全文获取类型
收费全文 | 5314篇 |
免费 | 149篇 |
国内免费 | 18篇 |
专业分类
化学 | 3879篇 |
晶体学 | 16篇 |
力学 | 62篇 |
数学 | 965篇 |
物理学 | 559篇 |
出版年
2020年 | 49篇 |
2019年 | 53篇 |
2018年 | 36篇 |
2016年 | 92篇 |
2015年 | 91篇 |
2014年 | 100篇 |
2013年 | 163篇 |
2012年 | 159篇 |
2011年 | 224篇 |
2010年 | 113篇 |
2009年 | 104篇 |
2008年 | 213篇 |
2007年 | 207篇 |
2006年 | 211篇 |
2005年 | 241篇 |
2004年 | 184篇 |
2003年 | 163篇 |
2002年 | 162篇 |
2001年 | 76篇 |
2000年 | 73篇 |
1997年 | 64篇 |
1996年 | 60篇 |
1995年 | 67篇 |
1994年 | 48篇 |
1993年 | 51篇 |
1992年 | 65篇 |
1991年 | 44篇 |
1990年 | 67篇 |
1989年 | 37篇 |
1988年 | 61篇 |
1987年 | 64篇 |
1986年 | 40篇 |
1985年 | 93篇 |
1984年 | 55篇 |
1983年 | 59篇 |
1982年 | 67篇 |
1981年 | 62篇 |
1980年 | 48篇 |
1979年 | 64篇 |
1978年 | 68篇 |
1977年 | 51篇 |
1976年 | 73篇 |
1975年 | 72篇 |
1974年 | 52篇 |
1973年 | 46篇 |
1972年 | 39篇 |
1971年 | 32篇 |
1970年 | 45篇 |
1967年 | 36篇 |
1966年 | 41篇 |
排序方式: 共有5481条查询结果,搜索用时 15 毫秒
81.
Abstract Starting from the kanamycin A 4″,6″-ditriflate 6 und -6″-brosy-late-4″-triflate 8, respectively, the following derivatives were prepared: 4″,6″-dideoxy-4″,6″-difluoro-4″-epi- (20), 4″,6″-dideoxy-4″-fluoro-4″-epi- (22), 6″-deoxy-6″-fluoro-4″-epi- (19), and 6″-deoxy-4″-epi-kanamycin A (21). C NMR and antibacterial data are given. 相似文献
82.
83.
Tyler B. J. Pinter Karl J. Koebke Vincent L. Pecoraro 《Angewandte Chemie (International ed. in English)》2020,59(20):7678-7699
The relationship between protein structure and function is one of the greatest puzzles within biochemistry. De novo metalloprotein design is a way to wipe the board clean and determine what is required to build in function from the ground up in an unrelated structure. This Review focuses on protein design efforts to create de novo metalloproteins within alpha‐helical scaffolds. Examples of successful designs include those with carbonic anhydrase or nitrite reductase activity by incorporating a ZnHis3 or CuHis3 site, or that recapitulate the spectroscopic properties of unique electron‐transfer sites in cupredoxins (CuHis2Cys) or rubredoxins (FeCys4). This work showcases the versatility of alpha helices as scaffolds for metalloprotein design and the progress that is possible through careful rational design. Our studies cover the invariance of carbonic anhydrase activity with different site positions and scaffolds, refinement of our cupredoxin models, and enhancement of nitrite reductase activity up to 1000‐fold. 相似文献
84.
Anna V. Davies Keegan P. Fitzpatrick Rick C. Betori Karl A. Scheidt 《Angewandte Chemie (International ed. in English)》2020,59(23):9143-9148
As a key element in the construction of complex organic scaffolds, the formation of C?C bonds remains a challenge in the field of synthetic organic chemistry. Recent advancements in single‐electron chemistry have enabled new methods for the formation of various C?C bonds. Disclosed herein is the development of a novel single‐electron reduction of acyl azoliums for the formation of ketones from carboxylic acids. Facile construction of the acyl azolium in situ followed by a radical–radical coupling was made possible merging N‐heterocyclic carbene (NHC) and photoredox catalysis. The utility of this protocol in synthesis was showcased in the late‐stage functionalization of a variety of pharmaceutical compounds. Preliminary investigations using chiral NHCs demonstrate that enantioselectivity can be achieved, showcasing the advantages of this protocol over alternative methodologies. 相似文献
85.
Henriette N. Tobiesen Dr. Lars A. Leth Marc V. Iversen Dr. Line Næsborg Dr. Søren Bertelsen Prof. Dr. Karl Anker Jørgensen 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(42):18648-18652
The first stereoselective, near-equimolar, and metal-free oxidative bioconjugation of amino acids and oligopeptides to aldehydes is presented. Based on a newly developed organocatalytic oxidative concept, the C-terminal and side-chain carboxylic acid functionalities of amino acids and oligopeptides are shown to couple in a stereoselective manner to α-branched aldehydes catalyzed by a chiral primary amine and a quinone as oxidizing agent. The oxidative coupling generally proceeds in high yield. For aspartic acid, selective coupling of the side-chain, or the C-terminal carboxylic acid, is demonstrated depending on the protection strategy. The stereoselective, oxidative bioconjugation concept is extended to a series of oligopeptides where coupling to carboxylic acid functionalities is presented. Bioorthogonal linker molecules for further functionalization are obtained by merging the oxidative coupling strategy with the click concept. It is demonstrated that the configuration of the new stereocenter is determined exclusively by the organocatalyst. 相似文献
86.
