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991.
Jocelino Sato Vicente Francisco De Souza Neto 《Annals of Global Analysis and Geometry》2006,29(3):221-240
We classify the zero scalar curvature O(p+1)×O(q+1)-invariant hypersurfaces in the euclidean space ℝ
p+q+2, p,q > 1, analyzing whether they are embedded and stable. The Morse index of the complete hypersurfaces show the existence of embedded, complete and globally stable zero scalar curvature O(p+1)×O(q+1)-invariant hypersurfaces in ℝ
p+q+2, p+q≥ 7, which are not homeomorphic to ℝ
p+q+1. Such stable examples provide counter-examples to a Bernstein-type conjecture in the stable class, for immersions with zero scalar curvature.
Mathematics Subject Classifications (2000): 53A10, 53C42,49005. 相似文献
992.
Combretastatins A-4 and AVE-8062A have been synthesized by coupling 3,4,5-trimethoxyphenylacetylene with the corresponding iodomethoxyphenol and bromomethoxyaniline, followed by hydrolysis of the thermally-stable Ti(II)-alkyne complexes. 相似文献
993.
B Alcaide P Almendros C Aragoncillo R Rodríguez-Acebes 《The Journal of organic chemistry》2001,66(15):5208-5216
Metal-mediated carbonyl allylation, allenylation, and propargylation of optically pure azetidine-2,3-diones were investigated in both anhydrous and aqueous environments. Different metals promoters showed varied regioselectivities on product formation during allenylation/propargylation reactions of the keto-beta-lactams. The stereochemistry of the new C3-substituted C3-hydroxy quaternary center was controlled by placing a chiral auxiliary at C4. In this way, the coupling of azetidine-2,3-diones with a variety of propenyl-, propynyl-, and allenylmetal reagents offers a convenient asymmetric entry to potentially bioactive 3-substituted 3-hydroxy-beta-lactams. 相似文献
994.
Marta Teijeira Yagamare Fall Francisco Santamarta Emilia Tojo 《Tetrahedron letters》2007,48(45):7926-7929
A new efficient and rapid experimental method and product isolation procedure as well as easy recyclable reaction media is expected to contribute to the development of a green strategy for the synthesis of furan diol from d-glucal. We determine the best conditions of reaction, minimizing the toxicity and finding an environmentally more acceptable process, using [Bmim][MSO4]/InCl3·3H2O as a new solvent system under extremely mild conditions, providing high yields with a dramatic enhancement of reaction rate. 相似文献
995.
Headspace single drop microextraction coupled to gas chromatography-mass spectrometry yielded a simple, fast and virtually solventless analytical protocol used for the headspace analysis of aqueous samples contaminated with methylcyclopentadienyl-manganese tricarbonyl (MMT). Initially, several experimental parameters were controlled and optimized and the optimum conditions found were 2.5 μl octane microdrop exposed for 20 min to the headspace of a 10 ml aqueous sample (15 ml vial) containing 20% (w/v) NaCl and stirred at 1250 rpm. The calculated calibration curves gave a high level of linearity for MMT with correlation coefficients >0.9995 after conducting a 3-day study. The limit of detection was calculated to be 0.21 μg l−1. The proposed method achieved an enrichment factor of the order of 2100 and a 53% recovery after extracting the spiked aqueous solution for 20 min under the optimized experimental conditions. The repeatability and intra-day reproducibility of the proposed method, expressed as relative standard deviation were 8.4 and 6.4%, respectively. Finally, analysis of spiked tap and wastewater samples revealed that matrix had little effect upon extraction. 相似文献
996.
