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   98814篇
  免费   17753篇
  国内免费   10187篇
化学   67392篇
晶体学   965篇
力学   6320篇
综合类   581篇
数学   11074篇
物理学   40422篇
  2024年   370篇
  2023年   2222篇
  2022年   3570篇
  2021年   3872篇
  2020年   4267篇
  2019年   3810篇
  2018年   3476篇
  2017年   3091篇
  2016年   4940篇
  2015年   4693篇
  2014年   5743篇
  2013年   7384篇
  2012年   8965篇
  2011年   9152篇
  2010年   6057篇
  2009年   5887篇
  2008年   6320篇
  2007年   5542篇
  2006年   5172篇
  2005年   4201篇
  2004年   3229篇
  2003年   2555篇
  2002年   2330篇
  2001年   1986篇
  2000年   1772篇
  1999年   2050篇
  1998年   1767篇
  1997年   1739篇
  1996年   1740篇
  1995年   1482篇
  1994年   1265篇
  1993年   1098篇
  1992年   942篇
  1991年   847篇
  1990年   714篇
  1989年   549篇
  1988年   412篇
  1987年   346篇
  1986年   349篇
  1985年   288篇
  1984年   162篇
  1983年   124篇
  1982年   110篇
  1981年   64篇
  1980年   47篇
  1979年   18篇
  1976年   1篇
  1974年   1篇
  1957年   31篇
  1922年   1篇
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
81.
2,6-O-diethyl-β-cyclodextrins were prepared and immobilized onto silica gel to develop a fluorosensor for dipyridamole. The analytical performance characteristics of the proposed sensor for analysis of dipyridamole were as follows: the detection limit was 0.8 nmol/L with a relative standard deviation of 1.4% for 10 determinations of 50 nmol/L of dipyridamole. The modification of β-CD improved the sensitivity and selectivity for measuring dipyridamole. The recommended method has been successfully tested for the determination of dipyridamole in clinical samples (urine and pharmaceutical preparations). Received: 23 January 1997 / Revised: 14 April 1997 / Accepted: 21 April 1997  相似文献   
82.
The Pd-catalyzed annulation of arynes by 2-halobiaryls and related vinylic halides provides a very efficient, high yielding synthesis of polycyclic aromatic and heteroaromatic hydrocarbons. This process appears to involve the catalytic, stepwise coupling of two very reactive substrates, an aryne and an organopalladium species, to generate excellent yields of cross-coupled products.  相似文献   
83.
84.
A gas chromatographic method for monitoring diacetyl guanfubase A in plasma is described. The procedure involved a single solvent extraction of drug from rabbit plasma into ethyl acetate with guanfubase A as an internal standard. The extract was analyzed subsequently on a gas chromatograph equipped with a hydrogen flame ionization detector. The recovery was 86.43% +/- 6.90% (+/- SD); the RSD of within-day and between-day was 2.81%-5.26% and 5.22%-8.24%, respectively; the regression line was linear over the concentration range of 25-200 micrograms/mL, the limit of detection was 10 micrograms/mL. No endogeneous interference was found in chromatograms of the biological samples. This method was applied to the pharmacokinetic study of diacetyl guanfubase A in rabbits.  相似文献   
85.
Reactive oxygen species (ROS) can damage DNA. Although a number of single nucleobase lesions induced by ROS have been structurally characterized, only a few intrastrand cross-link lesions have been identified and characterized, and all of them involve adjacent thymine and guanine or adenine. In mammalian cells, the cytosines at CpG sites are methylated. On the basis of the similar reactivity of 5-methylcytosine and thymine toward hydroxyl radical and the similar orientation of adjacent thymine guanine (TG) and 5-methylcytosine guanine (mCG) in B-DNA, we predict that the cross-link lesion, which was identified in TG and has a covalent bond formed between the 5-methyl carbon atom of T and the C8 carbon atom of G, should also form at mCG site. Here, we report for the first time the independent generation of 5-(2'-deoxycytidinyl)methyl radical, and our results demonstrate that this radical can give rise to the predicted novel intrastrand cross-link lesion in dinucleoside monophosphates d(mCG) and d(GmC). Furthermore, we show that the cross-link lesion can also form in d(mCG) from gamma irradiation under anaerobic conditions.  相似文献   
86.
