全文获取类型
收费全文 | 50673篇 |
免费 | 4889篇 |
国内免费 | 10673篇 |
专业分类
化学 | 46701篇 |
晶体学 | 1878篇 |
力学 | 638篇 |
综合类 | 425篇 |
数学 | 3320篇 |
物理学 | 13273篇 |
出版年
2024年 | 166篇 |
2023年 | 971篇 |
2022年 | 1795篇 |
2021年 | 1795篇 |
2020年 | 1853篇 |
2019年 | 1674篇 |
2018年 | 1382篇 |
2017年 | 1745篇 |
2016年 | 1836篇 |
2015年 | 1607篇 |
2014年 | 2143篇 |
2013年 | 4632篇 |
2012年 | 3206篇 |
2011年 | 3468篇 |
2010年 | 2974篇 |
2009年 | 3587篇 |
2008年 | 3416篇 |
2007年 | 3490篇 |
2006年 | 3306篇 |
2005年 | 2889篇 |
2004年 | 2743篇 |
2003年 | 2291篇 |
2002年 | 1903篇 |
2001年 | 1558篇 |
2000年 | 1462篇 |
1999年 | 1177篇 |
1998年 | 1002篇 |
1997年 | 868篇 |
1996年 | 762篇 |
1995年 | 771篇 |
1994年 | 683篇 |
1993年 | 546篇 |
1992年 | 507篇 |
1991年 | 387篇 |
1990年 | 262篇 |
1989年 | 226篇 |
1988年 | 193篇 |
1987年 | 132篇 |
1986年 | 98篇 |
1985年 | 113篇 |
1984年 | 90篇 |
1983年 | 36篇 |
1982年 | 67篇 |
1981年 | 91篇 |
1980年 | 64篇 |
1979年 | 67篇 |
1978年 | 45篇 |
1977年 | 50篇 |
1976年 | 30篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Possible use of room temperature ionic liquids (RTILs) as chelate extraction solvent was evaluated by using 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim][PF6]). These RTILs showed high extraction performance for divalent metal cations with 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (Htta). The extracted metals were back-extracted into 1 mol dm−3 nitric acid quantitatively. Furthermore, the extracted species were estimated as neutral hydrated complexes M(tta)2(H2O)n (n= 1 or 2) for M = Ni, Cu and Pb and anionic complexes M(tta)3− for M = Mn, Co, Zn and Cd. 相似文献
152.
An efficient synthesis of 6-alkyl-3-cyano-2(1H)-pyridinethiones by the reactions of the sodium salts of -ketoaldehydes with cyanothioacetamide was developed. Pyridinethiones undergo selectiveS-alkylation with haloacetonitriles and haloacetophenones followed by cyclization to the corresponding thieno[2,3-b]pyridines.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 727–731, April, 1995.This study was financially supported by the Russian Foundation for Basic Research (Project No. 94-03-08-823). 相似文献
153.
Ab initio molecular orbital calculations employing a 4-31G basis set have been used to study the minimum energy paths for the formation of HCO+, COH+, and HCOH2+ from CO by protonation. The protonation of N2 to give NNH+ and HNNH2+ and of NO+ to form HNO2+ and NOH2+ have also been investigated. All species formed have linear equilibrium geometries and the minimum energy path for approach of the proton is along the line-of-centers of the heavy atoms. Energy barriers to the formation of the various species are given, where appropriate, and changes in geometry, ordering of molecular orbitals and orbital occupancy are discussed. 相似文献
154.
