation of lithium alkenylamides 2 undergo a stereoselective cyclization to give cis-1-methyl-2,5-disubstituted pyrrolidines 4. Their cis stereochemistry was confirmed by a comparison with trans-1,2-dimethyl-5-phenylpyrrolidine, the structure of which was established by X-ray crystallographic analysis of its quarternary ammonium bromide 6.  相似文献   
84.
Using a hydroxyl group appended free base porphyrin derivative (HP), porphyrin-introduced silica gels were synthesized by a sol-gel process. The HP content in the materials linearly increased with increasing the HP concentration. Meanwhile, free base tetraphenylporphyrin (TPP) with no hydroxyl groups were almost not incorporated into the silica gels. These facts suggested that the interaction between hydroxyl groups of the HP molecules and silica network is considerably strong. The UV-vis characters of HP-introduced materials were almost the same as pure HP molecules. The Beer's plot indicated that the HP molecules in the materials are dispersed.  相似文献   
85.
This paper describes a systematic study of addition reactions between the chiral Ni(II) complex of the Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone and (S)- or (R)-3-[(E)-enoyl]-4-phenyl-1,3-oxazolidin-2-ones as a general and synthetically efficient approach to beta-substituted pyroglutamic acids and relevant compounds. These reactions were shown to occur at room temperature in the presence of nonchelating organic bases and, most notably, with very high (>98% diastereomeric excess (de)) stereoselectivity at both newly formed stereogenic centers. The stereochemical outcome of the reactions was found to be overwhelmingly controlled by the stereochemical preferences of the Michael acceptors, and the chirality of the glycine complex influenced only the reaction rate. Thus, in the reactions of both the (S)-configured Ni(II) complex and the Michael acceptors, the reaction rates were exceptionally high, allowing preparation of the corresponding products with virtually quantitative (>98%) chemical and stereochemical yields. In contrast, reactions of the (S)-configured Ni(II) complex and (R)-configured Michael acceptors proceeded at noticeably lower rates, but the addition products were obtained in high diastereo- and enantiomeric purity. To rationalize the remarkably high and robust stereoselectivity observed in these reactions, we consider an enzyme-substrate-like mode of interaction involving a topographical match or mismatch of two geometric figures. Excellent chemical and stereochemical yields, combined with the simplicity and operational convenience of the experimental procedures, render the present method of immediate use for preparing various beta-substituted pyroglutamic acids and related compounds.  相似文献   
86.
The microbiological transformation of lithocholic acid (3α-hydroxy-5β-cholanic acid) into 3α, 15β-dihydroxy-5β-cholanic acid by Cunninghamella sp. is Investigated. The structure has been determined on the basis of two-dimensional 1H-NMR.  相似文献   
87.
The gyromagnetic ratios of the 4 1 + , 6 1 + , and 2 2 + states in186W were measured relative to that of the 2 1 + level by means of the transient field implantation perturbedγ-ray angular distribution technique. The nuclei in the states of interest were Coulomb excited using a beam of 220-MeV63Cu projectiles and recoiled swiftly through a thin, polarized Fe foil. The present measurements yielded ratiosg(4 1 + )/g(2 1 + )=1.04±0.07,g(6 1 + )/g(2 1 + )=1.03 ±0.20 andg(2 2 + )/g(2 1 + )=0.63±0.13. The sizable disparity between the measuredg-factors of the ground- and excited-band is examined within the context of the interacting boson approximation model.  相似文献   
88.
Antianoxic action and active constituents of evodiae fructus   总被引:1,自引:0,他引:1  
In order to develop new drugs from natural products, constituents of natural medicines were examined for their effectiveness in the KCN-induced anoxia model in mice. Methanol extract from a Chinese medicine, evodia (fruits of Evodia rutaecarpa Benth. or E. officinalis Dode), had a significant effect in the KCN-induced anoxia model in mice and therefore the active constituents were further examined. The results indicated that the antianoxic action of evodia was found in the fraction containing evodiamine. Further analysis of the active constituent indicated that evodiamine and rutaecarpine, indole-alkaloids found in large amounts in the Chinese medicine evodia, were mainly responsible for the antianoxic action.  相似文献   
89.
90.
Phenotyping of erythrocyte acid phosphatase (EAP) and esterase D (ESD) by cellulose acetate membrane isoelectric focusing (CAM-IEF) under a nonequilibrium condition is described. In an attempt to improve the method of CAM-IEF, we shortened the electrode distance to provide a higher field strength at a given (low) voltage. Various carrier ampholytes for EAP typing and various chemical separators for ESD typing were also tested. Good separations were obtained after 30 min IEF for EAP typing and 25 min for ESD typing. When applied to blood stains and stored for various periods at room temperature, the stains up to 8 months old could still be phenotyped for EAP and those up to 4 weeks old for ESD. CAM-IEF is suitable for routine forensic work of EAP and ESD phenotyping.  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