全文获取类型
收费全文 | 854篇 |
免费 | 21篇 |
国内免费 | 7篇 |
专业分类
化学 | 417篇 |
晶体学 | 2篇 |
力学 | 40篇 |
数学 | 192篇 |
物理学 | 231篇 |
出版年
2019年 | 9篇 |
2018年 | 10篇 |
2017年 | 10篇 |
2016年 | 11篇 |
2015年 | 17篇 |
2014年 | 11篇 |
2013年 | 31篇 |
2012年 | 46篇 |
2011年 | 37篇 |
2010年 | 22篇 |
2009年 | 8篇 |
2008年 | 20篇 |
2007年 | 33篇 |
2006年 | 35篇 |
2005年 | 23篇 |
2004年 | 21篇 |
2003年 | 24篇 |
2002年 | 21篇 |
2001年 | 17篇 |
2000年 | 20篇 |
1999年 | 11篇 |
1998年 | 15篇 |
1997年 | 10篇 |
1996年 | 19篇 |
1995年 | 16篇 |
1994年 | 20篇 |
1993年 | 27篇 |
1992年 | 25篇 |
1991年 | 19篇 |
1990年 | 23篇 |
1989年 | 13篇 |
1988年 | 20篇 |
1987年 | 10篇 |
1986年 | 11篇 |
1985年 | 20篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 16篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 9篇 |
1974年 | 5篇 |
1973年 | 9篇 |
1970年 | 6篇 |
1867年 | 6篇 |
1863年 | 4篇 |
排序方式: 共有882条查询结果,搜索用时 15 毫秒
21.
A Rosenthal D Cech V.P Veiko T.S Orezkaja E.A Romanova A.A Elov V.G Metelev E.S Gromova Z.A Shabarova 《Tetrahedron letters》1984,25(39):4353-4356
In order to investigate the interaction of Eco RII restriction and modification enzymes with synthetic DNA fragments three nonadeoxyribonucleotides containing the modified bases uracil, 5-bromouracil and 5-methylcytosine were synthesized according to the phosphate tri-ester approach using TPS/l-methylimidazole as the condensation agent. The patterns of these modified DNA fragments obtained by Maxam/Gilbert sequence technique are presented. 相似文献
22.
Zusammenfassung Bisher waren die Untersuchungen über Erwartungswert-Streuungs-effiziente Wertpapiermischungen auf den Fall nichtsingulärer Kovarianzmatrizen beschränkt. Der Fall singulärer Kovarianzmatrizen war bisher nicht konstruktiv behandelt worden. Es wird in dem vorstehenden Papier in einem allgemeinen Ansatz, der auch den Fall singulärer Kovarianzmatrizen zuläßt, die allgemeine Gültigkeit des Separationstheorems und des Zwei-Fonds-Theorems nachgewiesen.
Summary Previous studies of mean-variance-efficient portfolios got constructive results in the case of non-singular covariance matrices only, singular covariance matrices were not treated in a constructional way. The present paper proves the Separation Theorem and the Two-Fonds-Theorem within a general framework including the case of singular covariance matrices.相似文献
23.
Michael I. Rosenthal Anthony W. Czarnik 《Journal of inclusion phenomena and macrocyclic chemistry》1991,10(1):119-126
The ability of cyclodextrins to effect rapid transacylations of bound substrates has been well studied. One important difference between cyclodextrin and enzyme-mediated transacylation is the pH required. Because the pK
a of a cyclodextrin secondary-side hydroxyl group is about 12, transacylations are accelerated in the presence of cyclodextrin under basic conditions (pH > 10.5). In 1988, our group reported the synthesis of cyclodextrin with attached cyclen-Co(III) complexes; significant acceleration in the reaction withp-nitrophenyl acetate was observed only with the primary side derivative. Of course, metalloenzymes utilize M2+ and not M3+ catalytic centers; in addition, large rate accelerations in the transacylations of both activated and unactivated substrates have been observed previously in systems utilizing M2+ ions (e.g., Zn, Cu, Ni) as well as M3+ ions (e.g. Co, Ir, Cr). In this paper, we describe the ability of CD-cyclen-M2+ conjugates to transacylate activated esters, amides, and phosphates. In addition, the ability of the apoenzyme mimic to effect transacylations was examined. 相似文献
24.
