全文获取类型
收费全文 | 1810篇 |
免费 | 78篇 |
国内免费 | 5篇 |
专业分类
化学 | 1323篇 |
晶体学 | 2篇 |
力学 | 10篇 |
数学 | 271篇 |
物理学 | 287篇 |
出版年
2023年 | 10篇 |
2022年 | 25篇 |
2021年 | 23篇 |
2020年 | 31篇 |
2019年 | 37篇 |
2018年 | 15篇 |
2017年 | 20篇 |
2016年 | 43篇 |
2015年 | 66篇 |
2014年 | 58篇 |
2013年 | 79篇 |
2012年 | 104篇 |
2011年 | 110篇 |
2010年 | 79篇 |
2009年 | 65篇 |
2008年 | 91篇 |
2007年 | 110篇 |
2006年 | 88篇 |
2005年 | 104篇 |
2004年 | 64篇 |
2003年 | 65篇 |
2002年 | 71篇 |
2001年 | 47篇 |
2000年 | 33篇 |
1999年 | 25篇 |
1998年 | 33篇 |
1997年 | 25篇 |
1996年 | 28篇 |
1995年 | 18篇 |
1994年 | 31篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 18篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 5篇 |
1987年 | 22篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 21篇 |
1981年 | 16篇 |
1980年 | 13篇 |
1979年 | 11篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1973年 | 6篇 |
1970年 | 6篇 |
1926年 | 4篇 |
排序方式: 共有1893条查询结果,搜索用时 15 毫秒
101.
102.
[reaction: see text] Enantioselective Ir-catalyzed intramolecular allylic aminations and etherifications are described. Up to 97% ee was achieved using catalysts prepared by in situ activation of mixtures of phosphorus amidites and [Ir(COD)Cl]2. Sequential aminations of bis-allylic carbonates, involving an inter- followed by an intramolecular reaction, gave trans-N-benzyl-2,5-divinylpyrrolidine and trans-N-benzyl-2,6-divinylpiperidine with > or = 99% ee. New phosphorus amidites as well as improved conditions for intermolecular aminations are reported. 相似文献
103.
Hagen A Steiner M Raschke MB Lienau C Hertel T Qian H Meixner AJ Hartschuh A 《Physical review letters》2005,95(19):197401
The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode. 相似文献
104.
Schreiner E Nicolini C Ludolph B Ravindra R Otte N Kohlmeyer A Rousseau R Winter R Marx D 《Physical review letters》2004,92(14):148101
The temperature-dependent behavior of a solvated oligopeptide, GVG(VPGVG), is investigated. Spectroscopic measurements, thermodynamic measurements, and molecular dynamics simulations find that this elastinlike octapeptide behaves as a two-state system that undergoes an "inverse temperature" folding transition and reentrant unfolding close to the boiling point of water. A molecular picture of these processes is presented, emphasizing changes in the dynamics of hydrogen bonding at the protein/water interface and peptide backbone librational entropy. 相似文献
105.
106.
A short and efficient synthesis of aristocularines, involving the sequential construction of phosphorylated 4-alkoxyisoindolinones, Horner-type reaction, and ultimate cyclization by diaryl ether coupling, is disclosed. The success of this new conceptual approach is demonstrated by the total synthesis of the aristocularine alkaloid aristoyagonine. 相似文献
107.
Zapf A Jackstell R Rataboul F Riermeier T Monsees A Fuhrmann C Shaikh N Dingerdissen U Beller M 《Chemical communications (Cambridge, England)》2004,(1):38-39
A practical synthesis of a novel class of phosphine ligands, phosphino substituted N-aryl pyrroles (PAP ligands), has been developed. These ligands are applied in the palladium-catalyzed coupling of a variety of aryl and heteroaryl chlorides with phenylboronic acid showing exceedingly high turnover numbers at mild reaction temperatures and even at room temperature. 相似文献
108.
Identification and analysis of the core biosynthetic machinery of tubulysin, a potent cytotoxin with potential anticancer activity 总被引:4,自引:0,他引:4
Myxobacteria are well known for their biosynthetic potential, especially for the production of cytotoxic compounds with potential anticancer activities. The tubulysins are currently in preclinical development. They are produced in very low quantities, and genetic manipulation of producing strains has never been accomplished. We report the development of a mariner-based transposon mutagenesis system for Angiococcus disciformis An d48. Extracts from a library of 1200 mutants were analyzed for the presence of tubulysin by a microscopic cell nucleus fragmentation bioassay. The transposition sites of four tubulysin-negative mutants were identified by vector recovery, which led to the identification and the sequencing of the corresponding core biosynthetic gene locus. Sequence analysis of more than 80,000 bp reveals an unusual multimodular hybrid polyketide synthase/peptide synthetase assembly line with a variety of unprecedented features. 相似文献
109.
Millisecond pulsars (MSPs) probably achieve their fast rotation by mass transfer from their companion stars in low-mass x-ray binaries (LMXBs). The lack of MSPs and LMXBs rotating near breakup has been attributed to the accretion torque being balanced, at fast rotation, by gravitational radiation, perhaps caused by an unstable oscillation mode. It has been argued that internal dissipation involving hyperons may cause LMXBs to evolve into a quasisteady state, with nearly constant rotation rate, temperature, and mode amplitude. We show that MSPs descending from these LMXBs spend a long time in a similar state, as extremely steady sources of gravitational waves and thermal x rays, while they spin down due to gravitational radiation and the standard magnetic torque. Observed MSP braking torques already place meaningful constraints on this scenario. 相似文献
110.
Szimtenings M Olt S Haase A 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,161(1):70-76
An NMR flow quantification technique applicable to metabolite flow in plants is presented. It combines flow sensitive magnetization preparation with slice selective spectroscopy. Flow encoded NMR spectroscopy is described to quantify, for the first time, flow velocities of metabolites in plants non-invasively. Flow sensitivity is introduced by magnetization preparation based on a stimulated echo experiment, prior to slice selective spectroscopy. For flow quantification eight different flow-weighted spectra are collected. With this flow preparation very slow flow velocities down to 0.1mm/s can be detected and small amounts of flowing metabolites can be observed despite the large background signal of stationary and flowing water. Important sequence optimization steps include appropriate choice of experimental parameters used for flow encoding as well as complete balancing of eddy currents from the flow encoding gradients. The method was validated in phantom experiments and applied in vivo. Examples of quantitative flow measurements of water and metabolites in phantoms and plants are provided to demonstrate the reliability and the performance of flow encoded spectroscopy. 相似文献