全文获取类型
收费全文 | 3313篇 |
免费 | 42篇 |
国内免费 | 3篇 |
专业分类
化学 | 2420篇 |
晶体学 | 11篇 |
力学 | 42篇 |
数学 | 475篇 |
物理学 | 410篇 |
出版年
2018年 | 26篇 |
2017年 | 26篇 |
2016年 | 33篇 |
2015年 | 32篇 |
2014年 | 30篇 |
2013年 | 84篇 |
2012年 | 80篇 |
2011年 | 104篇 |
2010年 | 76篇 |
2009年 | 77篇 |
2008年 | 103篇 |
2007年 | 119篇 |
2006年 | 99篇 |
2005年 | 113篇 |
2004年 | 112篇 |
2003年 | 91篇 |
2002年 | 97篇 |
2001年 | 53篇 |
2000年 | 78篇 |
1999年 | 73篇 |
1998年 | 57篇 |
1997年 | 60篇 |
1996年 | 56篇 |
1995年 | 55篇 |
1994年 | 42篇 |
1993年 | 64篇 |
1992年 | 61篇 |
1991年 | 47篇 |
1990年 | 30篇 |
1989年 | 37篇 |
1988年 | 45篇 |
1987年 | 46篇 |
1986年 | 54篇 |
1985年 | 70篇 |
1984年 | 52篇 |
1983年 | 46篇 |
1982年 | 53篇 |
1981年 | 58篇 |
1980年 | 43篇 |
1979年 | 62篇 |
1978年 | 48篇 |
1977年 | 44篇 |
1976年 | 42篇 |
1975年 | 34篇 |
1974年 | 43篇 |
1973年 | 25篇 |
1972年 | 26篇 |
1971年 | 24篇 |
1970年 | 24篇 |
1934年 | 21篇 |
排序方式: 共有3358条查询结果,搜索用时 15 毫秒
231.
232.
233.
Marko P. Sušnik Michael Schnürch Marko D. Mihovilovic Kurt Mereiter Peter Stanetty 《Monatshefte für Chemie / Chemical Monthly》2009,140(4):423-430
Abstract A short and efficient sequence for the synthesis of a series of 4-(2-amino-5-thiazolyl)-pyrimidine-2-amines was developed.
1-Phenyl-2-(6-pyrimidinyl)-ethanones, obtained via Weinreb’s methodology, were used in a Hantzsch thiazole cyclization reaction,
followed by introduction of the aniline moieties via nucleophilic substitution.
Graphical abstract
相似文献
234.
Rolf Meyer Zu Kcker Gerlinde Frenzen Bernhard Neumüller Kurt Dehnicke Jrg Magull 《无机化学与普通化学杂志》1994,620(3):431-437
Syntheses and Crystal Structures of the Phosphaneimine Complexes MCl2(Me3SiNPMe3)2 with M = Zn and Co, and CoCl2(HNPMe3)2 The molecular complexes MCl2(Me3SiNPMe3)2 (M = Zn, Co) have been prepared by the reaction of the dichlorides of zinc and cobalt with Me3SiNPMe3 in CH3CN and CH2Cl2, respectively, whereas the complex CoCl2(HNPMe3)2 has been prepared by the reaction of CoCl2 with NaF in boiling acetonitrile in the presence of Me3SiNPMe3. All complexes were characterized by IR spectroscopy and by crystal structure determinations. The complexes MCl2(Me3SiNPMe3)2 crystallize isotypically. ZnCl2(Me3SiNPMe3)2: Space group P212121, Z = 4, 2677 observed unique reflections, R = 0.024. Lattice dimensions at ?70°C: a = 1243.6; b = 1319.0; c = 1464.7 pm. CoCl2(Me3SiNPMe3)2: Space group P212121, Z = 4, 3963 observed unique reflections, R = 0,071. Lattice dimensions at ?80°C: a = 1236.3; b = 1317.4; c = 1457.6 pm. CoCl2(HNPMe3)2 · CH2Cl2: Space group Pbca, Z = 8, 1354 observed unique reflections, R = 0.055. Lattice dimensions at ?80°C: a = 1247.3; b = 998.4; c = 2882.4 pm. All complexes have monomeric molecular structures, in which the metal atoms are coordinated in a distorted tetrahedral fashion by the two chlorine atoms and by the nitrogen atoms of the phosphaneimine molecules. 相似文献
235.
Kurt M. Anstreicher 《Mathematical Programming》1997,76(1):245-263
We consider the construction of small step path following algorithms using volumetric, and mixed volumetric-logarithmic, barriers.
We establish quadratic convergence of a volumetric centering measure using pure Newton steps, enabling us to use relatively
standard proof techniques for several subsequently needed results. Using a mixed volumetric-logarithmic barrier we obtain
an O(n
1/4
m
1/4
L) iteration algorithm for linear programs withn variables andm inequality constraints, providing an alternative derivation for results first obtained by Vaidya and Atkinson. In addition,
we show that the same iteration complexity can be attained while holding the work per iteration to O(n
2
m), as opposed to O(nm
2), operations, by avoiding use of the true Hessian of the volumetric barrier. Our analysis also provides a simplified proof
of self-concordancy of the volumetric and mixed volumetric-logarithmic barriers, originally due to Nesterov and Nemirovskii.
