全文获取类型
收费全文 | 4996篇 |
免费 | 118篇 |
国内免费 | 6篇 |
专业分类
化学 | 3940篇 |
晶体学 | 9篇 |
力学 | 44篇 |
数学 | 652篇 |
物理学 | 475篇 |
出版年
2019年 | 42篇 |
2018年 | 37篇 |
2017年 | 29篇 |
2016年 | 104篇 |
2015年 | 65篇 |
2014年 | 99篇 |
2013年 | 144篇 |
2012年 | 147篇 |
2011年 | 155篇 |
2010年 | 131篇 |
2009年 | 120篇 |
2008年 | 164篇 |
2007年 | 159篇 |
2006年 | 182篇 |
2005年 | 201篇 |
2004年 | 170篇 |
2003年 | 138篇 |
2002年 | 182篇 |
2001年 | 110篇 |
2000年 | 109篇 |
1999年 | 105篇 |
1998年 | 85篇 |
1997年 | 95篇 |
1996年 | 96篇 |
1995年 | 78篇 |
1994年 | 89篇 |
1993年 | 91篇 |
1992年 | 92篇 |
1991年 | 86篇 |
1990年 | 67篇 |
1989年 | 89篇 |
1988年 | 87篇 |
1987年 | 84篇 |
1986年 | 71篇 |
1985年 | 98篇 |
1984年 | 111篇 |
1983年 | 64篇 |
1982年 | 89篇 |
1981年 | 112篇 |
1980年 | 84篇 |
1979年 | 104篇 |
1978年 | 70篇 |
1977年 | 68篇 |
1976年 | 59篇 |
1975年 | 79篇 |
1974年 | 52篇 |
1973年 | 73篇 |
1972年 | 51篇 |
1971年 | 43篇 |
1970年 | 52篇 |
排序方式: 共有5120条查询结果,搜索用时 15 毫秒
41.
Dieter Jakubke 《Fresenius' Journal of Analytical Chemistry》1958,160(1):6-10
Zusammenfassung Auf der Grundlage der Schwerlöslichkeit des Kupferchelats des 5-Oxy-4-aza-phenanthrens in wäßriger Lösung wurde eine quantitative Kupferbestimmung ausgearbeitet. Es konnte festgestellt werden, daß Chrom(III)-, Eisen(III)-, Aluminium(III)-, Wismut(III)-, Blei(II)- und Zink(II)-Ionen nicht stören.Meinem hochverehrten Lehrer, Herrn Prof. Dr. W. Langenbeck, möchte ich an dieser Stelle für seine rege Anteilnahme und seine Unterstützung bei der Durchführung dieser Arbeit meinen herzlichen Dank aussprechen. 相似文献
42.
The Li/K-derivative 6 is used to synthesize the title compounds ( 3a and 4a ) in enantiomerically pure form from (?)-(S)-propylene epoxide. The C,C bond forming key step leading to the hydroxyketone 7 is followed by cyclization (→, 8 ), Beckmann cleavage (→ 9b ) and hydrolysis to 3a (recently isolated from civet). Base treatment of 3a opens the ring ( 10 ) to give the hydroxyacid 1 which is cyclized to the macrolide 4a . The synthetic usefulness of the highly nucleophilic doubly reduced dienone system 6 as d5-reagent (see synthons 2) is thus demonstirated. 相似文献
43.
Gunter Silber Dieter Flockerzi Rajendra Singh Varma Ramamurthy Charubala Eugen Uhlmann Wolfgang Pfleiderer 《Helvetica chimica acta》1981,64(5):1704-1716
Nucleotides. XV. Synthesis and Properties of 2′O-t-Butyldimethylsilyl-5′-O-monomethoxytritylribonucleoside-3′-phosphotriesters, Starting Materials for Oligonucleotide Syntheses The syntheses of two types of fully blocked ribonucleoside 3′-phosphotriesters 6–14 have been achieved in excellent yields from 2′-O-t-butyldimethylsilyl-5′-O-monomethoxytrityl-ribonucleosides 1–5 by phosphorylation with 2-chloro- and 2,5-dichlorophenylphosphorodichloridate respectively and subsequent treatment by cyanoethanol to 6 , 8 , 10 , 12 and 14 and by p-nitrophenylethanol to 7 , 9 , 11 and 13 . These phosphotriesters are valuable starting materials for oligonucleotide syntheses due to the fact that the corresponding phosphotriesters 15–23 with free HO? C(5′) could be obtained by detritylation and the 3′-phosphodiester triethylammonium salts 24–32 by deblocking of the cyanoethyl and the 2,5-dichlorophenyl group respectively. All newly synthesized compounds have been characterized by UV.-and NMR.-spectra as well as C, H, N elementary analyses. 相似文献
44.
