全文获取类型
收费全文 | 4789篇 |
免费 | 133篇 |
国内免费 | 32篇 |
专业分类
化学 | 3105篇 |
晶体学 | 41篇 |
力学 | 93篇 |
数学 | 905篇 |
物理学 | 810篇 |
出版年
2023年 | 43篇 |
2022年 | 73篇 |
2021年 | 129篇 |
2020年 | 110篇 |
2019年 | 129篇 |
2018年 | 146篇 |
2017年 | 116篇 |
2016年 | 172篇 |
2015年 | 130篇 |
2014年 | 135篇 |
2013年 | 283篇 |
2012年 | 261篇 |
2011年 | 345篇 |
2010年 | 210篇 |
2009年 | 210篇 |
2008年 | 259篇 |
2007年 | 249篇 |
2006年 | 259篇 |
2005年 | 224篇 |
2004年 | 191篇 |
2003年 | 149篇 |
2002年 | 128篇 |
2001年 | 80篇 |
2000年 | 55篇 |
1999年 | 46篇 |
1998年 | 48篇 |
1997年 | 37篇 |
1996年 | 34篇 |
1995年 | 25篇 |
1994年 | 33篇 |
1993年 | 36篇 |
1992年 | 23篇 |
1991年 | 39篇 |
1990年 | 22篇 |
1989年 | 22篇 |
1987年 | 18篇 |
1986年 | 27篇 |
1985年 | 28篇 |
1984年 | 33篇 |
1983年 | 26篇 |
1982年 | 24篇 |
1981年 | 30篇 |
1980年 | 26篇 |
1979年 | 26篇 |
1978年 | 26篇 |
1977年 | 29篇 |
1976年 | 16篇 |
1975年 | 23篇 |
1974年 | 20篇 |
1955年 | 15篇 |
排序方式: 共有4954条查询结果,搜索用时 345 毫秒
41.
Joachim GarricJean-Michel Léger Axelle GrelardMasakazu Ohkita Ivan Huc 《Tetrahedron letters》2003,44(7):1421-1424
X-Ray crystallography and NMR show a strong preference for trans conformers of N′-phenyl or N′-(2-pyridyl) 2-pyridinecarboxylic acid hydrazides, stabilized by an NHNpyr. intramolecular hydrogen bond both in the solid state and in solution. This allows us to extrapolate that oligomers of this unit should adopt extended linear conformations. 相似文献
42.
Silylstannanes can be regioselectively added across terminal alkynes in a quantitative fashion in the presence of a palladium(0) catalyst immobilised in the [bmim]PF6 ionic liquid which can be recycled without loss of activity. 相似文献
43.
Rudovsky J Hermann P Botta M Aime S Lukes I 《Chemical communications (Cambridge, England)》2005,(18):2390-2392
A marked increase of relaxivity has been observed upon rigidifying the internal frame of Gd-containing PAMAM dendrimers: the effect has been attained by either protonation of the dendrimer or by forming supramolecular adducts with cationic polyaminoacids. 相似文献
44.
Reaction of [Nb(NMe(2))(5)] with 10 equiv of 2,6-Me(2)C(6)H(3)SH in toluene results in the formation of red crystals of [Nb(SC(6)H(3)Me(2)-2,6)(5)]. Crystal structure analysis of [Nb(SC(6)H(3)Me(2)-2,6)(5)] showed that the niobium center adopts a distorted trigonal bipyramidal geometry. Niobium disulfide, NbS(2), has been successfully prepared via a thio "sol-gel" process using [Nb(SC(6)H(3)Me(2)-2,6)(5)] as the metal source. In contrast, vapor phase thin-film studies revealed that [Nb(SC(6)H(3)Me(2)-2,6)(5)] functions as a single-source precursor to NbS films. 相似文献
45.
A. A. Altynnikov O. V. Kalinkina V. F. Anufrienko A. A. Shubin S. V. Vossel V. V. Zakharov N. N. Bulgakov 《Reaction Kinetics and Catalysis Letters》1994,52(2):261-267
The ESR spectrum of exchange-coupled Ti3+ ions of gge has been observed in dissolution products of metallic titanium in HCl, evidencing the formation of chloride clusters of mutually ordered Ti3+ ions in dz2-ground state in distorted tetrahedral coordination. The cooperative Jahn-Teller effect (CJTE) defines orbital ordering of the d1 of Ti3+ ions in the cluster. 相似文献
46.
