全文获取类型
收费全文 | 9088篇 |
免费 | 367篇 |
国内免费 | 20篇 |
专业分类
化学 | 6202篇 |
晶体学 | 68篇 |
力学 | 345篇 |
数学 | 1384篇 |
物理学 | 1476篇 |
出版年
2023年 | 51篇 |
2022年 | 67篇 |
2021年 | 122篇 |
2020年 | 160篇 |
2019年 | 160篇 |
2018年 | 186篇 |
2017年 | 149篇 |
2016年 | 310篇 |
2015年 | 248篇 |
2014年 | 280篇 |
2013年 | 575篇 |
2012年 | 579篇 |
2011年 | 658篇 |
2010年 | 353篇 |
2009年 | 335篇 |
2008年 | 586篇 |
2007年 | 609篇 |
2006年 | 545篇 |
2005年 | 532篇 |
2004年 | 406篇 |
2003年 | 376篇 |
2002年 | 296篇 |
2001年 | 145篇 |
2000年 | 138篇 |
1999年 | 118篇 |
1998年 | 89篇 |
1997年 | 72篇 |
1996年 | 113篇 |
1995年 | 98篇 |
1994年 | 83篇 |
1993年 | 76篇 |
1992年 | 68篇 |
1991年 | 47篇 |
1990年 | 46篇 |
1989年 | 43篇 |
1988年 | 28篇 |
1987年 | 30篇 |
1985年 | 63篇 |
1984年 | 44篇 |
1983年 | 31篇 |
1982年 | 54篇 |
1981年 | 44篇 |
1980年 | 54篇 |
1979年 | 42篇 |
1978年 | 45篇 |
1977年 | 39篇 |
1976年 | 50篇 |
1975年 | 47篇 |
1974年 | 30篇 |
1973年 | 28篇 |
排序方式: 共有9475条查询结果,搜索用时 15 毫秒
981.
We describe the design, synthesis, and characterization of a heterobivalent ligand (HBL) system that competitively inhibits allergen binding to mast cell bound IgE antibody, thereby inhibiting mast cell degranulation. HBLs are composed of a hapten conjugated to a nucleotide analog allowing simultaneous targeting of the antigen-binding site as well the "unconventional nucleotide binding site" on IgE Fab domains. Simultaneous bivalent binding to both sites provides HBLs with over 100-fold enhancement both in avidity for IgE(DNP) (K(d) = 0.33 μM) and in inhibition of allergen binding to IgE(DNP) (IC(50) = 0.45 μM) than the monovalent hapten (K(d)(mono) = 41 μM; IC(50)(mono) = 55.4 μM, respectively). In cellular assays, HBL2 effectively inhibits mast cell degranulation (IC(50) = 15 μM), whereas no inhibition is detected by the monovalent hapten. In conclusion, this study establishes the use of multivalency in a novel HBL design to inhibit mast cell degranulation. 相似文献
982.
The intermediacy of the geminate base-proton pair (A*···H(+)) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH(+)* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base-proton pair A*···H(+). The geminate pair becomes detectable because of the increase in the recombination rate constant, k(rec), of (A*···H(+)) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH(+)*, k(d), and recombination, k(rec), of A*···H(+)) from the diffusion controlled rates (dissociation, k(diss), and formation, k(diff)[H(+)], of A*···H(+)), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k(d) and k(rec)) as well as on proton diffusion/migration (k(diss) and k(diff)). 相似文献
983.
Agopcan S Çelebi-Ölçüm N Üçışık MN Sanyal A Aviyente V 《Organic & biomolecular chemistry》2011,9(23):8079-8088
In this study, the origins of diastereoselectivity in the hydrogen bonding assisted Diels-Alder reactions of chiral dienes with achiral dienophiles have been investigated with density functional methods. The distortion/interaction model has been applied to shed light on the origins of selectivity. C9-Substituted chiral anthracene templates (R = (CH(3))(OCH(3))(H), R = (CH(3))(OH)(H), R = (CH(3))(CH(2)CH(3))(H) and R = (-CH(2)-C(CH(3))(OCH(3))(H)) are used to rationalize the role of a stereogenic center and H-bonding on the product distribution ratio. Even though hydrogen bonding increases the reactivity of the diene, the stereoselectivity is reduced because of the hydrogen bonding capacity of both diastereomeric transition states. The interaction energies of the studied anthracene templates with N-methyl maleimide at the transition state correlate linearly with an increase in reactivity. The selectivity is determined by both favorable distortion and interaction energies. The π-facial selectivity induced by the presence of a chiral auxiliary in 1-substituted 1,3-pentadienes (R1 = (CH(3))(OCH(3))(H) and R1 = (CH(3))(OH)(H)) has also been modeled in order to rationalize the role of the stereogenic center and H-bonding on the stereoselectivity of an aliphatic diene. In both parts, the product distribution ratios calculated from Boltzmann distributions based on Gibbs free energies are in reasonable agreement with the experimental results. Finally the role of OH-substituted five-membered pyrrolidine on C9 of anthracene is investigated since the successful usage of the conformationally rigid pyrrolidines in asymmetric synthesis is well known. Overall, both in the acyclic system and in anthracene, the facilitation due to H-bonding is reflected in the interaction energies: the higher the difference in interaction energies in the transition structures of the two diastereomers, the more selective the H-bonding assisted Diels-Alder reaction is. 相似文献
984.
