全文获取类型
收费全文 | 295篇 |
免费 | 3篇 |
专业分类
化学 | 163篇 |
晶体学 | 1篇 |
力学 | 14篇 |
数学 | 47篇 |
物理学 | 73篇 |
出版年
2023年 | 2篇 |
2022年 | 8篇 |
2021年 | 11篇 |
2020年 | 7篇 |
2018年 | 4篇 |
2017年 | 2篇 |
2016年 | 10篇 |
2015年 | 8篇 |
2014年 | 2篇 |
2013年 | 13篇 |
2012年 | 12篇 |
2011年 | 17篇 |
2010年 | 7篇 |
2009年 | 9篇 |
2008年 | 6篇 |
2007年 | 17篇 |
2006年 | 16篇 |
2005年 | 16篇 |
2004年 | 16篇 |
2003年 | 15篇 |
2002年 | 7篇 |
2001年 | 8篇 |
2000年 | 9篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 4篇 |
1992年 | 3篇 |
1990年 | 2篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1983年 | 5篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1970年 | 2篇 |
1942年 | 2篇 |
1941年 | 1篇 |
1939年 | 1篇 |
1936年 | 1篇 |
1935年 | 1篇 |
1931年 | 1篇 |
1909年 | 1篇 |
排序方式: 共有298条查询结果,搜索用时 0 毫秒
71.
ATPase activity and phosphorylation by [gamma-32P] ATP of isolated plasma membrane of alveolar macorphages are stimulated in a parallel fashion by physiologic concentrations of Ca2+, with half-maximal activating effect of this ion at (3--7) X 10(-7) M. For various membrane preparations, a direct proportionality exists between Ca2+-dependent ATPase activity and amount of 32P incorporated. Labeling of membrane attains the steady-state level by 10 sec at 0 degrees C, and is rapidly reversed by adenosine diphosphate (ADP), K+ decreases the amount of membrane-bound 32P, mainly by enhancing the rate of dephosphorylation of the 32P-intermediate. Hydroxylamine causes a release of about 90% of 32P bound to the membrane, thus indicating that the 32P-intermediate contains an acyl-phosphate bond. When the labeled plasma membrane is solubilized and electrophoresed on acrylamide gels in the presence of sodium dodecyl sulphate, the radioactivity appears to be largely associated with a single protein fraction of 132,000 +/- 2,000 aarent molecular weight. These features of the macrophage Ca2+-ATPase suggest that the enzyme activity might be part of a surface-localized Ca1+-extrusion system, participating in the regulation of Ca2+-dependent activities of the macrophage. 相似文献
72.
The ligand exchange rate constants for the reactions [Pt(bph)(SR2)2] + 2*SR2 --> [Pt(bph)(*SR2)2] + 2SR2 (bph = 2,2'-biphenyl dianion; R = Me and Et) and cis-[PtPh2(SMe2)2] + 2*SMe2 --> cis-[PtPh2(*SMe2)2] + 2SMe2 have been determined in CDCl3 as a function of ligand concentration and temperature, by 1H NMR isotopic labeling and magnetization transfer experiments. The rates of exchange show no dependence on ligand concentration and the kinetics are characterized by largely positive entropies of activation. The kinetics of displacement of the thioethers from [Pt(bph)(SR2)2] with the dinitrogen ligands 2,2'-bipyridine and 1,10-phenanthroline (N-N) to yield [Pt(bph)(N-N)], carried out in the presence of sufficient excess of thioether and N-N to ensure pseudo-first-order conditions, follow a nonlinear rate law k(obsd) = a[N-N]/(b[SR2] + [N-N]). The general pattern of behavior indicates that the rate-determining step for substitution is the dissociation of a thioether ligand and the formation of a three-coordinated [Pt(bph)(SR2)] intermediate. The value of the parameter a, which measures the rate of ligand dissociation, is constant and independent of the nature of N-N, and it is in reasonable agreement with the value of the rate of ligand exchange at the same temperature. Theoretical ab initio calculations were performed for both [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], and for their three-coordinated derivatives upon the loss of one SMe2 ligand. The latter optimize in a T-shaped structure. Calculations were performed in the HF approximation (LANL2DZ basis set) and refined by introducing the correlation terms (Becke3LYP model). The activation enthalpies from the optimized vacuum-phase geometries are 52.3 and 72.2 kJ moll compared to the experimental values in CDCl3 solution, 80 +/- 1 and 93 +/- 1 kJ mol(-1) for [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], respectively. The electrostatic potential maps of both parent compounds show a remarkable concentration of negative charge over the platinum atom which exerts a repulsion force on an axially incoming nucleophile. On the other hand, the strength of the organic carbanions trans to the leaving group and the stabilization of the T-shaped intermediate in the singlet ground state may also rationalize the preference for the dissociative mechanism. All of the kinetic and theoretical data support the latter hypothesis and indicate, in particular, that dissociation from the complex containing the planar 2,2'-biphenyl dianion is easier than from its analogue with single aryl ligands. Electron back-donation from filled d orbitals of the metal to empty pi* of the in-plane cyclometalated rings is weak or absent and is not operative in promoting an associative mode of activation. 相似文献
73.
