全文获取类型
收费全文 | 11478篇 |
免费 | 68篇 |
国内免费 | 32篇 |
专业分类
化学 | 4662篇 |
晶体学 | 14篇 |
力学 | 713篇 |
数学 | 4031篇 |
物理学 | 2158篇 |
出版年
2024年 | 117篇 |
2023年 | 690篇 |
2022年 | 418篇 |
2021年 | 411篇 |
2020年 | 1455篇 |
2019年 | 1067篇 |
2018年 | 926篇 |
2017年 | 753篇 |
2016年 | 732篇 |
2015年 | 562篇 |
2014年 | 737篇 |
2013年 | 2832篇 |
2012年 | 495篇 |
2011年 | 26篇 |
2010年 | 22篇 |
2009年 | 10篇 |
2008年 | 26篇 |
2007年 | 22篇 |
2006年 | 16篇 |
2005年 | 69篇 |
2004年 | 79篇 |
2003年 | 29篇 |
2002年 | 11篇 |
2001年 | 8篇 |
2000年 | 1篇 |
1998年 | 8篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 10篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1973年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
22.
23.
W. Z. Chen Y. F. Zhao 《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):2054-2063
In this article, we used the Atherton–Todd reaction to synthesize amino acid methyl ester 5′-phosphoamidates of uridine as prodrugs. Their structures were confirmed by 1H NMR, 31P NMR, 13C NMR, IR, and mass spectrometry. Supplemental materials are available for this article. Go to the publisher′s online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. 相似文献
24.
Duc-Truc Pham Philip Clements Christopher J. Easton John Papageorgiou Bruce L. May 《Supramolecular chemistry》2013,25(6):510-519
This study shows that stereochemical factors largely determine the extent to which 6-(4′-t-butylphenylamino)-naphthalene-2-sulphonate, BNS? and its dimer, (BNS? )2, are complexed by β-cyclodextrin, βCD, and a range of linked βCD dimers. Fluorescence and 1H NMR studies, respectively, show that BNS? and (BNS? )2 form host–guest complexes with βCD of the stoichiometry βCD.BNS? (10? 4 K 1 = 4.67 dm3 mol? 1) and βCD.BNS2 2 ? (10? 2 K 2′ = 2.31 dm3 mol? 1), where the complexation constant K 1 = [βCD.BNS? ]/([βCD][BNS? ]) and K 2′ = [βCD. (BNS? )2]/([βCD.BNS? ][BNS? ]) in aqueous phosphate buffer at pH 7.0, I = 0.10 mol dm3 at 298.2 K. (The dimerisation of BNS? is characterised by 10? 2 K d = 2.65 dm3 mol? 1.) For N,N-bis((2AS,3AS)-3A-deoxy-3A-β-cyclodextrin)succinamide, 33βCD2su, N-((2AS,3AS)-3A-deoxy-3A-β-cyclodextrin)-N′-(6A-deoxy-6A-β-cyclodextrin)urea, 36βCD2su, N,N-bis(6A-deoxy-6A-β-cyclodextrin)succinamide, 66βCD2su, N-((2AS,3AS)-3A-deoxy-3A-β-cyclodextrin)-N′-(6A-deoxy-6A-β-cyclodextrin)urea, 36βCD2ur, and N,N-bis(6A-deoxy-6A-β-cyclodextrin)urea, 66βCD2ur, the analogous 10? 4 K 1 = 11.0, 101, 330, 29.6 and 435 dm3 mol? 1 and 10? 2 K 2′ = 2.56, 2.31, 2.59, 1.82 and 1.72 dm3 mol? 1, respectively. A similar variation occurs in K 1 derived by UV–vis methods. The factors causing the variations in K 1 and K 2 are discussed in conjunction with 1H ROESY NMR and molecular modelling studies. 相似文献
25.
26.
Jacek E. Nycz 《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):2605-2612
The present study shows new aspects of the synthesis of polyhalogenoarylphosphanes. The sterically hindered anions Ph(R)P-Y? (1a–c, Y = O, lone pair; R = Ph, But) have been used to show the complexity of the reaction between phosphorus nucleophiles and hexahalogenobenzenes or 9-bromofluorene (E3). The Ph(But)P-O? (1a) anion reacts with hexachlorobenzene (E1), hexafluorobenzene (E2), or E3 to give Ph(R)P(O)X (4a–c, X = F, Cl, Br) with the release of the corresponding carbanion as a nucleofuge, followed by side reactions. In contrast, the lithium phosphides Ph(R)PLi (1b,c) react with hexahalogenobenzenes to give the corresponding diphosphanes 5a,b as the main product and traces of P-arylated products, i.e., Ph(R)P-C6X5 (10a,b, X = Cl, F). Unexpectedly, Ph(But)PLi (1b) reacts with an excess of 9-bromofluorene to give only halogenophosphane Ph(But)P-X. 相似文献
27.
《Green Chemistry Letters and Reviews》2013,6(4):335-339
Abstract Hydrotalcite containing magnesium and aluminium (Mg–Al HT) with a molar ratio of Mg(II)/Al(III) = 2.5 has been prepared by a co-precipitation method using the effluent of a Friedel–Crafts acylation reaction. The HT was calcined at 500°C and reconstructed with deionized water. The synthesized HT was characterized by XRD and FT-IR spectroscopy and was successfully used as a catalyst in the Knoevenagel reaction of aldehydes and active methylene compounds. The catalyst was found to be reusable. 相似文献
28.
Friedel–Crafts acetylation at the 4-position of 3-phenylsydnone (1) was achieved via thermal heating overnight in moderate to good yields by employing various metal triflate catalysts (5–20 mol%), lithium perchlorate (0–20 mol%), and acetic anhydride (4 equivalents) using either acetonitrile or acetic anhydride, in excess, as solvent. Six commercially available, homogeneous metal triflate catalysts were investigated, and optimal conditions were determined. The best yields overall were achieved with indium triflate. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications ® to view the free supplemental file. 相似文献
29.
Formation of 1-aryl-4-oxo-cyclohexa(e)nonecarboxylates from the Diels–Alder cycloaddition of 2-trimethylsilyloxy-1,3-butadiene and Danishefsky diene with aryl- and pyridylacrylates and further conversion thereof to spirocycles is described. This provides an efficient method for spiro oxindoles, azaoxindoles, and dihydroisoquinolones.
30.
1,1‐Bis(4‐alkylthiophenyl)‐1‐alkenes were conveniently and efficiently prepared from alkyl phenyl sulfides and acyl chlorides via a tandem Friedel–Crafts acylation and alkylation in the presence of anhydrous aluminum chloride. The scope, limitation, and mechanism of the tandem reaction were also discussed. 相似文献