全文获取类型
收费全文 | 807篇 |
免费 | 13篇 |
国内免费 | 2篇 |
专业分类
化学 | 684篇 |
晶体学 | 2篇 |
力学 | 6篇 |
数学 | 81篇 |
物理学 | 49篇 |
出版年
2022年 | 5篇 |
2021年 | 8篇 |
2020年 | 8篇 |
2019年 | 13篇 |
2017年 | 4篇 |
2016年 | 21篇 |
2015年 | 20篇 |
2014年 | 17篇 |
2013年 | 43篇 |
2012年 | 39篇 |
2011年 | 47篇 |
2010年 | 35篇 |
2009年 | 34篇 |
2008年 | 39篇 |
2007年 | 44篇 |
2006年 | 29篇 |
2005年 | 38篇 |
2004年 | 36篇 |
2003年 | 36篇 |
2002年 | 20篇 |
2001年 | 10篇 |
2000年 | 11篇 |
1999年 | 10篇 |
1998年 | 10篇 |
1997年 | 10篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 16篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 17篇 |
1981年 | 7篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1973年 | 6篇 |
1972年 | 4篇 |
1971年 | 4篇 |
1968年 | 5篇 |
排序方式: 共有822条查询结果,搜索用时 15 毫秒
741.
Abstract— Slow destruction of the far-red-absorbing form of phytochrome (Pfr ), which has been observed in light-grown oat and maize, occurs in light- and dark-grown Amaranthus, Pharbitis , and Brassica seedlings as well. Destruction of Pfr in these seedlings shows two phases: if a high level of Pfr is produced in dark-grown seedlings, the destruction is fast in the beginning and then slows after a low Pfr level has been reached. Slow Pfr destruction is predominant in light-grown tissue. 相似文献
742.
The variations of concentrations of volatile halocarbons (CHCl3, CCl4, CH2Cl—CH2Cl, CCl3—CH3, CHCl—CCl2, and CCl2=CCl2) in rural and urban air are demonstrated by two series of air sampling carried out in Southern Germany. The interpretation of the changes in concentrations for the individual compounds is based on specific local input for the chlorinated solvents and on changing weather conditions during the sampling periods.Dedicated to Professor J. F. K. Huber on the occasion of his 65th birthday 相似文献
743.
Richard D. Adams István T. Horváth Brigitte E. Segmüller 《Journal of organometallic chemistry》1984,262(2):243-252
Thermal degradation of the cluster compound Os3(CO)8(PPh2H)(μ3-S)2 (I) at 125°C leads to decarbonylation and formation of the new ligand bridged hexanuclear cluster Os6(CO)14(μ-PPh2)2(μ3-S)3(μ4-S) (II) in 11% yield. Space Group: P, No. 2, a 10.427(5), b 13.552(3), c 17.919(3) Å, α 84.87(2), β 75.41(3), γ 78.43(3)°, V 2399(2) Å3Z = 2, ?calc 2.82 g cm?3. The structure was solved by the heavy atom method and refined (3223 reflections) to the final residuals R = 0.042 and Rw = 0.036. The molecule consists of two sulfido bridged open triosmium clusters which are linked by a bridging sulfido ligand and a bridging diphenylphosphino ligand. 相似文献
744.
Hubert Schmidbaur Julian M. Wallis Reinhold Nowak Brigitte Huber Gerhard Müller 《欧洲无机化学杂志》1987,120(11):1837-1843
Arene Complexes with a Half-Sandwich Structure: The 1:1 Complexes of Mesitylene with SbCl3, SbBr3, BiCl3, and BiBr3 1,3,5-Trimethylbenzene forms stable 1:1 complexes with SbCl3, SbBr3, BiCl3, and BiBr3 of the stoichiometry C6H3Me3·EX3 ( 1 – 4 ). According to the results of X-ray structure analyses of compounds 2 and 3 , one arene molecule is coordinated to each antimony or bismuth atom characterizing these adducts as half-sandwich species. To a good approximation the mesitylene molecules are centered over the metal atoms, but deviations from strict η6-hapticity are larger for antimony than for bismuth. – Despite some obvious analogies in many features of the structures, 2 and 3 are not isostructural. Differences appear with regard to the halogen bridging between the EX3 moieties giving rise to the formation of two-dimensional networks (EX3)n covered above and below by mesitylene molecules. The structural and sequential principles of the layers differ in a characteristic way for 2 and 3 . 相似文献
745.
