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   37571篇
  免费   400篇
  国内免费   147篇
化学   18891篇
晶体学   688篇
力学   1625篇
综合类   2篇
数学   3685篇
物理学   13227篇
  2022年   340篇
  2021年   382篇
  2020年   331篇
  2019年   335篇
  2018年   537篇
  2017年   452篇
  2016年   741篇
  2015年   448篇
  2014年   748篇
  2013年   1640篇
  2012年   1564篇
  2011年   1941篇
  2010年   1377篇
  2009年   1388篇
  2008年   1616篇
  2007年   1569篇
  2006年   1430篇
  2005年   1266篇
  2004年   1182篇
  2003年   1013篇
  2002年   965篇
  2001年   1355篇
  2000年   970篇
  1999年   810篇
  1998年   615篇
  1997年   583篇
  1996年   516篇
  1995年   476篇
  1994年   448篇
  1993年   392篇
  1992年   466篇
  1991年   470篇
  1990年   462篇
  1989年   424篇
  1988年   444篇
  1987年   430篇
  1986年   375篇
  1985年   441篇
  1984年   471篇
  1983年   344篇
  1982年   387篇
  1981年   379篇
  1980年   319篇
  1979年   401篇
  1978年   379篇
  1977年   394篇
  1976年   392篇
  1975年   343篇
  1974年   324篇
  1973年   340篇
排序方式: 共有10000条查询结果,搜索用时 23 毫秒
101.
Skorko-Trybula  Z.  Boguszewska  Z. 《Mikrochimica acta》1976,66(3-4):335-342
Summary A method has been developed for determining boron in antimony compounds in the range 10–7–10–6%. The method comprises the isolation of the boron by distillation as methyl borate and its determination fluorimetrically with dibenzoylmethane. For the three materials analysed, SbCl3, Sb2O3, and SbCl5, individual procedures of dissolution as well as distillation were developed. The results obtained are reproducible, the mean deviation being about 12% at boron concentrations of 10–7–10–6%.
Zusammenfassung Ein Verfahren zur Bestimmung von 10–7 bis 10–6% Bor in Antimon-verbindungen wurde ausgearbeitet. Es umfaßt die Abtrennung des Bors durch Destillation als Methylborat und dessen fluorimetrische Messung mit Dibenzoylmethan. Für SbCl3, Sb2O3 und SbCl5 wurden jeweils eigene Arbeitsweisen der Auflösung und Destillation angegeben. Die Analysenergebnisse sind reproduzierbar; die mittlere Abweichung beträgt etwa 12% für Borkonzentrationen von 10–7 bis 10–6%.
  相似文献   
102.
Conclusions The IR spectra of 5- and 8-hydroxyfurocoumarins and their esters, 5,8-disubstituted furocoumarins containing hydroxyl, methoxyl, and alkoxyl groups, and 4,5-dihydrofurocoumarins differ with respect to the intensity and number of the bands in the 1630–1500 cm–1 region of the spectrum. The integral intensities of the absorption bands in this frequency region differ markedly in the derivatives mentioned.Khimiya Prirodnykh Soedinenii, Vol. 5, No. 5, pp. 355–359, 1969  相似文献   
103.
104.
Temperature programmed reduction (TPR) has been used to study the redox behavior of Ni(II) ions in nickel sodium mordenite (NiNaM) and decationated nickel mordenite (NiHM). The TPR profiles suggest that Ni(II) ions occupy nonequivalent sites with different cooridination states in the mordenite. The reducibility of Ni(II) depends strongly on the zeolite acidity.
() NiII NiNaM NiHM. . .
  相似文献   
105.
