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   56161篇
  免费   4916篇
  国内免费   11253篇
化学   49688篇
晶体学   1916篇
力学   727篇
综合类   565篇
数学   4059篇
物理学   15375篇
  2024年   162篇
  2023年   1029篇
  2022年   1880篇
  2021年   1822篇
  2020年   1897篇
  2019年   1735篇
  2018年   1406篇
  2017年   1802篇
  2016年   1897篇
  2015年   1648篇
  2014年   2225篇
  2013年   4956篇
  2012年   3419篇
  2011年   3616篇
  2010年   3158篇
  2009年   3782篇
  2008年   3622篇
  2007年   3807篇
  2006年   3552篇
  2005年   3129篇
  2004年   3001篇
  2003年   2432篇
  2002年   2031篇
  2001年   1671篇
  2000年   2354篇
  1999年   1795篇
  1998年   1626篇
  1997年   959篇
  1996年   862篇
  1995年   847篇
  1994年   745篇
  1993年   599篇
  1992年   559篇
  1991年   425篇
  1990年   288篇
  1989年   247篇
  1988年   211篇
  1987年   147篇
  1986年   109篇
  1985年   137篇
  1984年   103篇
  1983年   44篇
  1982年   77篇
  1981年   113篇
  1980年   78篇
  1979年   77篇
  1978年   59篇
  1977年   59篇
  1976年   36篇
  1973年   37篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
All the investigated reactions of 5-cyano-1,4,6,9-trioxathia-5-phosphaspiro[4,4]nonanes with compounds containing a labile hydrogen atom proceed with the loss of HCN, predominant opening of the oxathiophospholane ring, and formation of 2-oxo-1,3,2-dioxaphospholane derivatives.A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Science Center, Russian Academy of Sciences, 420083 Kazan. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, p. 2670–2673, November, 1992.  相似文献   
102.
The composition range and (composite modulated) structure of compounds within the wide range non-stoichiometric LaSb2Snx, 0.1?x?0.75, solid solution is carefully investigated via a combined electron diffraction, XRD and electron probe microanalysis study. Evidence for metastability of the LaSb2Snx phase at the low x composition end of the solid solution is presented. Direct evidence is found for a reasonably (although by no means perfectly) well ordered Sn sub-structure which is, in general, mutually incommensurable with respect to a very well ordered underlying LaSb2 sub-structure along both a and c directions. The overall (3+2)-d superspace group symmetry is given along with a discussion of the consequences as regards the arrangement of the Sn atoms. The Sn sub-structure c-axis cell dimension shows very little variation with composition x providing direct experimental evidence of the importance of Sn-Sn metallic bonding (along one-dimensional [001] Sn strings) for the stability of the phase.  相似文献   
103.
Methods for the synthesis of polyfunctionalN-phosphoryl- andN-phosphonoyldiazeneN-oxides containing hydroxyl, acetoxyl, and nitrate groups, and dibromoallyl and dibromopropyl fragments have been developed. The molecular structure ofN-(2,2-dimethyl-5-nitro-1,3-dioxan-5-yl)-N-[methoxy(phenyl)phosphoryl)diazeneN-oxide was established by X-ray structural analysis.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1284–1289, July, 1994.  相似文献   
104.
The ENDOR spectrum of ~(14)N-~(63)Cu-HAP complex in DMSO/EtOH (5:1) freezing solution at 20 K has been studied using orientational selective method in this paper. The anisotropic superhyperfine coupling tensor and qusdrupole coupling tensor of ligand ~(14)N nucleus, and the superhyperfine coupling tensor of various ~1H nuclei have been measured precisely. Comparing the superhyperfine coupling tensor of ~(14)N-nucleus with previous work shows that the analytical method of spectrum is reasonable and the data are reliable in our previous work.  相似文献   
105.
It has been established that the interaction of N1-(2-hydroxyphenylmethylthieno[2,3-b]pyrid-3-yl)arylamides with hydrazine hydrate leads to thieno[2,3-b]pyridine-2,3-diamines. It was shown that the reaction of the latter with acetylacetone and acetoacetic ester occurs regioselectively at the amino group in position 3 of the thiophene ring. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1400–1408, September, 2007.  相似文献   
106.
