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   171314篇
  免费   1552篇
  国内免费   507篇
化学   88249篇
晶体学   3039篇
力学   7154篇
综合类   3篇
数学   19514篇
物理学   55414篇
  2020年   1398篇
  2019年   1552篇
  2018年   1980篇
  2017年   2065篇
  2016年   3184篇
  2015年   1889篇
  2014年   3112篇
  2013年   7659篇
  2012年   5747篇
  2011年   6877篇
  2010年   5021篇
  2009年   5002篇
  2008年   6533篇
  2007年   6428篇
  2006年   6076篇
  2005年   5502篇
  2004年   5091篇
  2003年   4592篇
  2002年   4450篇
  2001年   5218篇
  2000年   3861篇
  1999年   2967篇
  1998年   2481篇
  1997年   2385篇
  1996年   2237篇
  1995年   2113篇
  1994年   2091篇
  1993年   2000篇
  1992年   2304篇
  1991年   2244篇
  1990年   2193篇
  1989年   2121篇
  1988年   2119篇
  1987年   2122篇
  1986年   2019篇
  1985年   2629篇
  1984年   2683篇
  1983年   2381篇
  1982年   2492篇
  1981年   2307篇
  1980年   2284篇
  1979年   2426篇
  1978年   2462篇
  1977年   2388篇
  1976年   2436篇
  1975年   2360篇
  1974年   2327篇
  1973年   2504篇
  1972年   1623篇
  1971年   1337篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
771.
Eight nonprotamine proteins have extracted from the nuclei of the cells of the gonads of the Russian sturgeon with 0.35 M sodium chloride solution followed by fractionation of carboxymethyl-Sephadex G-25 and desalting on Bio-Gel P-2, and their amino acid compositions have been determined.  相似文献   
772.
Summary A gas Chromatographic determination of traces of alkali metal O,O-diethyl phosphorodithioates is described. The salts were converted into a volatile derivative by alkylation with diazomethane. The product was identified by gas chromatography-mass spectrometry. With aqueous samples a linear relationship of peak height to amount of salt was obtained in the range 0.04–1 ng and for urine samples a calibration curve was constructed. The detection limit (signal to noise ratio 4/1) was 40 pg of salt. The procedure was successfully used for monitoring phosalone absorption by occupationally exposed persons.
Zusammenfassung Eine gaschromatographische Bestimmung von Spuren Alkalisalz des Diäthyldithiophosphorsäureesters wurde beschrieben. Die Salze wurden durch Alkylieren mit Diazomethan in ein flüchtiges Derivat übergeführt. Dessen Identifizierung erfolgte mittels eines gaschromatographisch-massenspektrometrischen Systems. In Wasserproben wurde ein lineares Verhältnis der Peakhöhen und der Salzmenge festgestellt und für Urinproben wurde eine Eichkurve ermittelt. Die Nachweisgrenze (Signal zu Rausch 41) wurde bei 40 pg Salz festgestellt. Die Methode wurde für die Kontrolle der Absorption von Phosalon bei beruflich exponierten Personen erfolgreich angewendet.


Presented at the Fourth International Congress of Pesticide Chemistry (IUPAC), Zürich, July 24–28, 1978.  相似文献   
773.
The infrared and Raman spectra of gaseous and solid (CH3)3 GeNCO and solid (CH3)3GeNCS have been recorded over the frequency range 20–4000 cm?1 . The Raman spectra of the liquids have also been recorded. Assignments of the normal modes have been made on the bases of band types, Raman depolarization values, and characteristic frequencies. Spectral data indicate that (CH3)3 GeNCO is non-linear in all phases and that (CH3)3GeNCS has a linear or quasi-linear heavy atom skeleton in the fluid phases.  相似文献   
774.
