全文获取类型
收费全文 | 1637篇 |
免费 | 79篇 |
国内免费 | 8篇 |
专业分类
化学 | 1265篇 |
晶体学 | 5篇 |
力学 | 9篇 |
数学 | 141篇 |
物理学 | 304篇 |
出版年
2023年 | 29篇 |
2022年 | 10篇 |
2021年 | 49篇 |
2020年 | 62篇 |
2019年 | 59篇 |
2018年 | 44篇 |
2017年 | 25篇 |
2016年 | 56篇 |
2015年 | 35篇 |
2014年 | 54篇 |
2013年 | 69篇 |
2012年 | 149篇 |
2011年 | 175篇 |
2010年 | 66篇 |
2009年 | 51篇 |
2008年 | 108篇 |
2007年 | 111篇 |
2006年 | 103篇 |
2005年 | 100篇 |
2004年 | 79篇 |
2003年 | 58篇 |
2002年 | 48篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 8篇 |
1998年 | 11篇 |
1997年 | 12篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 9篇 |
1993年 | 8篇 |
1992年 | 3篇 |
1991年 | 8篇 |
1990年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1983年 | 2篇 |
1982年 | 7篇 |
1981年 | 14篇 |
1980年 | 9篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 7篇 |
1971年 | 3篇 |
1896年 | 1篇 |
排序方式: 共有1724条查询结果,搜索用时 640 毫秒
991.
Synthesis and Biological Evaluation of the Antimicrobial Natural Product Lipoxazolidinone A 下载免费PDF全文
Jonathan J. Mills Kaylib R. Robinson Troy E. Zehnder Prof. Joshua G. Pierce 《Angewandte Chemie (International ed. in English)》2018,57(28):8682-8686
Natural products have historically been a major source of antibiotics and therefore novel scaffolds are constantly of interest. The lipoxazolidinone family of marine natural products, with an unusual 4‐oxazolidinone heterocycle at their core, represents a new scaffold for antimicrobial discovery; however, questions regarding their mechanism of action and high lipophilicity have likely slowed follow‐up studies. Herein, we report the first synthesis of lipoxazolidinone A, 15 structural analogues to explore its active pharmacophore, and initial resistance and mechanism of action studies. These results suggest that 4‐oxazolidinones are valuable scaffolds for antimicrobial development and reveal simplified lead compounds for further optimization. 相似文献
992.
Dr. Le Wang Douglas W. Agnew Xiao Yu Prof. Dr. Joshua S. Figueroa Prof. Dr. Seth M. Cohen 《Angewandte Chemie (International ed. in English)》2018,57(2):511-515
The development of catalysts capable of fast, robust C?H bond amination under mild conditions is an unrealized goal despite substantial progress in the field of C?H activation in recent years. A Mn‐based metal–organic framework (CPF‐5) is described that promotes the direct amination of C?H bonds with exceptional activity. CPF‐5 is capable of functionalizing C?H bonds in an intermolecular fashion with unrivaled catalytic stability producing >105 turnovers. 相似文献
993.
The development and optimization of bismuth(III) triflate-promoted regioselective 1,4- and 1,6-additions of electron-rich heteroarenes to cyclic, β,β-disubstituted enones and dienones is described. Additions of a range of heteroarenes, including furan, thiophene, pyrrole, and indole nucleophiles, to cyclic, β,β-disubstituted enones occur to form all-carbon quaternary centers in up to 88% yield. In addition, regioselective 1,6-additions of electron-rich heteroarenes to 3-vinyl-2-cyclohexenone occur to produce a variety of δ-heteroarylated, β,β-disubstituted enones in up to 93% yield. The high 1,6-selectivity for these reactions is attributed to the increased steric bulk at the β-position relative to the δ-position, and no competing 1,4-conjugate addition is observed. 相似文献
994.
Witkowski FX Leon LJ Penkoske PA Clark RB Spano ML Ditto WL Giles WR 《Chaos (Woodbury, N.Y.)》1998,8(1):94-102
The measurement of cardiac transmembrane potential changes with voltage sensitive dyes is in increasing use. Detection of these very small fluorescent alterations using large multiplexed arrays, such as charge coupled device (CCD) cameras at high sampling rates, has proven challenging and usually requires significant averaging to improve the signal-to-noise ratio. To minimize the damage of living tissue stained with voltage sensitive dyes, excitation photon exposure must be limited, with the inevitable consequence of diminishing the fluorescence that is generated. State-of-the-art high frame rate CCD cameras have read noise levels in the 5-10 e(-) rms range, which is at least two orders of magnitude above that required to detect voltage sensitive dye alterations at individual pixels corresponding to 1 mm(2) heart regions illuminated with levels of 100 mW/cm(2) at frame rates approaching 1000 frames/sec. Image intensification is thus required prior to photon quantification. We report here the development of such a data acquisition system using commercially available hardware. Additionally, in the past ten years, a mathematical theory of multiresolution has been developed, and new building blocks called wavelets, allow a signal to be observed at different resolutions. Wavelet analysis also makes possible a new method of extricating signals from noise. We have incorporated spatially adaptive filters based on wavelet denoising of individual pixels to significantly reduce the multiple noise sources present in the acquired data. (c) 1998 American Institute of Physics. 相似文献
995.