In order to investigate the arsenic level in serum and packed cells of patients with renal insufficiency, total arsenic (As) concentrations were determined with hydride generation atomic absorption spectrometry (HGAAS) in serum (S) and packed cells (PC) of 31 non-dialyzed patients. The accuracy of the method was tested by the analysis of arsenic in 3 certified reference materials. Patients showed a three-fold increase of arsenic concentrations in serum and a two-fold increase of arsenic in packed cells compared with controls. Patients (n=10) with higher serum creatinine (>2.0 mg/dL), urea (>0.70 g/L) and urinary protein (mean+/-SD: 1.12+/-0.82 g/L) showed higher arsenic concentrations (5.8+/-3.3 microg/L in serum and 18.0+/-16.7 microg/kg in packed cells) compared with those with lower creatinine (<1.6 mg/dL), urea (<0.6 g/L) and urinary protein (mean+/-SD: 0.27+/-0.82 g/L) (n=16, serum arsenic 1.2+/-1.2 microg/L, packed cells arsenic 2.6+/-1.9 microg/kg). The significant differences (both p < 0.001) in S and PC-arsenic levels of patients in group I and II implies a relationship between the arsenic level and the degree of chronic renal insufficiency.  相似文献   
87.
Toralactone 9-O-beta-d-glucopyranosyl-(1-->6)-beta-d-glucopyranosyl-(1-->3)-beta-d-glucopyranosyl-(1-->6)-beta-d-glucopyranoside (1, cassiaside C(2)), isolated from Cassia obtusifolia L. and showing strong antiallergic activity, was concisely synthesized employing glycosyl trifluoroacetimidates as glycosylation agents. The unique naphtho-alpha-pyrone structure of toralactone (5) was constructed by condensation of orsellinate 8 with pyrone 9 in the presence of LDA as developed by Staunton and co-workers. The naphthol of toralactone showed minimal reactivity as an acceptor and was screened with various glycosyl donors. It is finally concluded that sacrifice of an excess amount of the trifluoroacetimidate or trichloroacetimidate donors (6f/6g, 6.0 equiv) in the presence of a catalytic amount of TMSOTf (0.05 and 0.3 equiv, respectively) afforded excellent yields of the coupling product, which was otherwise only a minor product under a variety of conditions examined.  相似文献   
88.
合成了五种新型双仲胺型氦杂冠醚,研究了它们对过渡金属离子进行液—液萃取,并用原子吸收法测定其浓度。实验结果表明:这类冠醚对Ag ̄+、Pd ̄(2+)、和Pt ̄(4+)等贵金属离子有较强的选择配位性能,对选择性分离这些金属离子有重要的意义。  相似文献   
89.
Sun H  Yang L  Zhang D  Sun J 《Talanta》1997,44(11):1979-1986
A method has been described for the direct determinations of trace cadmium using derivative atom trapping flame atomic absorption spectrometry with an improved water-cooled stainless steel trapping equipment. The characteristic concentration (gave a derivative absorbance of 0.0044) and detection limit (3sigma) of cadmium were 0.028 and 0.02 ng ml(-1) when collecting for a 1 min, respectively, which were 992 and 145-fold better than those of the conventional flame atomic absorption spectrometry. The detection limit and sensitivity of the proposed method for a 2 min collection time were 1 and 2 orders of magnitude higher than those of conventional flame atomic absorption spectrometry. The present method was applied to the determinations of cadmium in water samples with a recovery range of 91 approximately 111% and a relative standard deviation of 4.7 approximately 5.6%.  相似文献   
90.
Three-dimensional time-dependent quantum wave packet calculation for the O((1)D)+HBr reaction has been carried out using an accurate ab initio global potential energy surface [K. A. Peterson, J. Chem. Phys. 113, 4598 (2000)]. The calculations show that the initial state-selected reaction probabilities are dominated by resonance structures, and the lifetime of the resonance is generally in the subpicosecond time scale. The energy dependence of the reaction cross section is computed, which manifests still resonance structures, and is a decreasing function of the translational energy. The thermal rate constants are also computed, which are nearly independent on the temperature. The calculation results are discussed and compared to similar reaction with deep well.  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