This paper was first presented at the 1994 Faculty Research Seminar “Optimization in Theory and Practice”, at the University
of Iowa Center for Advanced Studies. 相似文献
236.
Ryding MJ Ruusuvuori K Andersson PU Zatula AS McGrath MJ Kurtén T Ortega IK Vehkamäki H Uggerud E 《The journal of physical chemistry. A》2012,116(20):4902-4908
Molecular cluster ions H(+)(H(2)O)(n), H(+)(pyridine)(H(2)O)(n), H(+)(pyridine)(2)(H(2)O)(n), and H(+)(NH(3))(pyridine)(H(2)O)(n) (n = 16-27) and their reactions with ammonia have been studied experimentally using a quadrupole-time-of-flight mass spectrometer. Abundance spectra, evaporation spectra, and reaction branching ratios display magic numbers for H(+)(NH(3))(pyridine)(H(2)O)(n) and H(+)(NH(3))(pyridine)(2)(H(2)O)(n) at n = 18, 20, and 27. The reactions between H(+)(pyridine)(m)(H(2)O)(n) and ammonia all seem to involve intracluster proton transfer to ammonia, thus giving clusters of high stability as evident from the loss of several water molecules from the reacting cluster. The pattern of the observed magic numbers suggest that H(+)(NH(3))(pyridine)(H(2)O)(n) have structures consisting of a NH(4)(+)(H(2)O)(n) core with the pyridine molecule hydrogen-bonded to the surface of the core. This is consistent with the results of high-level ab initio calculations of small protonated pyridine/ammonia/water clusters. 相似文献
237.
Copper(I) Clusters with Trifunctional Silyl Amide Ligands. Synthesis and Crystal Structures of [Li(THF)4]2[Cu10{RSi(NPh)3}4] (R = Me,Ph, Vin)
下载免费PDF全文
![点击此处可从《无机化学与普通化学杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Treatment of CuCl with the lithiated silyl amides RSi(NLiPh)3 (R = Me, Ph, Vin) in THF as solvent led to the formation of the novel CuI cluster compounds [Li(THF)4]2[Cu10{RSi(NPh)3}4]. For each of the three compounds the X‐ray crystal structure analysis revealed similar Si4N12Cu10 cores which are based on cubane like Cu8 cores bearing two additional peripheral copper atoms. The copper atoms are coordinated nearly linearly by the μ5‐bridging silyl amide ligands with Cu–N distances in the range of 187.1(3) to 194.5(4) pm and N–Cu–N angles of 171.6(1) to 178.7(1)°. For each of the compounds the structural parameters are very similar which indicates that the structures are barely influenced by the different steric requirements of the organic groups bound to silicon. 相似文献
238.
239.
Different mean field homogenization methods are applied to a short glass fiber reinforced polybutylene terephthalate. The different models' predictions for the anisotropic effective elastic properties are assessed and compared to experimental data from tensile tests. Additionally, the estimation of fiber stresses and the influence of the fiber length distribution is studied. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
240.
Manzano BR Jalón FA Ortiz IM Soriano ML Torre FG Elguero J Maestro MA Mereiter K Claridge TD 《Inorganic chemistry》2008,47(2):413-428
The ligands 4,6-bis(pyrazol-1-yl)pyrimidine (bpzpm), 4,6-bis(3,5-dimethylpyrazol-1-yl)pyrimidine (bpz(*)pm), 4,6-bis(4-methylpyrazol-1-yl)pyrimidine (Mebpzpm), and 3,6-bis(3,5-dimethylpyrazol-1-yl)pyridazine (ppdMe) were synthesized and were made to react with Cu(I) centers in the presence of different counteranions. Different [2 x 2] metallic grids were obtained. With ligands bpzpm, bpz*pm, and Mebpzpm, a new type of grid was obtained where the facing ligands were divergent and two counteranions (BF(4-) or PF(6-)) were hosted in the resulting cavities and exhibit C-H...F and anion...pi interactions in the solid state. The presence of methyl groups on the pyrazolyl rings induced several distortions in the structure. In complexes with the ligand ppdMe, there were found two groups of parallel ligands in the grid, and the cavities generated were smaller. The counteranions were situated outside the grid, and the facing ligands exhibited aromatic pi-pi stacking interactions. Anion-pi interactions involving the pyridazine ring were found. The behavior in solution of the new derivatives with a special emphasis on the cation-anion interactions was studied by UV-vis and NMR spectroscopy. Diffusion NMR experiments performed for some complexes allowed us to conclude that weak cation-anion interactions exist in solution, with the counteranions undergoing fast exchange on the diffusion time scale between the free and ion-paired states. 相似文献