全文获取类型
收费全文 | 12086篇 |
免费 | 400篇 |
国内免费 | 80篇 |
专业分类
化学 | 8269篇 |
晶体学 | 74篇 |
力学 | 364篇 |
数学 | 2162篇 |
物理学 | 1697篇 |
出版年
2023年 | 72篇 |
2022年 | 132篇 |
2021年 | 292篇 |
2020年 | 224篇 |
2019年 | 227篇 |
2018年 | 196篇 |
2017年 | 174篇 |
2016年 | 431篇 |
2015年 | 333篇 |
2014年 | 356篇 |
2013年 | 694篇 |
2012年 | 844篇 |
2011年 | 938篇 |
2010年 | 553篇 |
2009年 | 496篇 |
2008年 | 815篇 |
2007年 | 759篇 |
2006年 | 719篇 |
2005年 | 677篇 |
2004年 | 558篇 |
2003年 | 461篇 |
2002年 | 443篇 |
2001年 | 155篇 |
2000年 | 124篇 |
1999年 | 97篇 |
1998年 | 118篇 |
1997年 | 116篇 |
1996年 | 160篇 |
1995年 | 84篇 |
1994年 | 106篇 |
1993年 | 92篇 |
1992年 | 76篇 |
1991年 | 72篇 |
1990年 | 85篇 |
1989年 | 71篇 |
1988年 | 60篇 |
1987年 | 71篇 |
1986年 | 45篇 |
1985年 | 72篇 |
1984年 | 82篇 |
1983年 | 52篇 |
1982年 | 61篇 |
1981年 | 41篇 |
1980年 | 52篇 |
1979年 | 42篇 |
1978年 | 35篇 |
1977年 | 33篇 |
1976年 | 16篇 |
1974年 | 21篇 |
1973年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Enantioselectivity in the gold(I)-catalyzed aldol reaction with chiral ferrocenylamine ligands is strongly dependent upon both the steric and electronic effects of the substrates. In the reaction of pyridine-2-, 3-, and 4-carbaldehydes with ethyl 2-isocyanoacetate, surprisingly and significantly different enantioselectivities were observed in the formation of the cis- and trans-dihydro-oxazoles that must be due to electronic rather than steric effects. The first example of double stereodifferentiation in the gold(l)-catalyzed aldol reaction is reported. 相似文献
122.
The interaction of aryldiazonium ions with some Schiff-base complexes of cobalt and ruthenium have been studied. With cobalt, one-electron oxidation of [Co(II)Salen] occurred; with [Co(I)Salen] the corresponding Co(III)-aryl complexes were isolated. In the case of ruthenium oxidation also occurs, [Ru(Salen)(CO)py] gave the corresponding monocation. The results, especially for ruthenium, are in contrast to the stabilisation of both nitrosyl and aryldiazonium adducts in analogous porphyrin complexes. 相似文献
123.
Miguel A. Vzquez Segura Josefa Donoso Francisco Muoz INS Fernandez
de
Pirola Francisco Garcia Blanco Gerardo R. Echevarria 《Photochemistry and photobiology》1993,57(Z1):923-928
Abstract— Absorption and fluorescence spectra of 5′-deoxypyridoxal (DPL) in various pure solvents and mixtures were recorded both at room temperature and over the range10–65°C. The areas under the absorption bands were analyzed to obtain the mole fraction (fN, fz) of two tautomers (the zwitterionic, Z, and neutral, N, forms) in the ground state. The following spectral parameters were determined from the fluorescence spectra: Stokes shift (Δv), fluorescence quantum yield of the neutral form (QN), fluorescence ratio of the neutral to the zwitterionic form (øN/øZ) and the rate constant of tautomerization (k1) from Z to N in the excited state. Some of these parameters (fN, Δv, QN, k1) were found to depend on the proton donor character of the solvent, whereas others (øN/øZ) depended on its dipole moment. Thus, the absorption and fluorescence spectra of DPL allow one to obtain information on the polarity and the concentration of –OH groups on its environment. 相似文献
124.