Dennis A. Buschmann Dr. H. Martin Dietrich Dr. David Schneider Dr. Verena M. Birkelbach Dr. Christoph Stuhl Prof. Dr. Karl W. Törnroos Dr. Cäcilia Maichle-Mössmer Prof. Dr. Reiner Anwander 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(47):10834-10840
Tetramethylaluminato/halogenido(X) ligand exchange reactions in half-sandwich complexes [CpRLa(AlMe4)2] are feasible in non-coordinating solvents and provide access to large coordination clusters of the type [CpRLaX2]x. Incomplete exchange reactions generate the hexalanthanum clusters [CpR6La6X8(AlMe4)4] (CpR=Cp*=C5Me5, X=I; CpR=Cp′=C5H4SiMe3, X=Br, I). Treatment of [Cp*La(AlMe4)2] with two equivalents Me3SiI gave the nonalanthanum cluster [Cp*LaI2]9, while the exhaustive reaction of [Cp′La(AlMe4)2] with the halogenido transfer reagents Me3GeX and Me3SiX (X=I, Br, Cl) produced a series of monocyclopentadienyl rare-earth-metal clusters with distinct nuclearity. Depending on the halogenido ion size the homometallic clusters [Cp′LaCl2]10 and [Cp′LaX2]12 (X=Br, I) could be isolated, whereas different crystallization techniques led to the aggregation of clusters of distinct structural motifs, including the desilylated cyclopentadienyl-bridged cluster [(μ-Cp)2Cp′8La8I14] and the heteroaluminato derivative [Cp′10La10Br18(AlBr2Me2)2]. The use of the Cp′ ancillary ligand facilitates cluster characterization by means of NMR spectroscopy. 相似文献
87.
Sheetal Saxena Alexander Summa Dominik Elsässer Michael Rüger Karl Mannheim 《The European Physical Journal C - Particles and Fields》2011,71(11):1815
As the largest mass concentrations in the local Universe, nearby clusters of galaxies and their central galaxies are prime
targets in searching for indirect signatures of dark matter annihilation (DMA). We seek to constrain the dark matter annihilation
emission component from multi-frequency observations of the central galaxy of the Virgo cluster. The annihilation emission
component is modeled by the prompt and inverse-Compton gamma rays from the hadronization of annihilation products from generic
weakly interacting dark matter particles. This component is fitted to the excess of the observed data above the spectral energy
distribution (SED) of the jet in M87, described with a best-fit synchrotron-self-Compton (SSC) spectrum. While this result
is not sufficiently significant to claim a detection, we emphasize that a dark matter “double hump signature” can be used
to unambiguously discriminate the dark matter emission component from the variable jet-related emission of M87 in future,
more extended observation campaigns. 相似文献
88.
89.
Dr. Łukasz Albrecht Dr. Hao Jiang Prof. Dr. Karl Anker Jørgensen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(2):358-368
This Concept article summarizes recent progress in the field of hydrogen‐bonding aminocatalysis using proline‐derived systems. The aminocatalysts available in the literature are categorized by the incorporated hydrogen‐bonding scaffold and its mode of recognition. Both mono‐ and double‐hydrogen‐bonding motifs are discussed and examples of their application in asymmetric synthesis are given. 相似文献
90.
The Electron Transfer Series [MoIII(bpy)3]n (n=3+, 2+, 1+, 0, 1−), and the Dinuclear Species [{MoIIICl(Mebpy)2}2(μ2‐O)]Cl2 and [{MoIV(tpy.)2}2(μ2‐MoO4)](PF6)2⋅4 MeCN 下载免费PDF全文
Dr. Mei Wang Dr. Thomas Weyhermüller Prof. Karl Wieghardt 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(29):9037-9044
The electronic structures of the five members of the electron transfer series [Mo(bpy)3]n (n=3+, 2+, 1+, 0, 1?) are determined through a combination of techniques: electro‐ and magnetochemistry, UV/Vis and EPR spectroscopies, and X‐ray crystallography. The mono‐ and dication are prepared and isolated as PF6 salts for the first time. It is shown that all species contain a central MoIII ion (4d3). The successive one‐electron reductions/oxidations within the series are all ligand‐based, involving neutral (bpy0), the π‐radical anion (bpy.)1?, and the diamagnetic dianion (bpy2?)2?: [MoIII(bpy0)3]3+ (S=3/2), [MoIII(bpy.)(bpy0)2]2+ (S=1), [MoIII(bpy.)2(bpy0)]1+ (S=1/2), [MoIII(bpy.)3] (S=0), and [MoIII(bpy.)2(bpy2?)]1? (S=1/2). The previously described diamagnetic dication “[MoII(bpy0)3](BF4)2” is proposed to be a diamagnetic dinuclear species [{Mo(bpy)3}2(μ2‐O)](BF4)4. Two new polynuclear complexes are prepared and structurally characterized: [{MoIIICl(Mebpy0)2}2(μ2‐O)]Cl2 and [{MoIV(tpy.)2}2(μ2‐MoVIO4)](PF6)2?4 MeCN. 相似文献