全文获取类型
收费全文 | 5800篇 |
免费 | 125篇 |
国内免费 | 19篇 |
专业分类
化学 | 3696篇 |
晶体学 | 77篇 |
力学 | 231篇 |
数学 | 510篇 |
物理学 | 1430篇 |
出版年
2021年 | 34篇 |
2020年 | 43篇 |
2019年 | 46篇 |
2018年 | 47篇 |
2017年 | 27篇 |
2016年 | 89篇 |
2015年 | 99篇 |
2014年 | 77篇 |
2013年 | 276篇 |
2012年 | 224篇 |
2011年 | 258篇 |
2010年 | 149篇 |
2009年 | 124篇 |
2008年 | 221篇 |
2007年 | 234篇 |
2006年 | 251篇 |
2005年 | 224篇 |
2004年 | 190篇 |
2003年 | 171篇 |
2002年 | 189篇 |
2001年 | 132篇 |
2000年 | 124篇 |
1999年 | 90篇 |
1998年 | 84篇 |
1997年 | 125篇 |
1996年 | 124篇 |
1995年 | 95篇 |
1994年 | 112篇 |
1993年 | 133篇 |
1992年 | 121篇 |
1991年 | 71篇 |
1990年 | 66篇 |
1989年 | 97篇 |
1988年 | 85篇 |
1987年 | 79篇 |
1986年 | 78篇 |
1985年 | 93篇 |
1984年 | 102篇 |
1983年 | 78篇 |
1982年 | 78篇 |
1981年 | 79篇 |
1980年 | 70篇 |
1979年 | 72篇 |
1978年 | 69篇 |
1977年 | 69篇 |
1976年 | 57篇 |
1975年 | 79篇 |
1974年 | 72篇 |
1973年 | 59篇 |
1972年 | 31篇 |
排序方式: 共有5944条查询结果,搜索用时 171 毫秒
271.
R. L. Jones M. J. Elder J. A. Ewen 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4):827-843
Abstract Substituted metallocene ligands containing cyclopentadiene ring fused to either selenophene or benzo[b]tellurophene were prepared following methods previously developed for analogs containing sulfur. 5-Methyl-4,5-dihydro-cyclopenta[b]-selenophen-6-one ( 2 ) and 3-Methyl-3,4-dihydro-benzo[b]cyclopenta[d]telluraphen-2-one ( 7 ) (major isomers) were prepared by polyphosphoric acid catalyzed Friedel-Crafts acylation/Nazarov cyclization of methacrylic acid onto selenophene and respectively benzo[b]tellurophene. Following reduction of the ketone to alcohol, then dehydration, the chalcogen-containing cyclopentadiene olefins were prepared. The olefins were deprotonated with n-butyllithium followed by either bridging with dichlorodimethylsilane, deprotonation and metallation, or deprotonation and direct metallation. In this manner, isomeric mixtures of –rac/-meso dimethylsilanediylbis(η5-5-Methyl-cyclopenta[b]selenophen-yl)zirconium dichloride ( 5 ) Bis(η5?2-methyl-cyclopenta[b][1]benzotelluraphen-yl)zirconium dichloride ( 9 ) were prepared and characterized. Complexes formed active olefin polymerization catalyst when activated with methylalumoxane. Polymerization results with ethylene and propylene are included. 相似文献
272.
Jordi Contreras J. Idris Jones 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-2):67-68
Abstract Various sulfides are easily oxidized selectively to the corresponding sulfoxides in quantitative yields by iodosylarene (ArIO) catalyzed by metalloporphyrin (TPPM(III)Cl (M [dbnd] Fe, Mn)). The oxidation system is demonstrated to be a possible model for monooxygenase in the study of the stereochemistry of these sulfoxides. Metalloporphyrin-iodosylarene can initially equilibrate with the oxometalloporphyrin (TPPM(V)=O·Cl) formed in situ. The initial process may involve one-electron transfer from the sulfide to the intermediate oxometalloporphyrin followed by coupling of two resulting charged products, and/or nucleophilic attack of sulfide on oxometalloporphyrin oxygen. The overall reactions are depicted by paths with different electron demands from the results of Hammett plots. 相似文献
273.
Stephen Brand John Milton Martin F. Jones Christopher M. Rayner 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):367-368
A new method for the synthesis of cyclic and acyclic optically active S,O-acetals is reported, along with subsequent stereocontrolled transformations, culminating in a synthesis of the important antiviral nucleoside Lamivudine. 相似文献
274.