Site-directed photochemical disruption of the actin cytoskeleton by actin-binding Rose Bengal-conjugates 总被引:1,自引:0,他引:1
Conlon KA Rosenquist T Berrios M 《Journal of photochemistry and photobiology. B, Biology》2002,68(2-3):140-146
The in situ light-induced, non-enzymatic digestion of cytoskeletal actin by a xanthene dye conjugated to heavy meromyosin, anti-actin antibodies and/or anti-myosin antibodies is reported. The dye Rose Bengal was conjugated to either anti-actin antibodies, anti-myosin antibodies or heavy meromyosin. Under our experimental conditions, visible light induced the non-enzymatic breakdown of cytoskeletal actin when mammalian tissue culture cells were probed either with Rose Bengal-conjugated anti-actin and/or anti-myosin antibodies. Similar results were obtained when tissue culture cells were probed with Rose Bengal-conjugated heavy meromyosin before irradiation with visible light. The in situ photochemical reaction depended on the presence of actin-binding Rose Bengal-conjugates. 相似文献
125.
Very simple and selective methods are presented to determine the amino acid tryptophan (Trp) and the biogenic amine tryptamine (Tryp), both compounds with an indole-type molecular structure by the methodology named Heavy Atom Induced-Room Temperature Phosphorescence (HAI-RTP) which constitutes the first time that HAI-RTP has been used to detect compounds with non-naphthalenic structures in their molecules. Different variables affecting the phosphorescence signal (heavy atom perturber and sodium sulfite concentration) were carefully studied. The analytical curves give a linear dynamic range of 15-100 ng ml(-1) and a detection limit of 4 ng ml(-1) for Trp and 94-400 ng ml(-1) and 28 ng ml(-1) for Tryp. The methods have been successfully applied to the analysis of complex food matrices such as the presence of tryptophan in yoghurt and tryptamine in bottled beer. A single alkaline hydrolysis to release Trp from yoghurt proteins and two methods for extracting Tryp from beer samples are proposed and optimised. A total Trp content of 374 mg of Trp per kg of yoghurt was quantified by the standard addition method of calibration and a recovery of 90% was obtained for 250 ng ml(-1) of Tryp in spiked non-alcoholic beer samples. 相似文献
126.
Antonio García Martínez Santiago de la Moya Cerero Beatriz Lora Maroto 《Tetrahedron》2005,61(12):3055-3064
An empiric first approach to the knowledge about the structural factors influencing the catalytic behavior of conformationally flexible δ-amino-alcohol-based ligands, for the enantioselective addition of dialkylzincs to prochiral carbonyl groups, has been applied using the 1-(2-aminoethyl)norbornan-2-ol moiety as the model chiral system, and the asymmetrically catalyzed addition of diethylzinc to benzaldehyde as the test reaction. For this purpose, a selected small library of seven norbornane-based chiral ligands, bearing well-defined structural variations to allow a comparative study, that is, variation of the relative configuration and steric hindrance at the C(2), C(3) and/or C(7) norbornane positions, has been synthesized and probed in the mentioned test reaction. The experimental results obtained have been rationalized empirically using diastereomeric Noyori-like transition states, demonstrating that the conformational flexibility of the δ-amino-alcohol ligands, contrary to the more studied and rigid β-amino-alcohols, plays a crucial role on the catalytic behavior of such ligands (stereochemical sense and degree of the stereodifferentiation in the asymmetric process), which makes such structural factors, important for the improved design of new related chiral catalysts. In this sense, a robust crude empirical model for the prediction of the catalytic behavior of such δ-amino-alcohol-based ligands is proposed. 相似文献
127.