A highly efficient direct injection on-line guard cartridge extraction/tandem mass spectrometry (DI-GCE/MS/MS) method has been validated for high-throughput evaluation of cytochrome P450 (CYP) 3A4, 2D6 and 2E1 inhibition potential via cassette dosing of midazolam, dextromethorphan and chlorzoxazone using human hepatic microsomes and 96-well microtiter plates. Microsomal incubations were terminated with formic acid, centrifuged, and the resulting supernatants were injected for analysis by DI-GCE/MS/MS. Due to the novel use of an extremely short C(18) guard cartridge (4 mm in length), this method exhibits several advantages such as no sample preparation, excellent on-line extraction, short run time (2.5 min), and minimized source contamination and performance deterioration. The DI-GCE/MS/MS method demonstrates acceptable accuracy and precision for the simultaneous quantification of 1'-hydroxymidazolam, dextrorphan and 6-hydroxychlorzoxazone in microsomal incubations. The inhibition potential of CYP3A4, 2D6 and 2E1 has been evaluated using their known selective inhibitors. The IC(50) values measured by the cassette dosing approach (high-throughput) are consistent with those observed by an individual dosing regimen (conventional) and are all in good agreement with the literature values. The results suggest that the cassette probe-dosing strategy may provide an in vitro approach to minimize cost while maximizing throughput of CYP inhibition evaluation of new chemical entities in support of drug discovery and development.  相似文献   
106.
The reaction of alkyl 4-(2-acetylhydrazino)-2-methyl-3-thiophenecarboxylates with 3-methylbutan-2-one was carried out under the Fischer reaction conditions. The influence of Lewis acids and solvents on the process was studied. A convenient method for the synthesis of thieno [3.2-b]pyrrolenine derivatives was proposed.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 725–729, March, 2005.  相似文献   
107.
We present a comparative golden rule analysis of the dynamics of the intramolecur (IM) hydrogen atom and proton transfer in the photochemical cycles of 2-(2′-hydroxyphenyl)benzoxazole (HBO) and 5,8-dimehtyl-1-tetralone (DMT). Two major effects are taken into consideration: the promoting effect of the IM vibrations which are symmetrically coupled to the reaction coordinate,and the suppressing effect resulting from the reorganization of both the molecule and solvent.

Semiempirical quantum-chemical calculations at the AM1 level were carried out to study the energy levels of all states involved in the photochemical cycles, including the effects of solvation in a polar protic solvent in the case of DMT. Two rotamers EI and EII for the enol form of DMT were located corresponding to different positions of the H atom in the hydroxyl group. In the group state the first is more stable both in the gas phase and in polar protic solvents such as diethyl ether—isopentane—ethanol (5:2:5 by volume). Therefore the reketonization reaction is treated as one-step tunneling from the rotamer EI to the keto form, i.e. without the activated rotational equilibrium EI↔EII proposed by Grellmann and coworkers in an earlier study. The steep slope of the kinetic curve of this reaction is attributed to the additional activation energy resulting from the final reorganization of the low frequency oscillators, both intramolecular and solvent. For the dynamic calculations, the standard AM1 output (structural and force field data) was used as the input, and good agreement with the available kinetic experiments was reached for both compounds. No special reasons were found for the similarity of the kinetic curves for triplet excited-state intramolecular proton transfer in HBO and DMT.  相似文献   

108.
Shihabi ZK 《Electrophoresis》2000,21(14):2872-2878
Discontinuous buffers for capillary zone electrophoresis (CZE) can be used under less rigid conditions compared to those for isotachophoresis for stacking. They can be prepared simply by modifying the sample itself, either by addition of small inorganic ions, low conductivity diluents, or both, and also by adjusting its pH, meanwhile injecting a large volume on the capillary. Zwitterionic and organic-based buffers such as triethanolamine and tris(hydroxymethyl)aminomethane (Tris) are well suited for stacking due to their low conductivity, provided the buffer is discontinuous as demonstrated here. A simple mechanism based on discontinuous buffers is described to explain many of the observed stacking types in CZE, pointing out the many similarities to transient isotachophoresis.  相似文献   
109.
Pyrimidines     
The condensation of -substituted acetophenones with benzylidenebisurea in an acid medium has given 5-R-2-oxo-4,6-diphenyl-1,2,3,4-tetrahydropyrimidones. The analogous condensation of propiophenone forms 5-methyl-2-hydroxy-4,6-diphenylpyrimidine. The tetrahydro derivatives obtained are readily dehydrogenated to 5-R-2-hydroxy-4,6-diphenylpyrimidines.For part XX, see [1].  相似文献   
110.
Data on the kinematic viscosity of phosphoric acid solutions in technical-grade tributyl phosphate and in an industrial extract from a technological purification system based on tributyl phosphate, which contain 0–18% phosphoric acid in terms of P2O5, at 10–70°C are presented.  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