An improved method for the preparation and purification of ethyl 2-hydroxy-4-oxo-4H-pyrido[1,2-a]-pyrimidine-3-carboxylates is proposed. According to 1H and 13C NMR spectroscopic data the synthesized compounds exist in DMSO solution in the 2-hydroxy-4-oxo form while in the crystal (at least for the case of the unsubstituted derivative) X-ray analysis shows that it occurs in the bipolar 2,4-dioxo form. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 864–876, June, 2007.  相似文献   
107.
When 4-methyl-7-nitro-2,3,4,5-tetrahydro-1H-1,5-benzodiazepin-2-one were reacted with phosphorus pentasulfide, the corresponding benzodiazepine-2-thione and its thiol tautomer were formed, which via the 2-methylmercapto derivative were converted to 4-(2-acetylhydrazino)-2-methyl-8-nitro-2,3-dihydro-1H-1,5-benzodiazepine.  相似文献   
108.
The addition of secondary and primary amines to ethyl (1-amino-9,10-anthraquinon-2-yl)propynoate affords an easily separable mixture of the corresponding ethyl 3-dialkylaminoor 3-alkylamino-3-(1-amino-9,10-anthraquinon-2-yl)acrylate and 3-dialkylamino- or 3-alkylaminonaphthol[2,3-h]quinoline-2(1H), 7,12-trione (in ∼4: 1 ratio). Intramolecular cyclization of the resulting substituted ethyl acrylates results in the formation of 4-dialkylaminoor 4-alkylamino-2-chlorinated pyridine rings. Subsequent nucleophilic substitution of the chlorine atom gives 2-functionalized 4-dialkylamino- or 4-alkylaminonaphtho[2,3-h]quinoline-7,12-diones. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2327–2332, November, 1998.  相似文献   
109.
Summary 4-Chloro-3-aryl-coumarins and quinolones2 a–e undergo thermolytic ring closure by reaction with sodium azide in refluxing dimethyl formamide to yield indolo[3,2-c]coumarins and indolo[3,2-c]quinolin-6(5H)-ones6 a–e. In the case of the coumarin2 a the azido coumarin5 can be isolated. The mono- and diazacoumestrol-dimethylethers6 a–c are converted into the coumestrol analogues7 a–c and their diacetyl derivatives8 a–c.
Potentielle nichtsteroidale Östrogene und Antiöstrogene, 4. Mitt.: Organische Azide in der Heterocyclensynthese, Teil 13: Synthese von Aza- und Diazacumöstrolen über Azidzwischenstufen
Zusammenfassung 4-Chlor-3-arylcumarine und-chinolone2 a–e reagieren thermolytisch mit Natriumazid in siedendem Dimethylfomamid unter Ringschluß zu Indolo[3,2-c]cumarinen und Indolo[3,2-c]chinolin-6(5H)-onen6 a–e. Nur aus dem Cumarinderivat2 a kann das zwischenzeitlich gebildete Azidocumarin5 isoliert werden. Die so erhaltenen Mono- und Diazacumöstroldimethylether6 a–c werden in die entsprechenden Cumöstrole7 a–c und ihre Diacetylderivate8 a–c umgewandelt.
  相似文献   
110.
Samples of poly[1-(3-sulfopropyl)-2-vinyl-pyridinium-betaine] (PSPV) have been synthesized to high conversion by free radical polymerization in aqueous solution of the zwitterionic monomer SPV with several concentrations of the crosslinker N,N′-methylene-bis-acrylamide (MBA). The densities of the resultant xerogels increased regularly with the content of MBA. Hydrogels obtained by swelling them in water and aqueous KSCN solution were examined by gravimetric and dimensional analysis. The water contents increased with decreasing content of MBA, the value of 92.7 wt% at the lowest MBA content being higher than that for other zwitterionic hydrogels. Enhanced swelling occurred in 1 M aq. KSCN at each MBA content, the total swelling being 98.1 wt% at the lowest crosslinker content. Swelling increased with increasing temperature. An approximate procedure to formulating swelling equilibrium in term of the volume fraction of water in hydrogel, in conjunction with the van’t Hoff equation, yields a small positive value for the enthalpy of swelling. This is compared with values derived similarly for other hydrogels.  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