Oxidation of 5×10−3 M noradrenaline in aqueous phosphate buffer, pH 7.4, with K3Fe(CN)6, NaIO4 or Fe2+/EDTA/H2O2 followed by extraction with ethyl acetate and acetylation with Ac2O/Pyr led to a main reaction product which was isolated and identified as 4-[bis-(1H-5,6-diacetoxyindol-2-yl)methyl]-1,2-diacetoxybenzene, an unprecedented [bis-(indol-2-yl)methyl]-benzene derivative unsubstituted on the 3-position of the indole rings. This product was also obtained in 40% yield by reaction of 5,6-dihydroxyindole with 3,4-dihydroxybenzaldehyde. Other components of the oxidation mixture were 1-acetyl-3,5,6-triacetoxyindole, derived from noradrenolutin, and 5,6-diacetoxyindole, originating from cyclisation/dehydration of the o-quinone of noradrenaline, along with some 3,4-diacetoxybenzaldehyde. Inspection of the aqueous phase revealed the presence of 3,4-dihydroxymandelic acid and 3,4-dihydroxybenzaldehyde, derived from oxidative breakdown of the 2-amino-1-hydroxyethyl chain via a p-quinomethane intermediate. These results disclose new aspects of the oxidative chemistry of noradrenaline beyond the aminochrome stage and provide a route to novel [bis-(indol-2-yl)methyl]-benzene derivatives of potential pharmacological interest. 相似文献
155.
Suitably functionalized 3-carbomethoxy/cyano-2H-pyran-2-ones are excellent synthons for the synthesis of arenes and heteroarenes of therapeutic importance. The compounds 6-aryl-3-cyano-4-methylsulfanyl-2H-pyran-2-ones have been transformed into bridgedhead azolopyrimidines and imidazothiazines through thermal and base-induced ring transformation reactions with aminoazoles and imidazolidin-2-thiones, respectively. 相似文献
156.
157.
Solid solutions NaCl—CdCl2 were studied in an interval of CdCl2 concentrations of 0.05—3 mol.% by Raman spectroscopy. The molecular form Na2CdCl4 decomposes: Na2CdCl4 2Na+ + Cd2+ + 2Cl2
– + 2e–; 2Na + 2e– 2Na0. Free sodium atoms form color centers of crystal (F centers) in the region of jumpwise changing the partition coefficient of CdCl2 in NaCl (K jump). 相似文献
158.
Z. Stec 《Monatshefte für Chemie / Chemical Monthly》1993,124(3):331-332
Summary 2-Acetoxy- and 2-Hydroxy-2,3-dihydronaphtho[1,8-bc]pyranes were synthesized by decomposition of 1-hydroperoxy-1,2-dihydroacenaphthylene in acetic acid in the presence of perchloric acid.
Synthese von 2-Acetoxy- und 2-Hydroxy-2,3-dihydronaphtho[1,8-bc]pyran
Zusammenfassung 2-Acetoxy- und 2-Hydroxy-2,3-dihydronaphtho[1,8-bc]pyrane konnten durch Zersetzung von 1-Hydroperoxy-1,2-dihydroacenaphthylen in Essigsäure in Anwesenheit von Perchlorsäure synthetisiert werden.相似文献
159.
The reactions of [RuHCl(CO)(PPh3)3] and [(C6H6)RuCl2]2 with 2-benzoylpyridine have been examined, and two novel ruthenium(II) complexes – [RuCl(CO)(PPh3)2(C5H4NCOO)] and [RuCl2(C12H9NO)2] – have been obtained. The compounds have been studied by IR and UV–Vis spectroscopy, and X-ray crystallography. The molecular orbital diagrams of the complexes have been calculated with the density functional theory (DFT) method. The spin-allowed singlet–singlet electronic transitions of the compounds have been calculated with the time-dependent DFT method, and the UV–Vis spectra of the compounds have been discussed on this basis. 相似文献
160.
2-Trifluoromethyl-4H-thiochromene-4-thione obtained from 2-trifluoromethyl-4H-thiochromen-4-one and P2S5 reacts with aromatic amines, hydrazine hydrate, phenylhydrazine, and hydroxylamine at the C(4) atom of the chromene ring
to give the corresponding anils, azine, hydrazones, and oxime of thiochromone. 2-Trifluoromethyl-4H-thiochromen-4-one is oxidized by hydrogen peroxide in AcOH into 4-oxo-2-trifluoromethyl-4H-thiochromene 1,1-dioxide and reduced by NaBH4 to 2-trifluoromethyl-4H-thiochromen-4-ol or cis-2-(trifluoromethyl)thiochroman-4-ol. When treated with hydrazine hydrate, thiochromen-4-one gives 3(5)-(2-mercaptophenyl)-5(3)-trifluoromethylpyrazole.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 504–509, March, 2006. 相似文献