Pedro M. A. Pereira Joana R. Bernardo Luisa Bivar Roseiro Francisco Gírio Rafa M. ukasik 《Molecules (Basel, Switzerland)》2021,26(24)
Biomass pre-treatment is a key step in achieving the economic competitiveness of biomass conversion. In the present work, an imidazole pre-treatment process was performed and evaluated using wheat straw and eucalyptus residues as model feedstocks for agriculture and forest-origin biomasses, respectively. Results showed that imidazole is an efficient pre-treatment agent; however, better results were obtained for wheat straw due to the recalcitrant behavior of eucalyptus residues. The temperature had a stronger effect than time on wheat straw pre-treatment but at 160 °C and 4 h, similar results were obtained for cellulose and hemicellulose content from both biomasses (ca. 54% and 24%, respectively). Lignin content in the pre-treated solid was higher for eucalyptus residues (16% vs. 4%), as expected. Enzymatic hydrolysis, applied to both biomasses after different pre-treatments, revealed that results improved with increasing temperature/time for wheat straw. However, these conditions had no influence on the results for eucalyptus residues, with very low glucan to glucose enzymatic hydrolysis yield (93% for wheat straw vs. 40% for eucalyptus residues). Imidazole can therefore be considered as a suitable solvent for herbaceous biomass pre-treatment. 相似文献
997.
Ronald Oliveira Francisco Thiago Luiz Antonacci Oakes Júlio César Fabris José Alexandre Nogueira 《Brazilian Journal of Physics》2014,44(2-3):271-277
We show that relativistic contributions to the ground-state energy of the hydrogen atom from a minimal length introduced by a Lorentz-covariant algebra are more important than non-relativistic contributions; the non-relativistic approach is therefore unsuitable. We compare our result with experimental data to estimate an upper bound of the order 10?20m for the minimal length. 相似文献
998.
Laura Díaz-Casado Israel Serrano-Chacón Dr. Laura Montalvillo-Jiménez Francisco Corzana Agatha Bastida Dr. Andrés G. Santana Prof. Dr. Carlos González Dr. Juan Luis Asensio 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(20):6204-6212
Targeting the interface between DNA quadruplex and duplex regions by small molecules holds significant promise in both therapeutics and nanotechnology. Herein, a new pharmacophore is reported, which selectively binds with high affinity to quadruplex–duplex junctions, while presenting a poorer affinity for G-quadruplex or duplex DNA alone. Ligands complying with the reported pharmacophore exhibit a significant affinity and selectivity for quadruplex–duplex junctions, including the one observed in the HIV-1 LTR-III sequence. The structure of the complex between a quadruplex–duplex junction with a ligand of this family has been determined by NMR methods. According to these data, the remarkable selectivity of this structural motif for quadruplex–duplex junctions is achieved through an unprecedented interaction mode so far unexploited in medicinal and biological chemistry: the insertion of a benzylic ammonium moiety into the centre of the partially exposed G-tetrad at the interface with the duplex. Further decoration of the described scaffolds with additional fragments opens up the road to the development of selective ligands for G-quadruplex-forming regions of the genome. 相似文献
999.
Dr. Alberto Feliciano Juan L. Vázquez Luis J. Benítez-Puebla Iván Velazco-Cabral Dr. David Cruz Cruz Dr. Francisco Delgado Dr. Miguel A. Vázquez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(32):8233-8251
The structure of Fischer carbene complexes (FCCs) is electron deficient. If bearing an α,β-unsaturated system, it can generate a wide variety of compounds by undergoing many different transformations, including higher-order cycloadditions. The latter are described as pericyclic reactions in which more than six electrons participate. These reactions have been employed in various areas of organic synthesis, resulting in highly selective compounds with a broad range of scaffolds. The first studies on higher-order cycloadditions involving FCCs frequently yielded competing byproducts. Many groups have attempted to increase selectivity by exploring distinct reaction conditions, reagents and co-catalysts (e. g., metal-mediated cycloadditions). The present review is the first to focus exclusively on using higher-order cycloadditions involving FCCs to synthesize carbocycles and heterocycles. Based on two decades of reports, an analysis is made of the main aspects of the mechanisms proposed for higher-order cycloadditions and the structural diversity obtained by the substituent effect. 相似文献
1000.
Journal of Mathematical Chemistry - We argue that the differential transform method is merely the well known Taylor series approach in disguise. We show that the examples chosen by Qin and Lou have... 相似文献