Lisa M. Jones Hao Zhang Weidong Cui Sandeep Kumar Justin B. Sperry James A. Carroll Michael L. Gross 《Journal of the American Society for Mass Spectrometry》2013,24(6):835-845
As therapeutic monoclonal antibodies (mAbs) become a major focus in biotechnology and a source of the next-generation drugs, new analytical methods or combination methods are needed for monitoring changes in higher order structure and effects of post-translational modifications. The complexity of these molecules and their vulnerability to structural change provide a serious challenge. We describe here the use of complementary mass spectrometry methods that not only characterize mutant mAbs but also may provide a general framework for characterizing higher order structure of other protein therapeutics and biosimilars. To frame the challenge, we selected members of the IgG2 subclass that have distinct disulfide isomeric structures as a model to evaluate an overall approach that uses ion mobility, top-down MS sequencing, and protein footprinting in the form of fast photochemical oxidation of proteins (FPOP). These three methods are rapid, sensitive, respond to subtle changes in conformation of Cys?→?Ser mutants of an IgG2, each representing a single disulfide isoform, and may be used in series to probe higher order structure. The outcome suggests that this approach of using various methods in combination can assist the development and quality control of protein therapeutics. 相似文献
275.
Dr. Alastair J. J. Lennox Prof. Dr. Guy C. Lloyd‐Jones 《Angewandte Chemie (International ed. in English)》2013,52(29):7362-7370
The Suzuki–Miyaura coupling is one of the few transition‐metal‐catalyzed C? C bond‐forming reactions that have been used in applications ranging from discovery chemistry to manufacturing processes. Although coupling proceeds through the generic three‐stage ‘oxidative addition, transmetalation, reductive elimination’ sequence, there are a number of features that differentiate the Suzuki–Miyaura process from other transition‐metal‐catalyzed cross‐couplings. Most of these features are centered around, or are a consequence of, activation of the boron reagent for transmetalation through one or both of two distinct pathways. This review focuses on the evidence that has been presented for this ‘fork in the trail′, and the potential to apply such mechanistic insight to the design of reaction conditions. 相似文献
276.
Jeffrey J. Sweterlitsch Perminus Mungara Roger W. Jones Jay-lin Jane 《International Journal of Polymer Analysis and Characterization》2013,18(6):464-468
Soy protein–based polymers offer promising performance properties, but their characteristics are sensitively dependent on production conditions, so on-line monitoring could help provide the needed control during production. Mid-infrared spectroscopy combined with partial least squares offer the needed analysis, but the opacity of many materials in the mid-infrared range limits its conventional application. Transient infrared spectroscopy is a method of acquiring mid-infrared spectra from moving streams in real time that avoids the opacity problem. We apply transient infrared spectroscopy to a polymer of soy protein and polyisoprene-graft-maleic anhydride–modified natural rubber during its compounding extrusion to measure tensile strength and Young's modulus. 相似文献
277.
L-Histidine benzyl ester di-p-toluenesulphonate is, contrary to a previous report, easily prepared by a very simple direct esterification procedure. 相似文献
278.
Many antibiotics of the anthracycline family1 possess structures for which 3-methoxyphthalic anhydride (1) would be a suitable synthon2 in a total synthesis. A classic synthesis of (1) has employed 3-nitrophthalic acid3 but, in our hands, proceeded in low overall yield. Recent syntheses have utilized the Diels Alder reaction of dimethyl acetylenedicarboxylate (2) with 1-methoxy-1,3-cyclohexadiene,4 1,4-diacetoxybutadiene,5 2-acetoxyfuran,6 2-methoxyfuran7 or furan8 to obtain 3-hydroxyphthalic acid or a derivative. We now wish to report a convenient Diels-Alder synthesis of (1) from 3-methoxy-2-pyrone (3). 相似文献
279.
280.
Dr. Dongpeng Yan Dr. Dejan‐Krešimir Bučar Dr. Amit Delori Bhavnita Patel Dr. Gareth O. Lloyd Prof. William Jones Prof. Xue Duan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(25):8213-8219
Multi‐component organic nanocrystals that are comprised of two or more supramolecular building blocks can be used to extend the design and assembly scope of solid molecular materials. Herein, we report the use of ultrasonication to prepare halogen‐bonded stilbene‐based nano‐cocrystals that exhibit different photoemission properties, including one‐ and two‐phonon emission and fluorescence lifetimes, relative to those of macrodimensional crystals. The structural transformation from nano‐cocrystals into nanocrystals upon heating results in a luminescence red‐shift from greenish blue to yellow. The temperature‐dependent ratiometric luminescence may allow such nano‐cocrystals to be used as fluorescent sensors and thermosensitive materials. 相似文献