Werner W.K.R. Mederski Manfred BaumgarthMartina Germann Dieter KuxThomas Weitzel 《Tetrahedron letters》2003,44(10):2133-2136
An efficient one-step synthesis of 4-nitro-N-aryl substituted glutarimides, succinimides and maleimides in polyphosphoric acid is described together with the subsequent reduction to the corresponding anilines. The scope and limitation of this cyclocondensation are presented. 相似文献
45.
Zusammenfassung Die Systeme Co
x
Mg
1-x
TiO3 (I) und Co
x
Cd
1-x
TiO3 (II) wurden röntgenographisch und spektralphotometrisch untersucht. I stellt eine lückenlose Mischkristallreihe dar. Mit wachsendemx nehmen bei I die Gitterkonstantena
0undc
0geringfügig zu und die drei Hauptabsorptionsbanden verschieben sich ein wenig in Richtung IR. In II liegt keine lückenlose Mischkristallreihe vor. Sowohl röntgenographisch als auch spektralphotometrisch wurde für die an CoTiO3 gesättigte CdTiO3-Phase die Zusammensetzung Co0,3Cd0,7TiO3 (II a) und für die an CdTiO3 gesättigte CoTiO3-Phase die Zusammensetzung Co0,8Cd0,2TiO3 (II b) gefunden.Beim Übergang III (x=0,1) wird eine beträchtliche IR-Verschiebung beobachtet. Die IR-Verschiebung ist die Folge der Gitteraufweitung bei der Substitution von Mg2+ durch Cd2+. Hierbei nimmt derRacah-Parameter geringfügig zu infolge Verminderung der covalenten Anteile der Bindung Co-O.
Mit 4 Abbildungen
11. Mitt.:D. Reinen, Mh. Chem.96, 730 (1965). 相似文献
The systems Co x Mg 1-x TiO3 (I) and Co x Cd 1-x TiO3 (II) had been investigated spectrophotometrically and by X-ray diffraction. I represents mixed crystals without any miscibility gap. An increase ofx causes a small enhancement of the lattice unitsa 0andc 0of I and the three main absorption bands will be shifted slightly towards IR. II are mixed crystals with a broad miscibility gap. Spectrophotometrically and by X-ray diffraction it could be found that the CdTiO3-phase saturated with CoTiO3 and the CoTiO3-phase saturated with CdTiO3 have the compositions Co0,3Cd0,7TiO3 (II a) and Co0,8Cd0,2TiO3 (II b), respectively. The transition III (x=0,1) produces a considerable shift of the main absorption bands towards IR as a consequence of the expansion of the lattice caused by the substitution of Cd2+ for Mg2+. This shift is accompanied by a small increase of theRacah-Parameter owing to a decrease of covalency of the bonding Co-O.
Mit 4 Abbildungen
11. Mitt.:D. Reinen, Mh. Chem.96, 730 (1965). 相似文献
46.
Hafnium tetrachloride reacts with the tris(trimethylsilyl)silyl potassium tmen adduct (1) to form a [tris(trimethylsilyl)silyl]trichlorohafnium tmen complex (2); reaction of 2 with 2,6-dimethylphenylisonitrile leads to insertion into the silicon hafnium bond (4). 相似文献
47.
