全文获取类型
收费全文 | 4655篇 |
免费 | 175篇 |
国内免费 | 17篇 |
专业分类
化学 | 3821篇 |
晶体学 | 20篇 |
力学 | 57篇 |
数学 | 491篇 |
物理学 | 458篇 |
出版年
2024年 | 3篇 |
2023年 | 37篇 |
2022年 | 139篇 |
2021年 | 204篇 |
2020年 | 121篇 |
2019年 | 153篇 |
2018年 | 124篇 |
2017年 | 102篇 |
2016年 | 177篇 |
2015年 | 168篇 |
2014年 | 148篇 |
2013年 | 314篇 |
2012年 | 353篇 |
2011年 | 356篇 |
2010年 | 231篇 |
2009年 | 178篇 |
2008年 | 284篇 |
2007年 | 320篇 |
2006年 | 269篇 |
2005年 | 227篇 |
2004年 | 197篇 |
2003年 | 163篇 |
2002年 | 131篇 |
2001年 | 47篇 |
2000年 | 50篇 |
1999年 | 40篇 |
1998年 | 45篇 |
1997年 | 29篇 |
1996年 | 35篇 |
1995年 | 17篇 |
1994年 | 24篇 |
1993年 | 22篇 |
1992年 | 20篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 5篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1985年 | 8篇 |
1984年 | 14篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1937年 | 1篇 |
排序方式: 共有4847条查询结果,搜索用时 15 毫秒
31.
Aguado GP Moglioni AG García-Expósito E Branchadell V Ortuño RM 《The Journal of organic chemistry》2004,69(23):7971-7978
The hydrogenation reactions of several cyclobutyl enamides derived from (-)-alpha-pinene or (-)-verbenone have been investigated by using different catalysts. The chiralities of both the substrate and the catalyst as well as the Z/E stereochemistry of the double bond have been considered, and the observed diastereoselectivity has been rationalized. For enamides with the double bond separated from the cyclobutane by a methylene, the Wilkinson catalyst did not induce any diastereoselection, but excellent diastereoselectivity was observed when using Et-DuPHOS-Rh and ChiraPHOS-Rh. The configuration of the new stereogenic center was catalyst-dependent and can be rationalized according to the Halpern mechanism. For (Z)-enamides with the double bond directly linked to the cyclobutane ring, the chirality of the substrate governed the diastereoselection and the Halpern mechanism seemed not to be operative in the hydrogenation with ChiraPHOS, with the configuration of the new stereogenic center being determined by steric effects. On the contrary, the chirality of the catalyst was the factor determining the stereochemistry of the major products with alkyl-DuPHOS-Rh. Z/E stereochemistry influenced the stereodifferentiation, and a different behavior for each Z or E stereoisomer was found. For both (Z)- and (E)-enamides, some instances of match/mismatch between the chirality of the substrate and that of the catalyst were observed. As a result of all of these studies, a series of new cyclobutyl alpha-amino acids has been synthesized. These products are interesting to incorporate into conformationally constrained peptides. 相似文献
32.
Méndez LA Jiménez J Cerrada E Mohr F Laguna M 《Journal of the American Chemical Society》2005,127(3):852-853
The symmetric digold(II)dichloride bis(ylide) complex [Au2Cl2(mu-{CH2}2PPh2)2] reacts with acetylides to form the asymmetric heterovalent gold(I)/gold(III) complexes [AuI(mu-{CH2}2PPh2)2AuIII(CCR)2] [R = Ph, tBu, and SiMe3], the phenyl analogue of which was characterized by X-ray crystallography. These compounds represent the first examples of gold(III) complexes containing two acetylide ligands. [AuI(mu-{CH2}2PPh2)2AuIII(CCPh)2] undergoes a reversible comproportionation reaction upon treatment with [Ag(ClO4)tht] to give the symmetric digold(II) cationic complex [Au2(tht)2(mu-{CH2}2PPh2)2](ClO4)2. If this complex is treated with phenylacetylene in the presence of base, the heterovalent gold(I)/gold(III) complex is re-formed. This reversible interconversion between binuclear gold(I)/gold(III) and digold(II) bis(ylide) complexes is unprecedented. 相似文献
33.
Aviñó A Cubero E González C Eritja R Orozco M 《Journal of the American Chemical Society》2003,125(51):16127-16138
The structural, dynamical, and recognition properties of antiparallel DNA triplexes formed by the antiparallel d(G#G.C), d(A#A.T), and d(T#A.T) motifs (the pound sign and dot mean reverse-Hoogsteen and Watson-Crick hydrogen bonds, respectively) are studied by means of "state of the art" molecular dynamics simulations. Once the characteristics of the helix are defined, molecular dynamics and thermodynamic integration calculations are used to determine the expected stabilization of the antiparallel triplex caused by the introduction of 8-aminopurines. Finally, oligonucleotides containing 8-aminopurine derivatives are synthesized and tested experimentally using several approaches in a variety of systems. A very large stabilization of the triplex is found experimentally, as predicted by simulations. These results open the possibility for the use of oligonucleotides carrying 8-aminopurines to bind single-stranded nucleic acids by formation of antiparallel triplexes. 相似文献
34.
