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971.
Solution-phase mixture synthesis has efficiency advantages and favorable reaction kinetics. Applications of this technique, however, have been discouraged by the difficulty in obtaining individual, pure final products by using conventional separation and identification processes. Introduced here is a new strategy for mixture synthesis that addresses the separation and identification problems. Members of a series of organic substrates are paired with a series of fluorous tags of different chain lengths. The tagged starting materials are then mixed and taken through a multistep reaction process. Fluorous chromatography is used to demix the tagged product mixtures on the basis of the fluorine content of the tags to provide the individual pure components of the mixture, which are detagged to release the final products. The utility of fluorous mixture synthesis is demonstrated by the preparation of a 560-membered library of analogues of the natural product mappicine. A seven-component mixture is carried through a four-step mixture synthesis (two one-pot and two parallel steps) to incorporate two additional points of diversity onto the tetracyclic core. Methods for analysis and purification of the intermediates are established for the quality control of the mixture synthesis. 相似文献
972.
Ehresmann JO Wang W Herreros B Luigi DP Venkatraman TN Song W Nicholas JB Haw JF 《Journal of the American Chemical Society》2002,124(36):10868-10874
Assessing the degree of proton transfer from a Br?nsted acid site to one or more adsorbed bases is central to arguments regarding the strength of zeolites and other solid acids. In this regard certain solid-state NMR measurements have been fruitful; for example, some (13)C, (15)N, or (31)P resonances of adsorbed bases are sensitive to protonation, and the (1)H chemical shift of the Br?nsted site itself reflects hydrogen bonding. We modeled theoretically the structures of adsorption complexes of several bases on zeolite HZSM-5, calculated the quadrupole coupling constants (Q(cc)) and asymmetry parameters (eta) for aluminum in these complexes and then in turn simulated the central transitions of their (27)Al MAS NMR spectra. The theoretical line width decreased monotonically with the degree of proton transfer, reflecting structural relaxation around aluminum as the proton was transferred to a base. We verified this experimentally for a series of adsorbed bases by way of single-pulse MAS and triple quantum MQMAS (27)Al NMR. The combined theoretical and experimental approach described here provides a strategy by which (27)Al data can be applied to resolve disputed interpretations of proton transfer based on other evidence. 相似文献
973.
Quaternization reactions of poly(vinylpyridines) with alkyl halides show retardation in excess of that predicted by the classical second-order kinetics. Based on the classical collision and transition state theories, a kinetic model has been developed to quantify such retardation, in which the overall reaction rate is characterized by a rate constant k0 of the intrinsic reactivity between a pyridyl group and an alkyl halide group, and by a steric hindrance effect parameter α. The latter accounts for the degree to which the rate of collisions of reactants is reduced, or to which the freedom of movement of the reactants in the transition state is restricted as the reaction proceeds. The resulting kinetic expression has been validated using experimental results reported in the literature and those of our own. The functional dependence of k0 and α values on the nature of poly(vinylpyridines) and that of alkyl halides is explained. Other factors affecting k0 and α, including changes in macromolecular dimensions and/or in the distribution of residue environment, quality of solvent, and reaction temperature, are discussed. © 1993 John Wiley & Sons, Inc. 相似文献
974.
Complex [Zn2(bimb)2(mal)2(H2O)2]·4H2O (1) (mal=−OCOCH2COO−) was obtained by reaction of bidentate ligand 4,4′-bis(imidazole-1-ylmethyl)biphenyl (bimb) with zinc(II) salt of malonate, while the reaction of the same metal salt with 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (titmb) gives another novel complex [Zn2(titmb)2(mal)][mal]·12H2O (2). The structures of these complexes were determined by X-ray crystallography. The results revealed that 1 is a cyclic dinuclear complex in which the malonate groups act as terminators and prevent further aggregation, while 2 is a 2D honeycomb network in which each independent 2D sheet contains two sub-layers bridged by the malonate groups and complex 2 also contains free malonate as a counteranion connected to the 2D layer by C-H?O hydrogen bonds. The entirely different structure and topology of complexes 1 and 2, on the one hand, indicates that the nature of organic ligands affected the structures of assemblies greatly, and on the other, reveals the versatility of the malonate which can act as a bridging and/or blocking ligand. 相似文献
975.
976.
Halogenated aromatic compounds exist widely in soil and aqueous environment. The study of their transport and distribution is quite important for pollution control and risk assessment. In the present work the adsorption coefficients of 28 halogenated benzenes, anilines and phenols on yellow-brown soil were measured with batch equilibrium method, and a prediction model was developed through the quantitative structure-property relationship (QSPR) technique. Then the obtained model was tested with Monte Carlo simulation and Jacknife methods. The results indicated that it was robust enough to estimate soil adsorption behaviors for the tested compounds. Based on the obtained model it could be deduced that the adsorption of halogenated aromatics on yellow-brown soil was not a simple partitioning process but involved complicated interactions. 相似文献
977.
聚(甲基丙烯酸甲酯—二甲基二烯丙基氯化铵)的合成及表征 总被引:1,自引:0,他引:1
以水和乙醇作混合溶剂,用逐步滴加甲基丙烯酸甲酯的方法,通过自由基聚合,由溶解性和共聚活性差异较大的两种单体MMA和二甲基二烯丙基氯化铵合成了新型含阳离子共聚物聚,并用IR和^1H-NMR进行了分析表征。该共聚物溶于丙酮和水组成的二元溶剂,并与P(MMA-MAA)有较强的分子复合能力。 相似文献
978.
979.
The spectral evolution of the surface-enhanced Raman scattering (SERS) of 4-tert-butylbenzylmer-captan(4-tBBM)on gold nanopanlcles assembly under laser irradiation is reported.The reIative intensities of typical peaks in the spectrum of 4-tBBM gradually change with irradiation time.Comparison of the rate of spectral changes under several experimental conditions indicates that the surface plasmon resonance(SPR)induced heat in the gold nanoparticles assembly is the origin of the spectraI evolution.During the process of self-assembly,4-tBBM molecules do not form a compact ordered monolayer because of the spatial hindrance of the 4-tert-butyl end group.The heat induced by laser irradiation drives the 4-tBBM molecules to rearrange to a more stable orientation. 相似文献
980.
A room temperature ionic liquid N‐butylpyridinium hexafluorophosphate (BPPF6) was used as a binder to make an ionic liquid modified carbon paste electrode (IL‐CPE), which showed good characteristics such as simple preparation procedure, fast electrochemical response and good conductivity. The electrochemical oxidation of ascorbic acid (AA) on the new IL‐CPE was carefully studied. The oxidation peak potential of AA on the IL‐CPE appeared at 109 mV (vs. SCE), which was about 338 mV decrease of the overpotential compared to that obtained on the traditional carbon paste electrode (CPE) and the oxidation peak current was increased for about four times. The electrochemical parameters of AA on the IL‐CPE were calculated with the charge transfer coefficient (α) and the electrode reaction rate constant (ks) as 0.87 and 0.800 s?1, respectively. Based on the relationship of the oxidation peak current and the concentration of AA a sensitive analytical method was established with cyclic voltammetry. The linear range for AA determination was in the range from 1.0×10?5 to 3.0×10?3 mol/L with the linear regression equation as Ip (μA)=?2.52–0.064C (μmol/L) (n=13, γ=0.9942) and the detection limit was calculated as 8.0×10?6 mol/L (3σ). The proposed method was free of the interferences of coexisting substances such as dopamine (DA) and amino acids etc., and successfully applied to the vitamin C tablets determination. 相似文献