全文获取类型
收费全文 | 4731篇 |
免费 | 124篇 |
国内免费 | 5篇 |
专业分类
化学 | 2852篇 |
晶体学 | 56篇 |
力学 | 63篇 |
数学 | 437篇 |
物理学 | 1090篇 |
无线电 | 362篇 |
出版年
2021年 | 35篇 |
2020年 | 36篇 |
2019年 | 39篇 |
2016年 | 67篇 |
2015年 | 68篇 |
2014年 | 77篇 |
2013年 | 149篇 |
2012年 | 146篇 |
2011年 | 176篇 |
2010年 | 120篇 |
2009年 | 91篇 |
2008年 | 158篇 |
2007年 | 165篇 |
2006年 | 162篇 |
2005年 | 163篇 |
2004年 | 150篇 |
2003年 | 147篇 |
2002年 | 129篇 |
2001年 | 124篇 |
2000年 | 131篇 |
1999年 | 76篇 |
1998年 | 51篇 |
1997年 | 68篇 |
1996年 | 96篇 |
1995年 | 79篇 |
1994年 | 87篇 |
1993年 | 70篇 |
1992年 | 69篇 |
1991年 | 69篇 |
1990年 | 65篇 |
1989年 | 48篇 |
1988年 | 54篇 |
1987年 | 69篇 |
1986年 | 63篇 |
1985年 | 68篇 |
1984年 | 67篇 |
1983年 | 53篇 |
1982年 | 60篇 |
1981年 | 63篇 |
1980年 | 46篇 |
1979年 | 50篇 |
1978年 | 54篇 |
1977年 | 60篇 |
1976年 | 63篇 |
1975年 | 59篇 |
1974年 | 60篇 |
1973年 | 47篇 |
1968年 | 27篇 |
1967年 | 25篇 |
1957年 | 30篇 |
排序方式: 共有4860条查询结果,搜索用时 10 毫秒
811.
[reaction: see text] The enantioselective total synthesis of proansamitocin, a key biosynthetic intermediate of the highly potent antitumor agent ansamitocin P-3, is described which bears a diene-ene RCM as the key macrocyclization step. Feeding of proansamitocin to an AHBA block mutant Actinosynnema pretiosum (HGF073) yielded ansamitocin P-3 as well as dechloroansamitocin P-3, the latter also being formed upon fermentation in the presence of 3-amino-5-methoxybenzoic acid. 相似文献
812.
All higher life forms use oxygen and respiration as their primary energy source. The oxygen comes from water by solar-energy conversion in photosynthetic membranes. In green plants, light absorption in photosystem II (PSII) drives electron-transfer activation of the oxygen-evolving complex (OEC). The mechanism of water oxidation by the OEC has long been a subject of great interest to biologists and chemists. With the availability of new molecular-level protein structures from X-ray crystallography and EXAFS, as well as the accumulated results from numerous experiments and theoretical studies, it is possible to suggest how water may be oxidized at the OEC. An integrated sequence of light-driven reactions that exploit coupled electron-proton transfer (EPT) could be the key to water oxidation. When these reactions are combined with long-range proton transfer (by sequential local proton transfers), it may be possible to view the OEC as an intricate structure that is "wired for protons". 相似文献
813.
Klingele J Prikhod'ko AI Leibeling G Demeshko S Dechert S Meyer F 《Dalton transactions (Cambridge, England : 2003)》2007,(20):2003-2013
The pyrazole-based diamide ligand N,N'-bis(2-pyridylmethyl)pyrazole-3,5-dicarboxamide (H(3)L) has been structurally characterised and successfully employed in the preparation of [2 x 2] grid-type complexes. Thus, the reaction of H(3)L with Cu(ClO(4))2.6H(2)O or Ni(ClO(4))2.6H(2)O in the presence of added base (NaOH) affords the tetranuclear complexes [M(4)(HL(4))].8H(2)O (1: M = Cu, 2: M = Ni). Employment of a mixture of the two metal salts under otherwise identical reaction conditions leads to the formation of the mixed-metal species [Cu(x)Ni(4-x)(HL)(4)].8H(2)O (x相似文献
814.
815.