Ana?C.?F.?RibeiroEmail author Victor?M.?M.?Lobo Artur?J.?M.?Valente Eduarda?F.?G.?Azevedo M.??da?G.?Miguel H.?D.?Burrows 《Colloid and polymer science》2004,283(3):277-283
Differential mutual diffusion coefficients of n-alkyltrimethylammonium bromides [CH3(CH2)n–1N(CH3)3Br, CnTAB] (n=10, 12, 14, 16) have been measured in aqueous solutions at 298.15 K using a conductimetric cell and an automatic apparatus to follow diffusion. The cell is based on an open-ended capillary, and the technique follows the diffusion process by measuring the resistance of a solution inside the capillaries at various times. The electrical conductances of those solutions have also been measured to calculate the critical micellar concentration (cmc). Thermodynamic analysis of the data suggests that the free ion concentration decreases at concentrations above the cmc, in agreement with theoretical predictions. The obtained values of the micellization parameters were used to model the mutual diffusion coefficients of CnTAB aqueous solutions. 相似文献
128.
Triolo A Russina O Hardacre C Nieuwenhuyzen M Gonzalez MA Grimm H 《The journal of physical chemistry. B》2005,109(46):22061-22066
A detailed investigation on the nature of the relaxation processes occurring in a typical room temperature ionic liquid (RTIL), namely, 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF(6)]), is reported. The study was conducted using both elastic and inelastic neutron scattering over a wide temperature range from 10 to 400 K, accessing the dynamic features of both the liquid and glassy amorphous states. In this study, the inelastic fixed energy scan technique has been applied for the first time to this class of materials. Using this technique, the existence of two relaxation processes below the glass transition and a further diffusive process occurring above the glass-liquid transition are observed. The low temperature processes are associated with methyl group rotation and butyl chain relaxation in the glassy state and have been modeled in terms of two Debye-like, Arrhenius activated processes. The high temperature process has been modeled in terms of a Kohlraush-Williams-Watts relaxation, with a distinct Vogel-Fulcher-Tamman temperature dependence. These results provide novel information that will be useful in rationalizing the observed structural and dynamical behavior of RTILs in the amorphous state. 相似文献
129.
Karla J. Moreno Antonio F. Fuentes Javier García-Barriocanal Jacobo Santamaría 《Journal of solid state chemistry》2006,179(3):928-934
Different compositions in a solid solution of general formula Dy2(Ti1−yZry)2O7, showing high oxygen ion conductivity, have been successfully prepared at room temperature via mechanochemical synthesis. Stoichiometric mixtures of the constituent oxides were dry milled together in a planetary ball mill by using zirconia vials and balls. Chemical changes in the powder mixtures as a function of composition and milling time were followed by X-ray diffraction and revealed that, in all cases and after milling for 19 h, the powder mixtures consisted of a single phase. Electrical properties were measured on sintered pellets as a function of frequency, temperature and zirconium content, revealing an increase in conductivity of more than one order of magnitude for y?0.4, which, as observed in the similar Y2(Ti1−yZry)2O7, has been related with the onset of disordering of the anion sublattice. Despite increasing structural disorder with increasing Zr content, conductivity remains almost constant for y>0.6, reaching a maximum value of ∼5×10−3 for Dy2Zr2O7 at 900 °C. 相似文献
130.
Alexandra M. Antonio Kyle J. Korman Glenn P. A. Yap Eric D. Bloch 《Chemical science》2020,11(46):12540
Diverse strategies for the preparation of mixed-metal three-dimensional porous solids abound, although many of them lend themselves only moderate levels of tunability. Herein, we report the design and synthesis of surface functionalized permanently microporous coordination cages and their use in the isolation of mixed metal solids. Judicious alkoxide-based ligand functionalization was utilized to tune the solubility of starting copper(ii)-based cages and their resulting compatibility with the mixed-cage approach described here. We further prepared a family of isostructural molybdenum(ii) cages for a subset of the ligands. The preparation of mixed-metal cage solids proceeds under facile conditions where solutions of parent cages are mixed and product phases isolated. A suite of spectroscopic and characterization tools confirm the starting cages are intact in the amorphous product. Finally, we show that utilization of precise ligand functional groups can be used to prepare mixed cage solids that can be easily and cleanly separated into their constituent components through simple solvent washing or solvent extraction techniques.Surface-functionalized porous coordination cages can be used to create homogeneous mixed-cage alloys with high levels of tunability and processability. 相似文献