Joshua N. Goldberg 《Foundations of Physics》1985,15(4):439-450
The Hamiltonian for the Einstein equations is constructed on a outgoing null cone with the help of the usual null tetrad. The resulting null surface constraints are shown to be second class in the terminology of Dirac. These second class constraints are eliminated by use of the starring procedure of Bergmann and Komar. 相似文献
996.
[reaction: see text] The Z and E nitrones 38 and 39 from condensation of aldehyde 20 with hydroxylamine 36 underwent intramolecular dipolar cycloaddition to give the substituted 1-aza-7-oxobicyclo[2.2.1] heptanes 40 and 41 in a ratio of 2:1, respectively. Reductive N-O bond cleavage of 40 followed by carbonylation gave cyclic urea 47 in which inversion of the secondary alcohol was effected via an oxidation-reduction sequence. After conversion of the p-bromobenzyl ether 50 to azide 54, activation of the cyclic urea as its O-methylisourea and reduction of the azide led to spontaneous cyclization to afford the tricyclic nucleus 59 of cylindrospermopsin. Global deprotection, including hydrolysis of the 2,4-dimethyoxypyrimidine appendage to a uracil, and then monosulfation of the resultant diol 60 afforded a substance identical with natural (-)-7-epicylindrospermopsin (1). The asymmetric synthesis of (-)-7-epicylindrospermopsin defines its absolute configuration as 7S,8R,10S,12S,13R,14S. 相似文献
997.
The role of carrier mobility in holographic recording in LiNbO3 总被引:1,自引:0,他引:1
We investigate the role of carrier mobility in holographic recording in LiNbO3 crystals. Both normal holographic recording (single wavelength, single trap) and two-center recording are considered, and
the differences between the performances of the two methods are explained. We show that increasing mobility by using stoichiometric
crystals or by doping with Mg does not improve sensitivity considerably, but does reduce M/# by at least one order of magnitude.
Received: 22 February 2001 / Revised version: 5 March 2001 / Published online: 27 April 2001 相似文献
998.
We prove comparability invariance results for three classes of ordered sets: bounded tolerance orders (equivalent to parallelogram orders), unit bitolerance orders (equivalent to point-core bitolerance orders) and unit tolerance orders (equivalent to 50% tolerance orders). Each proof uses a different technique and relies on the alternate characterization. 相似文献
999.
Adedamola A. Opalade Joshua D. Parham Victor W. Day Timothy A. Jackson 《Chemical science》2021,12(38):12564
While alkylperoxomanganese(iii) (MnIII–OOR) intermediates are proposed in the catalytic cycles of several manganese-dependent enzymes, their characterization has proven to be a challenge due to their inherent thermal instability. Fundamental understanding of the structural and electronic properties of these important intermediates is limited to a series of complexes with thiolate-containing N4S− ligands. These well-characterized complexes are metastable yet unreactive in the direct oxidation of organic substrates. Because the stability and reactivity of MnIII–OOR complexes are likely to be highly dependent on their local coordination environment, we have generated two new MnIII–OOR complexes using a new amide-containing N5− ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H6Medpaq) ligand, we generated the [MnIII(OOtBu)(6Medpaq)]OTf and [MnIII(OOCm)(6Medpaq)]OTf complexes through reaction of their MnII or MnIII precursors with tBuOOH and CmOOH, respectively. Both of the new MnIII–OOR complexes are stable at room-temperature (t1/2 = 5 and 8 days, respectively, at 298 K in CH3CN) and capable of reacting directly with phosphine substrates. The stability of these MnIII–OOR adducts render them amenable for detailed characterization, including by X-ray crystallography for [MnIII(OOCm)(6Medpaq)]OTf. Thermal decomposition studies support a decay pathway of the MnIII–OOR complexes by O–O bond homolysis. In contrast, direct reaction of [MnIII(OOCm)(6Medpaq)]+ with PPh3 provided evidence of heterolytic cleavage of the O–O bond. These studies reveal that both the stability and chemical reactivity of MnIII–OOR complexes can be tuned by the local coordination sphere.A pair of room-temperature-stable MnIII–alkylperoxo complexes were characterized and shown to oxidize PPh3. Thermal decomposition studies provide evidence of both homolysis and heterolysis of the MnIII–alkylperoxo O–O bond. 相似文献
1000.