Antonino Corsaro Ugo Chiacchio Venerando Pistarà Roberto Romeo Felicia D'Andrea Emanuele Attolino 《Tetrahedron》2004,60(17):3787-3795
Completely protected 4-deoxy-α-L-threo-hex-4-enopyranosides 1c,d undergo the dichlorocarbene addition affording exclusively diastereomeric adducts 5c,d with the cyclopropane ring anti to the C-3 alkyloxy substituent, while the reaction with 3-unprotected derivatives 1a,b affords a mixture of syn and anti derivatives. Under the Simmons-Smith cyclopropanation adducts 2a-d with a syn stereochemistry are obtained. Starting from 5b, the cyclopropanated sugar 3b is obtained by reduction with LiAlH4, thus the two diastereomers 2b and 3b can be stereoselectively obtained through the two different pathways. For a useful comparison, 4-deoxy-β-L-threo-hex-4-enopyranoside 1e was also subjected to the above two cyclopropanation methods affording the expected cycloadduct 2e and a diastereomeric mixture of dichlorocycloadducts 4e and 5e (4e/5e=2.8:1). 相似文献
74.
Romeo Rizzi 《Journal of Graph Theory》1999,32(1):1-15
An r-graph is any graph that can be obtained as a conic combination of its own 1-factors. An r-graph G(V, E) is said to be indecomposable when its edge set E cannot be partitioned as E = E1 ∪ E2 so that Gi(V, Ei) is an ri-graph for i = 1, 2 and, for some r1, r2. We give an indecomposable r-graph for every integer r ≥ 4. This answers a question raised in [Seymour, Proc London Math Soc 38 (1979, 423–460], and has interesting consequences for the Schrijver System of the T-cut polyhedron to be given in [Rizzi, 1997, to appear]. A graph in which every two 1-factors intersect is said to be poorly matchable. Every poorly matchable r-graph is indecomposable. We show that for every r ≥ 4 that “being indecomposable” does not imply “being poorly matchable.” Next we give a poorly matchable r-graph for every r ≥ 4. The article provides counterexamples to some conjectures of Seymour. © 1999 John Wiley & Sons, Inc. J Graph Theory 32: 1–15, 1999 相似文献
75.
Romeo Rizzi 《Journal of Graph Theory》2000,33(3):138-139
We give a short proof of the following basic fact in matching theory: in a bipartite graph the maximum size of a matching equals the minimum size of a node cover. © John Wiley & Sons, Inc. J Graph Theory 33: 138–139, 2000 相似文献
76.
A new high-order accurate staggered semi-implicit space-time discontinuous Galerkin(DG)method is presented for the simulation of viscous incompressible flows on... 相似文献
77.