Aryl and Aryne Complexes of Group Six Transition Metals. Preparation of [Mo(p-C6H4CH3)6(Li · OC4H10)3] und [W(p-C6H4CH3)4(C6H3CH3)2(Li · OC4H10)4] and NMR Spectroscopic Investigation of the W Complex Molybdenum(V) chloride reacts with an etheral solution of p-tolyllithium (molar ratio 1:10) to yield a yellow, strongly paramagnetic hexatolylo complex ([MoTol6(Li · S)3]1) (μeff = 3.51 B.M.), while from tungsten(V) bromide and p-tolyllithium (molar ratio 1:11) a blackish violet, diamagnetic complex [WTol4Tn2(Li · S)4] is formed, containing two tolylene or tolyne groups for ligands. The 1H-NMR spectrum points to the Tol-ligands being influenced by the methyl groups of the Tn-ligands. 相似文献
746.
747.
748.
Brigitte I. Glnzer Zoltan Gyrgydek Bruno Bernet Andrea Vasella 《Helvetica chimica acta》1991,74(2):343-369
The piperidines 12 – 18 , piperidmose analogues of Neu5Ac ( 1 ) with a shortened side chain, were synthesized from N-acetyl-D -glucosamine via the azidoalkene 32 and tested as inhibitors of Vibrio cholerae sialidase. Deoxygenation at C(4) of the uronate 22 , obtained from the known D -GlcNAc derivative 20 , was effected by β-elimination (→ 23 ), exchange of the AcO at C(3) with a (t-Bu)Me2SiO group and hydrogenation (→ 26 ; Scheme 1). Chain extension of 26 by reaction with Me3SiCH2MgCl gave the D -ido-dihydroxysilane 28 , which was transformed into the unsaturated L -xylo-mesylate 29 and further into the L -lyxo-alcohol 30 , the mesylate 31 , and the L -xylo-azide 32 . The derivatives 29 – 31 prefer a sickle zig-zag and 32 mainly an extended zig-zag conformation (Fig. 2). The piperidinecarboxylate 15 was obtained from 32 by ozonolysis (→ 33 ), intramolecular reductive animation (→ 34 ), and deprotection, while reductive animation of 34 with glycolaldehyde (→ 35 ) and deprotection gave 16 (Scheme 2). An intramolecular azide-olefin cycloaddition of 32 yielded exclusively the fused dihydrotriazole 36 , while the lactone 39 did not cyclize (Scheme 3). Treatment of 36 with AcOH (→ 37 ) followed by hydrolysis (→ 38 ) and deprotection led to the amino acid 18 . To prepare the (hydroxymethyl)piperidinecarboxylates 12 and 17 , 32 was first dihydroxylated (Scheme 4). The L -gluco-diol 40 was obtained as the major product, in agreement with Kishi's rule. Silylation of 40 (→ 42 ), oxidation with periodinane (→ 44 ), and reductive animation gave the L -gluco-piperidine 45 . It was, on the one hand, deprotected to the amino acid 12 and, on the other hand, N-phenylated (→ 46 ) and deprotected to 17 . While 45 and 12 adopt a 2C5 conformation, the analogous N-Ph derivatives 46 and 17 adopt a 5C2 and a B3,6 conformation, respectively, on account of the allylic 1,3-strain. The conformational effects of this 1,3-strain are also evident in the carbamate 47 , obtained from 45 (Scheme 5), and in the C(2)-epimerized bicyclic ether 48 , which was formed upon treatment of 47 with (diethylamino)sulfur trifluoride (DAST). Fluorination of 40 with DAST (→ 49 ) followed by treatment with AcOH led to the D -ido-fluorohydrin 50 . Oxidation of 50 (→ 51 ) followed by a Staudinger reaction and reduction with NaBH3CN afforded the (fluoromethyl)piperidine 52 , while reductive amination of 51 with H2/Pd led to the methylpiperidine 55 , which was similarly obtained from the keto tosylate 54 and from the dihydrotriazole 36 . Deprotection of 52 and 55 gave the amino acids 13 and 14 , respectively. The aniline 17 does not inhibit V. cholerae sialidase; the piperidines 12 – 16 and 18 are weak inhibitors, evidencing the importance of an intact 1,2,3-trihydroxypropyl side chain. 相似文献
749.
The rates of homogeneous nucleation of orpiment (As2S3), claudetite (As2O3) and arsenic selenide (As2Se3) in their undercooled melts are much lower than the calculated values. From the slow nucleation rates results the extremely low tendency towards crystallization of the pure undercooled melts. By seeding crystals or under conditions favouring a heterogeneous nucleation, crystalline As2S3 and As2Se3 were obtained from the melt. Orpiment shows a retarded melting behaviour and can be superheated beyond its melting point of 591 K. 相似文献
750.
Both enantiomers of the polyketide-derived lactone citreofuran (4) have been prepared. The enantiopure building blocks were obtained on a chemoenzymatic route ((S)-6) and by a chiral pool synthesis ((R)-6). The crucial step in the synthesis, a macrolactonization, was accomplished under Mitsunobu conditions. 相似文献