Transition Metal Phosphido Complexes. VIII. X-Ray Diffraction Studies of Transition Metal Phosphorus Four- and Six-Membered Ring Complexes. Structures of [(CO)4MnPH2]2, [(CO)4MnPH2]3, and [cpNiPH2]3 [(CO)4MnPH2]2 1 crystallizes triclinic in the space group P1 with a = 680.4 pm, b = 706.4 pm, c = 919.1 pm, α 110.5°, β = 91.92°, γ 115.65°, and Z = 1 formula unit. The molecule exhibits a centrosymmetrical structure. The bond angles within the planar four-membered (Mn? P)2-ring are 76.1° at the Mn atoms and 103.9° at the P atoms, respectively. The average Mn? P bond distance is found to be 235.1 pm. [(CO)4MnPH2]3 2 crystallizes monoclinic in the space group P2/n with a = 905.2 pm, b = 974.8 pm, c = 1264.2 pm, β = 109.1°, and Z = 2 formula units. The framework of the six-membered (Mn? P)3-ring can be described as having a twist boat conformation. The average endocyclic bond angles are with 89.1° at the Mn atoms and 130.1° at the P atoms, respectively, largely widened compared to 1 . The average Mn? P bond distance, which is found to be 238.5 pm, is also slightly increased compared to 1 . [cpNiPH2]3 3 crystallizes rhombohedral in the space group R3. The cell constants (hexagonal setting) are a = b = 1686.1 pm, c = 561.1 pm and Z = 3 formula units. The six-membered (Ni? P)3-ring exhibits a chair conformation. The endocyclic bond angles are with 92.3° at the Ni atoms and 124.3° at the P atoms, respectively, comparable with those of the six-membered ring compound 2 . The Ni? P bond distance is found to be 215.2 pm. The eyclopentadienyl ligands are disordered and have been refined as rigid groups. 相似文献
48.
For the first time, the nuclear magnetic resonance (NMR) spin-spin coupling mechanism is decomposed into one-electron and electron-electron interaction contributions to demonstrate that spin-information transport between different orbitals is not exclusively an electron-exchange phenomenon. This is done using coupled perturbed density-functional theory in conjunction with the recently developed J-OC-PSP [=J-OC-OC-PSP: Decomposition of J into orbital contributions using orbital currents and partial spin polarization)] method. One-orbital contributions comprise Ramsey response and self-exchange effects and the two-orbital contributions describe first-order delocalization and steric exchange. The two-orbital effects can be characterized as external orbital, echo, and spin transport contributions. A relationship of these electronic effects to zeroth-order orbital theory is demonstrated and their sign and magnitude predicted using simple models and graphical representations of first order orbitals. In the case of methane the two NMR spin-spin coupling constants result from totally different Fermi contact coupling mechanisms. (1)J(C,H) is the result of the Ramsey response and the self-exchange of the bond orbital diminished by external first-order delocalization external one-orbital effects whereas (2)J(H,H) spin-spin coupling is almost exclusively mitigated by a two-orbital steric exchange effect. From this analysis, a series of prediction can be made how geometrical deformations, electron lone pairs, and substituent effects lead to a change in the values of (1)J(C,H) and (2)J(H,H), respectively, for hydrocarbons. 相似文献
49.
Günther Kraft Dieter Lindenberger Heinz Beck 《Fresenius' Journal of Analytical Chemistry》1976,282(2):119-121
Zusammenfassung Die Intensität des flammenabsorptionsspektrometrischen Cr-Signals hängt in einer sehr komplexen Weise von der Wertigkeit des Chroms und der Flammentemperatur ab. Ferner ist ein ausgeprägter Kalium-Einfluß vorhanden. Alle diese Schwierigkeiten können am besten dadurch eliminiert werden, daß in der Lachgasflamme gearbeitet und die Analysenlösung mit einer höheren K-Konzentration gepuffert wird.
Effect of valency on the determination of chromium by flame absorption spectrometry
The intensity of the chromium signal in AAS depends in a very complex manner strongly on the valency of the Cr and the flame temperature. It is furthermore influenced by potassium, if present. The best way to overcome all these interferences is to use the nitrous oxide flame and to buffer the solution with a higher concentration of potassium.相似文献
50.
Dieter Sellmann 《Angewandte Chemie (International ed. in English)》1974,13(10):639-649
Compared to the large number of CO complexes, N2 complexes are still rare. In certain cases they may be formed from N2 gas and metal compounds under physiological conditions, and are therefore frequently considered as possible intermediates in N2 assimilation. However, despite numerous attempts it has not yet been possible to reduce the N2 ligand of a fully characterized N2 complex to NH3. This negative evidence recently prompted the discoverers of the first dinitrogen complex, the [Ru(NH3)5N2]2⊕ ion isolated in 1965, to express doubt whether such complexes really do play a part in the enzymatic reduction of N2. The latest findings nevertheless incline toward a somewhat more optimistic view. Thus, the mild partial reduction of the N2 ligand in metal complexes with inert gas configuration, hitherto considered nonreducible, justifies the hope that a suitable system permitting the catalytic reduction of molecular nitrogen under normal conditions will one day be found. Even apart from any technical potential, the N2 complexes constitute an interesting chapter of modern inorganic chemistry. 相似文献