Hybrids of vinblastine and phomopsin, designed by a molecular modelling study, were elaborated in order to target tubulin. The key step of the synthesis (fragmentation and insertion of vindoline) was mediated by an internal N-carboxyanhydride (or O-acylcarbamate). This reaction was diastereospecific and addition of silver salts could reverse the diastereoselectivity. Even if the synthesized compounds are inactive, this synthesis represents an original example of a C-N fragmentation mediated by a N-carboxyanhydride. 相似文献
985.
Costa M Areias F Castro M Brea J Loza MI Proença F 《Organic & biomolecular chemistry》2011,9(11):4242-4249
A one-pot procedure was developed for the synthesis of novel 3-[amino(methoxy)methylene]-2-oxo-3,4-dihydro-2H-chromen-4-yl)-3-cyanoacetamides and chromeno[3,4-c]pyridine-1-carbonitriles from the reaction of 2-oxo-2H-chromene-3-carbonitriles and cyanoacetamides. These chromene derivatives were identified as new scaffolds for adenosine receptors and the hits 3a, 3c, 5a, and 5b were found. 相似文献
986.
Rose-Munch F Marti A Cetiner D Tranchier JP Rose E 《Dalton transactions (Cambridge, England : 2003)》2011,40(7):1567-1575
First Suzuki-Miyaura coupling reactions applied to (η(5)-chloro-cyclohexadienyl)Mn(CO)3 complexes are described and lead to the syntheses of (η(5)-aryl-cyclohexadienyl)Mn(CO)3 and of cationic (η(6)-arene)Mn(CO)3 complexes after rearomatization. The structures of two of the new complexes have been investigated by X-ray diffraction study. 相似文献
987.
Pieters G Gaucher A Marrot J Maurel F Naubron JV Jean M Vanthuyne N Crassous J Prim D 《Organic letters》2011,13(16):4450-4453
The synthesis and properties of new chiral polycyclic architectures that display both helicity and a saddle-type shape are described. The enantiomers have been separated, and their absolute configuration was determined by VCD and ECD. The unprecedented molecular architecture is based on a cyclooctatriene core surrounded by an association of benzo[c]fluorene and ortho-phenylene units. 相似文献
988.
A general and convenient protocol for the electrophilic borylation of aryl Grignard reagents prepared from arylbromides by direct insertion of magnesium in the presence of LiCl or by Mg/Br exchange with (i)PrMgCl·LiCl has been developed. Various aryl boronic acids were synthesized in a straightforward manner in excellent yields at 0 °C. 相似文献
989.
The core of daphnilactone B-type and yuzurimine-type alkaloids was synthesized in only 16 steps from a known β-allyl-γ-butyrolactone. The key sequence of Vilsmeier-Haack cyclization and intramolecular azomethine ylide cycloaddition allowed the construction of, in a single step, three of the five rings common to all alkaloids found in both of these classes with perfect chemocontrol. 相似文献
990.
The stereoselective synthesis of both cis- and trans-β-fluorostyrene derivatives from a common intermediate, (Z)-1-aryl-2-fluoro-1-(trimethylsilyl)ethenes, is described. The trans isomers are obtained by a stereospecific replacement of the silyl group in the presence of water and a fluoride source, whereas the preparation of the cis isomers is achieved by a bromination/desilicobromination sequence followed by reduction of the newly created C-Br bond. A stereoselective transformation of both stereoisomers of β-fluorostyrene is also presented. 相似文献