Nonempirical LCAO MO SCF computations (in the ΔSCF formalism) have been carried out of binding energies and relaxation energies for an extensive series of oxygen-containing organic systems encompassing most of the common functionalities. The molecules for which experimental data are available for comparison demonstrate the adequacy of the treatment for describing the relative binding energies for both the Cls and Ols core levels. A sound basis is therefore provided for the discussion of relative core binding energies (Cls and Ols) for functionalities for which experimental data are not available, that is, hydroperoxides, peroxides, peroxyacids, and peroxyesters, a knowledge of which is essential for investigations of the surface oxidation of polymers by means of ESCA. Cls shifts are discussed in terms of primary and secondary substituent effects of oxygen, which greatly simplify the analysis of complex unresolved spectra, whereas for the Ols levels a similar but less straightforward situation exists. The relaxation energies associated with both Cls and Ols core ionization follow a dependence on the binding energy for a given structural type.  相似文献   
775.
Conclusions The combined use of the1H and13C NMR spectra makes it possible to unequivocally assign 1-methyl-4-octalin-l,2-dicarboxylic acids and their derivatives to the proper steric series.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1429–1431, June, 1977.  相似文献   
776.
The IR- and Raman Spectra of several compounds of the stoichiometryM 2 I M IIIF5H2O and their deuterated analogs have been interpreted on the basis of their crystal structures. The compounds studied were divided into two classes regarding the bonding of water molecules. The main spectroscopic difference between the two classes was found in the region of H2O vibrations in good agreement with theoretical predictions. The metal—fluorine and metal—oxygene vibrations gave no direct information concerning the two types of water.  相似文献   
777.
The acidolytic cleavage of a series of new N-aralkyloxycarbonyl protecting groups has been found to proceed as a first-order reaction, the rate of cleavage being dependent on the stability of the corresponding aralkyl carbonium ions. Some of the groups are cleaved at much the same rate as the N-trityl residue and up to 60000 times faster than the t-butyloxycarbonyl (Boc) group. The rate is also strongly influenced by the acidity of the reaction media. The relative rates at which aralkyloxycarbonyl and Boc groups are split off can be largely controlled by appropriate selection of the reaction conditions. The implications of these findings for peptide syntheses are discussed and the use of the 2-(p-diphenyl)-isopropyloxycarbonyl residue as an especially suitable N-protecting group is proposed.  相似文献   
778.
A new system CrCl3–C6H5CH2N(C2H5)3Cl–CH3CN for photooxygenation of alkanes is suggested. Unlike metal chlorides and oxocomplexes, it oxidizes alkanes to produce ketones and small amounts of alcohol. Adding of benzene, methylene chloride or ethanol to the cyclohexane solution rises the oxygenation rate and changes the ketone/alcohol ratio. With small amounts of hydroquinone the formation rate of cyclohexanone (but not of cyclohexanol) sharply decreases. Kinetic isotope effect in the oxidation of C6H12 and C6D12 is 1 for cyclohexanol and 2.9 for cyclohexanone. Cyclohexanol formation is assumed to follow a mechanism that does not involve free radicals. Free radicals can participate in the route toward ketone.
CrCl3–C6H5CH2N(C2H5)3Cl–CH3CN . , . , /. ( ) . C6H12 C6D12 1 2,9 . , , . , .
  相似文献   
779.
Recovered polyethylene is used with polyethylene waxes as a new binder for mineral granules. This binder is characterized and optimized mainly on a rheological basis. Its fluid index (I.F.), molecular distribution and crystallinity are especially studied. Its rheograms and tensile properties are compared with those of industrial polyethylene (I.F. = 250). The dimensional shrinkages of such binders during cooling from 190° to ambient temperature are measured.  相似文献   
780.
Alkylthio-substituted dienic 1,3-dioxolan-2-ones and bis(4,4-diorganyl-5-methyli-dene-1,3-dioxolan-2-ones) were obtained in high yields by the reaction of 2-methyl-6-alkylthio-5-hexen-3-yn-2-ols and tertiary diacetylenic glycols with carbon dioxide under pressure in the presence of catalytic amounts of triethylamine and monovalent copper salts.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1617–1619, December, 1979.Original article submitted January 24, 1979.  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