Gabriele Albertin Stefano Antoniutti Sonia Pizzol 《Journal of organometallic chemistry》2004,689(9):1639-1647
Chloro-complexes [OsCl(N-N)P3]BPh4 (1, 2) [N-N=2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen); P=P(OEt)3 and PPh(OEt)2] were prepared by allowing OsCl4(N-N) to react with zinc dust in the presence of phosphites. Treatment of the chloro-complexes 1, 2 with NaBH4 yielded, in the case of bpy, the hydride [OsH(bpy)P3]BPh4 (4) derivatives. Mono-phosphite [OsCl(bpy)2P]BPh4 (3) complexes were also prepared by reacting the [OsCl2(bpy)2]Cl compound with zinc dust in the presence of phosphite. Protonation reaction of the hydride [OsH(bpy)P3]+ (4) cations with Brønsted acid was studied and led to thermally unstable (above 0 °C) dihydrogen [Os(η2-H2)(bpy)P3]2+ (4*) derivatives. The presence of the H2 ligand is supported by variable-temperature NMR spectra and T1min measurements. Carbonyl [Os(CO)(bpy){P(OEt)3}3](BPh4)2 (5), nitrile [Os(CH3CN)(bpy){P(OEt)3}3](BPh4)2 (6), and hydrazine [Os(bpy)(NH2NH2){P(OEt)3}3](BPh4)2 (7) complexes were prepared by substituting the H2 ligand in the η2-H2 (4*) derivatives. Aryldiazene complex [Os(C6H5NNH)(bpy){P(OEt)3}3](BPh4)2 (8) was also obtained by allowing the hydride [OsH(bpy)P3]BPh4 to react with phenyldiazonium cation. 相似文献
78.
M. Terheggen H. Heinrich G. Kostorz D. Baetzner A. Romeo A.N. Tiwari 《Interface Science》2004,12(2-3):259-266
CdTe layers have been grown on CdS layers to produce thin-film photovoltaic devices. Because of the large lattice mismatch of roughly 10%, CdTe and CdS can only be joined at the expense of a high density of misfit dislocations. Additionally, after deposition the CdTe layer contains submicrometer sized, [111] oriented, columnar grains with a high density of stacking faults and microtwins resulting in a poor electrical performance of the p-n junction. The performance of these cells can be improved by depositing a CdCl2 layer on the CdTe absorber layer and subsequent annealing of the stack in air. This treatment induces interdiffusion of S and Te across the interface, which results in a better lattice match. During this anneal, CdTe is subject to grain growth, recovery and recrystallization. In samples annealed for different durations after different amounts of CdCl2 were applied, grain growth is completed during the first minutes of annealing. Subsequent diffusion of Cl is detected along the CdTe grain boundaries. The presence of Cl enhances the recrystallization of the CdTe layer, starting from the CdTe surface, while recovery of the CdTe layer, mostly by the reduction of microtwins, takes place at the interface. The simultaneous occurrence of recrystallization and recovery leads to a preferred alignment of grain boundaries in CdTe parallel to the interface. Electron beam induced current measurements show the detrimental effect of these grain boundaries on the charge carrier collection efficiency of the cell. Based on these results, a modified growth procedure is proposed. 相似文献
79.
Electroluminescence emission has been obtained in the compound CuGaS2 with In and Au contacts. Such emission is mainly ascribed to the injection of minority carriers from the In contact. The electroluminescent spectrum presents some peaks at energies lower than gap energy showing that electroluminescence is mainly due to a recombination into levels localized in the gap. A weak emission at an energy near the gap has also been observed. 相似文献
80.
G. Romeo M. C. Aversa P. Giannetto M. G. Vigorita P. Ficarra 《Magnetic resonance in chemistry : MRC》1979,12(10):593-597
The sterochemistry of some 1,3-dihydro-2H-1,4-benxodiazepin-2-ones, employed as psychotherapeutic agents, is deduced by proton magnetic resonance using the paramagnetic shift reagent Eu(fod)3. the lanthanide induced shifts are computer simulated on the basis of the geometric parameters of the protons in different model structures, having intermediate conformations between a cycloheptadiene- and a cyclohepatatriene-like system. N-Desmethyldiazepam shows a conformational equilibrium between two pseudoboat forms, while the 1-alkyl substituted derivatives exist, at room temperature, in olny one boat cycloheptatriene-like conformation. 相似文献