全文获取类型
收费全文 | 529篇 |
免费 | 22篇 |
国内免费 | 1篇 |
专业分类
化学 | 437篇 |
晶体学 | 2篇 |
力学 | 5篇 |
数学 | 52篇 |
物理学 | 56篇 |
出版年
2023年 | 4篇 |
2022年 | 7篇 |
2021年 | 9篇 |
2020年 | 9篇 |
2019年 | 5篇 |
2018年 | 2篇 |
2017年 | 3篇 |
2016年 | 27篇 |
2015年 | 19篇 |
2014年 | 15篇 |
2013年 | 29篇 |
2012年 | 36篇 |
2011年 | 36篇 |
2010年 | 37篇 |
2009年 | 26篇 |
2008年 | 30篇 |
2007年 | 32篇 |
2006年 | 41篇 |
2005年 | 23篇 |
2004年 | 30篇 |
2003年 | 18篇 |
2002年 | 11篇 |
2001年 | 12篇 |
2000年 | 9篇 |
1999年 | 5篇 |
1998年 | 4篇 |
1997年 | 10篇 |
1996年 | 12篇 |
1995年 | 6篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 6篇 |
1991年 | 2篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有552条查询结果,搜索用时 0 毫秒
21.
22.
Bernd Reif Valentin Wittmann Harald Schwalbe Christian Griesinger Karlheinz Wrner Kerstin Jahn-Hofmann Joachim W. Engels Wolfgang Bermel 《Helvetica chimica acta》1997,80(6):1952-1971
1-(2′-Deoxy-2′-fluororibofuranosyl)pyrimidines were synthesized and incorporated into an RNA oligonucleotide to give 5′-r[CfGCf(UfUfCfG)GCfG]-3′ (Cf: short form of C = 2′-deoxy-2′-fluorocytidine; Uf: short form of U = 2′-deoxy-2′-fluorouridine). The oligomer was investigated by means of UV, CD, and NMR spectroscopy to address the question of how F-labels can substitute 13C-labels in the ribose ring. Through-space (NOE) and through-bond (scalar couplings) experiments were performed that make use of the ameliorated chemical-shift dispersion induced by 19F as an alternative heteronucleus. A comparison of the structures of fluorinated vs. unmodified oligomer is given. It turns out that the fluorinated oligonucleotide exists in a 14:3 equilibrium between a hairpin and a duplex conformation, in contrast to the unmodified oligonucleotide which predominantly adopts the hairpin conformation. Furthermore, the fluorinated hairpin structure adopts two distinct conformations that differ in the sugar conformation of the U and C nucleoside units, as detected by the 19F-NMR chemical shifts. The role of the 2′-OH group as stabilizing element in RNA secondary structure is discussed. 相似文献
23.
We will demonstrate how optical tweezers can be combined with a microfluidic system to create a versatile microlaboratory. Cells are moved between reservoirs filled with different media by means of optical tweezers. We show that the cells, on a timescale of a few seconds, can be moved from one reservoir to another without the media being dragged along with them. The system is demonstrated with an experiment where we expose E. coli bacteria to different fluorescent markers. We will also discuss how the system can be used as an advanced cell sorter. It can favorably be used to sort out a small fraction of cells from a large population, in particular when advanced microscopic techniques are required to distinguish various cells. Patterns of channels and reservoirs were generated in a computer and transferred to a mask using either a sophisticated electron beam technique or a standard laser printer. Lithographic methods were applied to create microchannels in rubber silicon (PDMS). Media were transported in the channels using electroosmotic flow. The optical system consisted of a combined confocal and epi-fluorescence microscope, dual optical tweezers and a laser scalpel. 相似文献
24.
Fries G Wolf J Ilg K Walfort B Stalke D Werner H 《Dalton transactions (Cambridge, England : 2003)》2004,(12):1873-1881
A series of unsymmetrical 1,2-bis(phosphino)ethanes R(2)PCH(2)CH(2)PR'(2) and 1-arsino-2-phosphinoethanes R(2)AsCH(2)CH(2)PR'(2) mainly with bulky substituents R and R' were prepared from the cyclic sulfate by stepwise cleavage of the carbon-oxygen bonds by LiPR(2) and LiPR'(2) or LiAsR(2) and LiPR'(2), respectively. Analogously, racemic mixtures of R(2)PCH(2)CH(Me)PPh(2)(R =iPr, Cy ) as well as the enantiomers (R)-, (R)- and (R)-tBu(2)PCH(2)CH(Me)PPh(2)(R)- were obtained from the corresponding unsymmetrical cyclic sulfates and (S)-. On a similar route, the racemates of the 1,3-bis(phosphino)propanes R(2)PCH(2)CH(2)CH(Me)PPh(2)(R =iPr, tBu ), optically pure (R)- and (S,S)-iPr(2)PCH(Me)CH(2)CH(Me)PPh(2)(S,S)- were prepared. The reaction of [[RhCl([small eta](4)-C(8)H(12))](2)] with chelating ligands L-L, where L-L is R(2)PCH(2)P(men)(2)(R =iPr, Ph; men =(1S,2R,5S)-menthyl), Cy(2)AsCH(2)P(men)(2), or (R)-, (R)-, (R)-, (R)- and (S,S)-, in the presence of AgPF(6), gave the complexes [Rh(eta(4)-C(8)H(12))(L-L)]PF(6) which were used as pre-catalysts in the hydrogenation of the methyl ester of alpha-acetamidocinnamic acid (ACM). Depending on L-L, the solvent, the temperature and the pressure of H(2), optical yields of up to 69% ee were achieved. For two of the rhodium complexes, and, the molecular structures were determined by X-ray crystallography. 相似文献
25.
