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   25636篇
  免费   470篇
  国内免费   100篇
化学   14303篇
晶体学   161篇
力学   774篇
综合类   9篇
数学   3510篇
物理学   7449篇
  2022年   326篇
  2021年   404篇
  2020年   371篇
  2019年   329篇
  2018年   414篇
  2017年   382篇
  2016年   661篇
  2015年   535篇
  2014年   632篇
  2013年   995篇
  2012年   1281篇
  2011年   1480篇
  2010年   824篇
  2009年   787篇
  2008年   1237篇
  2007年   1224篇
  2006年   1095篇
  2005年   1526篇
  2004年   1560篇
  2003年   1089篇
  2002年   733篇
  2001年   606篇
  2000年   568篇
  1999年   351篇
  1998年   295篇
  1997年   293篇
  1996年   357篇
  1995年   323篇
  1994年   314篇
  1993年   330篇
  1992年   327篇
  1991年   278篇
  1990年   229篇
  1989年   212篇
  1988年   187篇
  1987年   185篇
  1986年   177篇
  1985年   229篇
  1984年   214篇
  1983年   183篇
  1982年   187篇
  1981年   185篇
  1980年   170篇
  1979年   152篇
  1978年   176篇
  1977年   142篇
  1976年   134篇
  1975年   114篇
  1974年   132篇
  1973年   122篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
41.
[reaction: see text]. A very simple methodology to stereoselectively achieve tricyclic isonucleosides (nucleobase = thymine, uracil, and 5-fluoruracil) and 3'-C-branched nucleosides (nucleobase = theophylline) was performed by means of a DBU-mediated addition process using a readily available 2-bromo sugar. The mechanism for these transformations implies the loss of both substituents at C-2 and C-3 on the sugar moiety, and although it seems that DBU is probably involved, its involvement has not yet been ascertained. Cytosine did not react under these conditions.  相似文献   
42.
A spectrophotometric method has been developed for the simultaneous determination ofo-cresol andm-cresol in water by reaction withp-aminophenol (PAP). Three different methodologies have been assayed; (i) batch analysis, after reaction in an alkaline medium in the presence of dissolved molecular oxygen as oxidizing agent, (ii) a stopped-flow procedure, carried out in the presence of KIO4 and (iii) a flow injection method based on the same approach. The batch procedure requires 22 min for the full development of colour witho-cresol and 12 min form-cresol. In the stopped-flow mode, using KIO4 and a reaction time of 12 min, better sensitivity can be obtained for both compounds and limits of detection of 10 g 1–1 foro-cresol and 30 g 1–1 form-cresol were found. The flow injection method has a lower sensitivity but permits more than 80 injections per hour. Based on the different maximum absorbance wavelengths obtained for the reaction products of PAP witho-cresol (614 nm) andm-cresol (632 nm), both compounds can be simultaneously determined in water samples and recoveries of 90 to 115% were found in spiked water samples of different types.  相似文献   
43.
In fluprednisolone and chloroprednisone acetate, the polarographic reduction of the carbon—halogen bond in position 6 occurs first. The carbanion—enolate formed is reduced at the dropping mercury electrode at more negative potentials than the conjugate acid. Controlled potential electrolysis at a mercury pool electrode where the carbanion—enolate can be protonated, yields the unsaturated ketone. Polarographic reduction of clobetasol-17-propionate and of clobetasone-17-butyrate results in cleavage of the C—Cl bond in the side-chain. This process is followed by reduction of the α,β-unsaturated ketone in the A-ring. Analytical methods for the determination of these compounds in ointments, creams and eye/ear drops gave results with standard deviations of 1–2%.  相似文献   
44.
A new reagentless system for sulphite (or sulphur dioxide) determination is reported based on the use of an organic conducting polymer, polyaniline, and its absorbance variation at 550 nm, depending on the sulphite concentration. After chemical polymerisation of aniline a very thin film of polyaniline is obtained. Although the response is not fully reversible, each film can be used for at least 10 measurements for low analyte concentrations (up to 0.5 mg l−1) and five measurements for higher sulphite concentrations. Moreover, the reproducibility, ease of preparation and low cost of the films, permit the use of a new disposable system for each measurement. When the change in absorbance at 550 nm was measured for 210 s (stabilisation time), the system showed a linear response, which ranged from 0.025 to 1.50 mg l−1 of sulphite. A theory with regard to the reaction mechanism between the polyaniline films and sulphite is also proposed. The system was applied to sulphite determination in wine samples and the results were in agreement with those obtained by the Official Method of Analysis (iodometric titration).  相似文献   
45.