Ivan Gutman Sabina Gojak Niko Radulović Boris Furtula 《Monatshefte für Chemie / Chemical Monthly》2006,137(3):277-284
Summary. In a recent work it was demonstrated that in linear hexagonal chains the distribution of π-electrons into rings (as computed
by means of the Randić–Balaban method) is uniform, irrespective of the nature of the terminal fragments. We now establish that an analogous, yet somewhat
more complex, uniformity in the π-electron distribution exists also in double linear hexagonal chains, as well as in some
other benzenoid systems. 相似文献
47.
James D. Korp Ivan Bernal Steven F. Watkins F. R. Fronczek 《Journal of heterocyclic chemistry》1982,19(3):459-462
The molecular structure of 10-(1,3-Dithiolan-2-ylidene)-10H-indeno[1,2-f] -1,2,3,4,5-pentathiepin S7C12H89 has been determined by single crystal X-ray diffraction. The crystals are triclinic, space group PI, with two molecules in a unit cell of dimensions α = 8.931(2), b = 9.387(2), c = 10.175(2) Å, α = 75.73(2), β = 73.35(1), γ = 64.37(2)°. The structure was solved by direct methods, and refined to a final R value of 3.3% for 1925 independent reflections. The molecule consists of an indene core with a nearly co-planar dithiolane and a fused pentasulfide chain. The S5C2 ring is in the chair configuration, with an average S? S distance of 2.052 Å. There is no variation of bond lengths as is frequently seen in multi-sulfur chains. The indenone ring shows no evidence of any delocalization, while the dithiolane ring is disordered at the two methylene positions. No attempt was made to resolve the disorder, since it is frequently seen and has been thoroughly investigated previously. 相似文献
48.
G. I. Borodkin I. R. Elanov R. V. Andreev M. M. Shakirov V. G. Shubin 《Russian Journal of Organic Chemistry》2006,42(3):406-411
The reaction of [2.2]paracyclophane with nitrosonium tetrachloroaluminate was studied by NMR (1H, 13C) spectroscopy using deuterium isotopic perturbation technique. The resulting cationic complexes containing one and two nitrosonium ions are involved in fast (on the NMR time scale) interconversion. Quantum-chemical calculations performed on the DFT level (using triple zeta basis set) indicate the higher stability of 2η single-charged π-complexes relative to σ complexes corresponding to the addition of NO+ ion at the ipso and ortho positions. The formation of the single-charged π-complex is energetically more favorable, compared to the double-charged π-complex. The affinity of NO+ for [2.2]paracyclophane is much greater than for p-xylene, presumably due to stacking interaction between the aromatic rings in the π-complex. 相似文献
49.
An inductively coupled plasma atomic emission spectrometry (ICP-AES) procedure has been developed and examined for the determination of boron content (0.01 up to about 2% B) in boron-alloyed steels such as POLDI ATA BOR (65% Fe, 19%Cr, 12% Ni, 1.5% Mn, 1% B), POLDI ATA BOR EXTRA (62% Fe, 18% Cr, 13% Ni, 2.5% Mo, 1.5% Mn, 1% B) and POLDI ATA BOR-R (75% Fe, 18% Cr, 3.5% Ti, 1.8% B). The steel sample is dissolved with a mixture of hydrochloric, nitric, sulfuric and phosphoric acids in a quartz vessel. Borides of alloyed metals, especially of iron and chromium, are quantitatively decomposed. The presence of phosphoric acid in a sample solution reduces the volatility of boric acid with water vapour.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria 相似文献
50.
Iliya Bl. Rashkov Ivan Gitsov 《Journal of polymer science. Part A, Polymer chemistry》1984,22(4):905-910
The anionic polymerization of δ-valerolactone initiated by KC24 in xylene or tetrahydrofuran was investigated. The influence of the polymerization conditions on the amount and composition of the oligomers fraction and high polymers formed was tested. Experiments were done with a potassium mirror and benzophenone-potassium as initiators of the polymerization to check the influence of the layered structure of the initiator. It was found that the amount of the oligomers formed under comparable conditions increases in the order KC24 (9%), potassium mirror (22%), benzophenone-potassium (56%). The highest yields of poly(δ-valerolactone) (over 90%) and highest intrinsic viscosities (1.0 dL/g and more) were achieved by the KC24-initiated polymerizations in xylene. 相似文献