González-Núñez ME Mello R Royo J Asensio G Monzó I Tomás F López JG Ortiz FL 《The Journal of organic chemistry》2005,70(9):3450-3457
[reaction: see text] Data on the apparent dipole moment of thianthrene-5-oxide (1) and (1)H NMR spectra in different solvents support the conformational mobility of 1, which flaps between two limit boat conformations with the sulfinyl group in pseudoequatorial and pseudoaxial positions, respectively. The conformational equilibrium of 1 occurs too fast for the (1)H NMR (500 MHz) time-scale even at -130 degrees C, and the equilibrium constant has not been determined. The apparent dipole moments of 1 in n-hexane and 1,4-dioxane and the (1)H NMR spectra of 1 and the model compounds cis- and trans-thianthrene-5,10-dioxides (2) and thianthrene (5) in different solvents and at various temperatures confirm that the relative position of the conformational equilibrium of 1 is solvent-dependent, and more polar solvents favor the conformation with the sulfoxide group in the pseudoaxial position (1(')(ax)). Variable-temperature (1)H NMR spectra have established the interconversion barrier of trans-2 and confirmed that the conformational equilibrium of cis-2 is strongly displaced toward the conformation with both sulfinyl groups in the pseudoequatorial position. The (1)H NMR data support the transannular interaction of the functional groups in 1 and trans-2. 相似文献
35.
Elena Levi Eli Lancry Yossi Gofer Doron Aurbach 《Journal of Solid State Electrochemistry》2006,10(3):176-184
A crystallographic approach was applied to elucidate the influence of the nature of the surface films on the electrochemical
behavior of Li and Mg intercalation compounds. This paper presents two examples: (1) protection of graphite electrodes by
Li2CO3 surface films, and (2) the unique electrochemical behavior of Mg-containing Chevrel phases (MgCP) obtained by different synthetic
routes. In the former case, the elucidation of the protection mechanism and the explanation of the high performance of such
protected electrodes are based on the analysis of possible Li-ion motion in the carbonate crystal structure. In the latter
case, a combination of synthesis, electrochemistry and XRD analysis was used to explain an unusual phenomenon: the difference
between the excellent electrochemical behavior of the Chevrel phase (CP) based on Cu-leached Cu2Mo6S8 (CuCP), and the poor electrochemical activity of the high-temperature synthesized MgCP, with the same phase composition.
It is shown that this phenomenon is caused by MgO formation on the surface of the latter material. The different surface chemistry
of the MgCPs obtained by the two different synthetic routes was substantiated by revealing the correlation between the electrochemical
activity and the chemical stability of these materials under ambient atmosphere conditions.
Dedicated to Prof. Mikhail A. Vorotyntsev on the occasion of his 60th birthday. 相似文献
36.
Elena V Chubarova Denis G. Samsonenko Maxim N. Sokolov Olga A. Gerasko Vladimir P. Fedin Javier G. Platas 《Journal of inclusion phenomena and macrocyclic chemistry》2004,48(1-2):31-35
A new inclusion compound which is a supramolecular adduct of cucurbit[8]uril with two guest molecules of phenylphosphonic acid, PhP(O)(OH)2, included into the cavity as ``two guests in host'' is reported. The guests match both size and hydrophilicity/hydrophobicity requirements. Two phenyl groups of molecules of PhP(O)(OH)2 are directed toward the center of the large hydrophobic cavity whereas the PO(OH)2 groups are outward-looking and bound with each hydrophilic portal of cucurbit[8]uril by a short hydrogen bond. 相似文献
37.
Eneyskaya EV Ivanen DR Shabalin KA Kulminskaya AA Backinowsky LV Brumer Iii H Neustroev KN 《Organic & biomolecular chemistry》2005,3(1):146-151
Transglycosylation catalyzed by a beta-D-xylosidase from Aspergillus sp. was used to synthesize a set of 4-methylumbelliferyl (MU) beta-1-->4-D-xylooligosides having the common structure [beta-D-Xyl-(1-->4)]2-5-beta-D-Xyl-MU. MU xylobioside synthesized chemically by the condensation of protected MU beta-D-xylopyranoside with ethyl 2,3,4-tri-O-acetyl-1-thio-beta-D-xylopyranoside was used as a substrate for transglycosylation with the beta-D-xylosidase from Aspergillus sp. to produce higher MU xylooligosides. The structures of oligosaccharides obtained were established by 1H and 13C NMR spectroscopy and electrospray tandem mass spectrometry. MU beta-D-xylooligosides synthesized were tested as fluorogenic substrates for the GH-10 family beta-D-xylanase from Aspergillus orizae and the GH-11 family beta-D-xylanase I from Trichoderma reesei. Both xylanases released the aglycone from MU xylobioside and the corresponding trioside. With substrates having d.p. 4 and 5, the enzymes manifested endolytic activities, splitting off MU, MUX, and MUX2 primarily. 相似文献
38.
Talapin DV Shevchenko EV Murray CB Kornowski A Förster S Weller H 《Journal of the American Chemical Society》2004,126(40):12984-12988
We demonstrate the self-organization of CdSe nanorods into nematic, smectic, and crystalline solids. Layered colloidal crystals of CdSe nanorods grow by slow destabilization of a nanocrystal solution upon allowing the diffusion of a nonsolvent into the colloidal solution of nanocrystals. The colloidal crystals of nanorods show characteristic birefringence, which we assign to specific spherulite-like texture of each nanorod assembly. To demonstrate the general character of nanorod self-assembly technique, CdSe/CdS heterostructure nanorods were organized into highly luminescent superlattices. 相似文献
39.
A model system for the synthesis of phloroglucinol containing natural products was synthesized. Key steps include a manganic acetate-mediated cyclization and the facile conversion of an alkene into a β-bromoenone. 相似文献
40.
The solid-phase synthesis of a 4500-member (30 x 15 x 10) tyrphostin library is demonstrated utilizing the Irori-directed sorting system. Fmoc-protected PL-Rink resin was used as the solid support. After Fmoc-deprotection, aryl aldehydes were attached to the resin through reductive amination. Acylation of the resulting secondary amines with cyanoacetic acid was followed by a Knoevenagel condensation with phenolic aldehydes. Mitsunobu coupling of primary alcohols to the resin-bound phenols yielded the final library of compounds 1. 相似文献