The cyclic sulfones1,2, and3 are lithiated in benzene with n-butyllithium. Lithiation is demonstrated to lead to α-mono, α,α′-di and/or α,α-di and α,α,α′ trimetallation. The lithiated sulfones are chlorinated with hexachloroethane (HCE). Some mechanistic aspects of the reaction of the lithiosulfones with vicinal dihalides are discussed.  相似文献   
46.
A simple physical picture is given of Fellner-Feldegg's thin cell method in time domain spectroscopy. From this picture an accurate analytical relation is derived for the total reflection coefficient.  相似文献   
47.
A non instrumental method for the semiquantitative determination of mercury in fish is described. The sample is acid digested and the mercury vapor released after chemical reduction with SnCl2. The mercury vapor is then collected on a detecting paper covered with an emulsion of Cu2I2, 3% carboxymethylcellulose and MgCl2 as moistener agent. The colored Cu2[HgI4] complex is formed and the color intensity is proportional to the mercury concentration in the original sample. Parameters such as sample mass/digestion solution volume ratio, emulsion composition, paper covering technique, carrier gas flow were investigated. Comparative studies using CVAAS and digital color analysis after scanning the papers confirmed the applicability of the method to real samples.  相似文献   
48.
To obtain further information concerning the interaction between Walsh-orbitols of ‘conjugated’ cyclopropane rings, the photoelectron spectra of the following compounds have been recorded: bicyclo[4.1.0]heptane 1 , cis- and trans-tricyclo[5.1.03, 5]octane 2, 3 , diademane 4 , trans-pentacyclo[3.3.2.02, 9.04, 10, 06, 8]decan 5 and bicyclo[4.1.0]heptene-2 6 . The first bands in the PE.-spectra of these compounds have been assigned on the basis of a ZDO HMO-approximation. For 2 and 4 the value for resonance integral between linked 2p atomic orbitals of two adjacent eclipsed cyclopropane rings is found to be ?1.73 eV.  相似文献   
49.
Summary A micro-determination method for potassium with EDTA is described. The perchlorate precipitate is reduced to chloride by heating the perchlorate with ammonium chloride, and the chloride is precipitated as silver chloride. This precipitate is dissolved in an ammoniacal solution of potassium tetracyanonickelate. By dissolution, two atoms of silver (or chlorine) displace one of nickel. The freed nickel is titrated with, EDTA and potassium is thus determined indirectly. The method is much more rapid than the classical gravimetric procedure, and the results are satisfactory.
Zusammenfassung Eine Mikromethode zur Bestimmung von Kalium mit ÄDTA wird beschrieben. Der Perchloratniederschlag wird durch Erhitzen mit Ammoniumchlorid reduziert und das Chlorid mit Silber gefällt. Dieser Niederschlag wird in einer ammoniakalischen Lösung von Kaliumtetracyanonickelat aufgelöst, wobei zwei Atome Silber ein Atom Nickel verdrängen. Das freigesetzte Nickel wird mit ÄDTA titriert und so das Kalium indirekt bestimmt. Das Verfahren ist rascher als die klassische gravimetrische Methode. Seine Resultate sind befriedigend.

Résumé On décrit une méthode de microdosage du potassium par l'EDTA. En chauffant le perchlorate avec du chlorure d'ammonium, on réduit le précipité de perchlorate en chlorure que l'on précipite à l'état de chlorure d'argent. On dissout ce précipité dans une solution ammoniacale de nickel-II tétracyanure de potassium. La mise en solution s'accompagne du déplacement d'un atome de nickel par deux atomes d'argent. On titre le nickel libéré par l'EDTA et l'on dose ainsi le potassium indirectement. La méthode est beaucoup plus rapide que le procédé gravimétrique habituel et les résultats sont satisfaisants.
  相似文献   
50.
The title compound, formula C15H20O2, is orthorhombic, P212121 witha=8.747(2),b=12.025(3),c=12.554(3)Å,Z=4, andD m =1.32(2)g/ml. The structural analysis shows that the compound corresponds to eudesma-4(15),7(11)-dien-8,12-olide, a sesquiterpene lactone previously isolated fromAster umbellatus but whose crystal structure was unknown.  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