Slaughter BD Unruh JR Price ES Huynh JL Bieber Urbauer RJ Johnson CK 《Journal of the American Chemical Society》2005,127(34):12107-12114
We used single-pair fluorescence resonance energy transfer (spFRET) measurements to characterize denatured and partially denatured states of the multidomain calcium signaling protein calmodulin (CaM) in both its apo and Ca(2+)-bound forms. The results demonstrate the existence of an unfolding intermediate. A CaM mutant (CaM-T34C-T110C) was doubly labeled with fluorescent probes AlexaFlour 488 and Texas Red at opposing globular domains. Single-molecule distributions of the distance between fluorophores were obtained by spFRET at varying levels of the denaturant urea. Multiple conformational states of CaM were observed, and the amplitude of each conformation was dependent on urea concentration, with the amplitude of an extended conformation increasing upon denaturation. The distributions at intermediate urea concentrations could not be adequately described as a combination of native and denatured conformations, showing that CaM does not denature via a two-state process and demonstrating that at least one intermediate is present. The intermediate conformations formed upon addition of urea were different for Ca(2+)-CaM and apoCaM. An increase in the amplitude of a compact conformation in CaM was observed for apoCaM but not for Ca(2+)-CAM upon the addition of urea. The changes in the single-molecule distributions of CaM upon denaturation can be described by either a range of intermediate structures or by the presence of a single unfolding intermediate that grows in amplitude upon denaturation. A model for stepwise unfolding of CaM is suggested in which the domains of CaM unfold sequentially. 相似文献
25.
26.
Normen Peulecke Andreas Ohff Wolfgang Baumann Rhett Kempe Vladimir V. Burlakov Uwe Rosenthal 《Journal of organometallic chemistry》1996,520(1-2):235-239
The reaction of Cp2Zr(L)(η2-Me3SiC2) (L = THF, py) with equimolar amounts of H2C = CMe-CHO at room temperature depends strongly on the ligands L and the solvents that are used. With L = THF, in the THF solution the insertion product 1 was isolated, whereas by conducting the reaction in n-hexane solution an alkyne substitution with 1,4-coordination of the methacrolein takes place and the binuclear complex [ 2 was obtained. In conttrast, with L = py (a stronger ligand) only a 1:1 ratio of 1 and 2 was observed in both THF and in n-hexane. At 50°C complex 1 was converted into 2 and the alkyne was eliminated quantitatively.
Complexes 1 and 2 were characterized by IR and NMR spectroscopical measurements and 1 by an additional X-ray structure determination. 相似文献
27.
28.
The synthesis and oxygen atom transfer (OAT) photoreactivity of a diiron(III) mu-oxo meso-tripentafluorophenyl bisporphyrin appended to a dibenzofuran spacer are presented. Reaction of 4,6-diformyldibenzofuran under standard Lindsey conditions furnishes the parent cofacial porphyrin architecture in a single step. These cofacial porphyrins photocatalyze the oxidation of sulfides and olefins using visible light and molecular oxygen as the terminal oxidant. High turnover numbers reflect the enhanced stability of the electron-deficient diiron(III) mu-oxo bisporphyrin core appended to a dibenzofuran spacer under aerobic conditions. 相似文献
29.
The syntheses of 3'-(aminoethyl)-2-biphenylpropionic acid (1) and 2-amino-3'-biphenylcarboxylic acid (2) are described. These residues were designed to nucleate beta-sheet structure in aqueous solution when incorporated into small, amphiphilic peptides in place of the backbone of the i + 1 and i + 2 residues of the beta-turn. N-Benzyl-3'-(2-(benzylamido)ethyl)-2-biphenylpropamide (3) and N-benzyl-(2-benzylamido)-3'-biphenylamide (4) were synthesized and studied as model compounds to investigate the hydrogen-bonding capabilities of residues 1 and 2, respectively. The X-ray crystal structure of 3 indicates that a 13-membered intramolecular hydrogen-bonded ring is formed, while the remaining amide proton and carbonyl are involved in intermolecular hydrogen bonding. Infrared and variable-temperature NMR experiments indicate that, in solution (CH(2)Cl(2)), 3 exists as an equilibrium mixture of the 13- and the 15-membered intramolecularly hydrogen-bonded conformers with the 15-membered ring conformer being favored. Amide 4 was shown to exist in solution (CH(2)Cl(2)) as an equilibrium mixture of the 11-membered intramolecular hydrogen-bonded ring and a nonbonded conformation. No contribution from the 9-membered hydrogen-bonded ring conformation was observed. The X-ray crystal structure of 4 indicated the absence of intramolecular hydrogen bonding in the solid state. 相似文献
30.