Thompson DW Fleming CN Myron BD Meyer TJ 《The journal of physical chemistry. B》2007,111(24):6930-6941
The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy=2,2'-bipyridine, py=pyridine, 4-Etpy=4-ethylpyridine, and tpy=2,2':6',2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, lambdaoo, contributes to the energy gap with lambdaoo approximately 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, Eo, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, Eo is temperature dependent above 200 K with partial differentialEo/ partial differentialT=2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of Eo for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with tau approximately 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by approximately 700 cm-1 in PMMA with the dd state at higher energy by DeltaH0 approximately 1000 cm-1. The "rigid medium stabilization effect" for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect. 相似文献
816.
Simone Dill Katharina Gibson Jochen Glaser Sonja Tragl H.‐Jürgen Meyer Prof. Dr. 《无机化学与普通化学杂志》2007,633(2):274-279
Synthesis and Structure of the Basic Alkaline Earth Nitrates Sr2(OH)3NO3 and Ba2(OH)3NO3 Sr2(OH)3NO3 and Ba2(OH)3NO3 were synthesized from mixtures of freshly prepared strontium or barium hydroxides and their corresponding nitrates in evacuated quartz glass ampoules at 420 °C and 360 °C, respectively. Single crystals of Sr2(OH)3NO3 were obtained in a solidified Sr(NO3)2 melt after subsequent heating and cooling cycles in air up to 600 °C. The crystal structure of the strontium compound was refined from single crystal and powder X‐ray data. Sr2(OH)3NO3 crystallizes hexagonally in the space group (No. 189) with Z = 1 and the lattice parameters a = 6.624(2) Å and c = 3.560(1) Å (single crystal data). The powder pattern of Ba2(OH)3NO3 was indexed isotypically to Sr2(OH)3NO3 with the lattice parameters a = 6.9260(1) Å and c = 3.8086(1) Å, and the crystal structure was refined from powder X‐ray data. Alkaline earth ions in the structures are surrounded trigonal‐prismatically by six hydroxide ions. These prisms are sharing their trigonal faces along [001] building up columns. These columns are connected in the ab‐plane by shared edges, and form hexagonal tunnels with the nitrate groups stacked inside. Infrared and thermoanalytical data of Sr2(OH)3NO3 are presented. 相似文献
817.
818.
819.
We report the enantioselective synthesis of pseudolaric acid B (1a), a diterpene acid isolated from the bark of Pseudolarix kaempferi Gordon, which displays interesting antifungal, antifertility, and cytotoxic activity against multidrug resistant cell lines. Our synthesis utilizes a highly efficient metal-catalyzed [5 + 2] vinylcyclopropane-alkyne intramolecular cycloaddition to construct the polyhydroazulene core of the natural product. Elaboration to the tricyclic scaffold of the pseudolaric acids was completed with an intramolecular alkoxycarbonyl radical cyclization to form the quaternary center and a highly diastereoselective cerium acetylide addition to a methyl ketone for introduction of the acid side chain. 相似文献
820.
Hessenauer-Ilicheva N Franke A Meyer D Woggon WD van Eldik R 《Journal of the American Chemical Society》2007,129(41):12473-12479
The use of synthetic iron(III) porphyrins as models for heme-type catalysts in biomimetic cytochrome P450 research has provided valuable information on the nature and reactivity of intermediates produced in the "peroxide shunt" pathway. This article reports spectroscopic detection of reactive intermediates formed in the epoxidation reaction of cis-stilbene with m-chloroperoxybenzoic acid catalyzed by a new mimic of cytochrome P450 with a substituted RSO3- group (1). The application of low-temperature rapid-scan stopped-flow techniques enabled the determination of equilibrium and rate constants for the formation and decay of all intermediates in the catalytic cycle of 1, including the rate constant for the formation (1*+)FeIV=O and for oxygen transfer to the substrate. Noteworthy, the reaction of (1*+)FeIV=O with cis-stilbene leads to an almost complete re-formation (95%) of the starting complex 1. The results show that complex 1 is a valuable catalyst with promising properties for further applications in a biomimetic approach toward mimicking oxygenation reactions of cytochrome P450. 相似文献