A Combined Experimental and Theoretical Study on the Stereodynamics of Monoaza[5]helicenes: Solvent‐Induced Increase of the Enantiomerization Barrier in 1‐Aza‐[5]helicene 下载免费PDF全文
Prof. Dr. Tullio Caronna Prof. Dr. Andrea Mele Prof. Dr. Antonino Famulari Dr. Daniele Mendola Prof. Dr. Francesca Fontana Dr. Markus Juza Dr. Matthias Kamuf Dr. Kerstin Zawatzky Prof. Dr. Oliver Trapp 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(40):13919-13924
Helicenes and heterohelicenes are attractive compounds with great potential in materials sciences to be used in optoelectronics as ligand backbones in enantioselective catalysis and as chiral sensors. The properties of these materials are related to the stereodynamics of these helical chiral compounds. However, little is known about features controlling stereodynamics in helicenes; in particular, for heterohelicenes the position of the heteroatom could be relevant in this respect. Herein the complete stereodynamic characterization of monoaza[5]helicenes is shown by enantioselective dynamic HPLC and DFT calculations. At variance with previous theoretical calculations, 1‐aza[5]helicene shows a surprisingly high enantiomerization barrier, which is triggered by specific solvent interactions. 相似文献
26.
Patze C Broedner K Rominger F Trapp O Bunz UH 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(49):13720-13725
We demonstrate that aldehyde-substituted donor-acceptor cruciforms [1,4-bis(arylethynyl)-2,5-distyrylbenzenes] are useful dosimeters for primary amines, primary diamines, and secondary amines. The 1,n-diamines are particularly reactive towards this dosimeter and can be detected in less than 100 ppm concentration. Using a single aldehyde-functionalized cruciform in seven different solvents allowed us to discern fourteen different amines by digital photography and statistical evaluation of the response patterns extracted as red, green, blue (RGB) values. 相似文献
27.
28.
Alexander Grechnikov Sergey Nikiforov Kerstin Strupat Alexander Makarov 《Analytical and bioanalytical chemistry》2014,406(13):3019-3023
A surface-assisted laser desorption/ionization (SALDI) source is coupled to the Orbitrap mass analyzer; the instrumental approach is tested for the analysis of rhenium (Re) and osmium (Os) complexes with 8-mercaptoquinoline. Silicon (Si) material obtained by laser treatment of monocrystalline Si is used as SALDI substrate. All studied complexes are detected as radical cations, with no protonated molecules. The comparison of SALDI, matrix-assisted laser desorption/ionization (MALDI), and direct laser desorption/ionization (LDI) on metal plates in the same instrumental setup demonstrated that the detection of the studied complexes using SALDI provides the highest sensitivity. The ability to analyze samples rapidly, high purity of spectra, and good analytical parameters make SALDI coupled to the Orbitrap mass analyzer a potentially powerful tool for the detection of Re and Os complexes and related organic, UV-absorbing compounds. Figure
? 相似文献
29.
Ralph C. Dougherty Derek Horton Kerstin D. Philips Joseph D. Wander 《Journal of mass spectrometry : JMS》1973,7(7):805-816
2-Acetamido-1,3,4,6-tetra-O-acetyl-2-deoxy-α-D-glucopyranose (I), and its analogs specifically mono (trideuterioacetylated) at O-1 (III), at N-2 (II), at O-4 (IV) and at O-6 (V), have been examined by high-resolution mass spectrometry. From the elemental compositions of the fragment ions, the mass-number shifts resulting from deuterium incorporation and analysis of metastable transitions, it has been possible to specify in detail the fragmentation pathways undergone by this molecule. The principal degradations of I proceed by initial rapid decomposition of the molecular ion (whose intensity is insignificant) by three routes: (i) by loss of the C-1 acetoxyl group as a radical to give the glycosyl cation (a), (ii) by loss of the 1-acetyl group as a radical to give an acyclic ion m/e 346 (b) and (iii) by loss of a C-6 fragment and acetic acid derived from the 3-acetate group to give m/e 241 (c). 相似文献
30.