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991.
A mu-oxo-type chiral bis-Ti(IV) oxide (S,S)-1 can be successfully utilized in the asymmetric 1,3-dipolar cycloaddition reactions between various nitrones 2 and acrolein to give the corresponding isoxazolidines with high to excellent enantioselectivities. For instance, the reaction between nitrone 2 (R = t-Bu) and acrolein in the presence of 10 mol % of bis-Ti(IV) catalyst (S,S)-1 in dichloromethane at -40 degrees C gave the corresponding endo cycloadduct with 97% ee. 相似文献
992.
The alternating group of degree 6 is located at the junctionof three series of simple non-commutative groups: simple sporadicgroups, alternating groups and simple groups of Lie type. Itplays a very special role in the theory of finite groups. Weshall study its new roles both in a finite geometry of a certainpentagon in the Leech lattice and also in the complex algebraicgeometry of K3 surfaces. 2000 Mathematics Subject Classification14J28, 11H06, 20D06, 20D08. 相似文献
993.
Highly stereoselective 1,3-dipolar cycloadditions of methacrolein and nitrones could be realized by the use of bis-titanium chiral Lewis acid catalyst. Key to the success is the introduction of bulky N-substituent on nitrone to attenuate the undesired Lewis acid-nitrone complexation. 相似文献
994.
Komatsu Y Watanabe R Ikishima H Nakano K Ichikawa Y Kotsuki H 《Organic & biomolecular chemistry》2012,10(15):2993-3001
A practical method for the synthesis of 1,3-aminohydroxyacetone synthons was developed, and their utility in the organocatalytic asymmetric aldol reaction was demonstrated in a short synthesis of aza-sugars. 相似文献
995.
By using the hybrid IMOMM(B3LYP:MM3) method, we examined the binap–RhI‐catalyzed oxidative‐addition and insertion steps of the asymmetric hydrogenation of the enamide 2‐acetylamino‐3‐phenylacrylic acid. We report a path that is energetically more favorable for the major enantiomer than for the minor enantiomer. This path follows the “lock‐and‐key” motif and leads to the major enantiomeric product via an energetically favorable binap–dihydride–RhIII–enamide complex. Our theoretical results are consistent with the mechanism that takes place via RhIII dihydride formation, that is, oxidative addition of H2 followed by enamide insertion. 相似文献
996.
Page AJ Yamane H Ohta Y Irle S Morokuma K 《Journal of the American Chemical Society》2010,132(44):15699-15707
The mechanism and kinetics of single-walled carbon nanotube (SWNT) nucleation from Fe- and Ni-carbide nanoparticle precursors have been investigated using quantum chemical molecular dynamics (QM/MD) methods. The dependence of the nucleation mechanism and its kinetics on environmental factors, including temperature and metal-carbide carbon concentration, has also been elucidated. It was observed that SWNT nucleation occurred via three distinct stages, viz. the precipitation of the carbon from the metal-carbide, the formation of a "surface/subsurface" carbide intermediate species, and finally the formation of a nascent sp(2)-hybidrized carbon structure supported by the metal catalyst. The SWNT cap nucleation mechanism itself was unaffected by carbon concentration and/or temperature. However, the kinetics of SWNT nucleation exhibited distinct dependences on these same factors. In particular, SWNT nucleation from Ni(x)C(y) nanoparticles proceeded more favorably compared to nucleation from Fe(x)C(y) nanoparticles. Although SWNT nucleation from Fe(x)C(y) and Ni(x)C(y) nanoparticle precursors occurred via an identical route, the ultimate outcomes of these processes also differed substantially. Explicitly, the Ni(x)-supported sp(2)-hybridized carbon structures tended to encapsulate the catalyst particle itself, whereas the Fe(x)-supported structures tended to form isolated SWNT cap structures on the catalyst surface. These differences in SWNT nucleation kinetics were attributed directly to the relative strengths of the metal-carbon interaction, which also dictates the precipitation of carbon from the nanoparticle bulk and the longevity of the resultant surface/subsurface carbide species. The stability of the surface/subsurface carbide was also influenced by the phase of the nanoparticle itself. The observations agree well with experimentally available data for SWNT growth on iron and nickel catalyst particles. 相似文献
997.
998.
The effect of sodium sulphate and temperature on the fining of float glass during the melting process as well as the cooling process was investigated by measuring the seed count and seed size distributions and analyzing the gas composition in the seed. The mechanism of fining in a tank furnace is discussed. 相似文献
999.
Enantiodivergent Atroposelective Synthesis of Chiral Biaryls by Asymmetric Transfer Hydrogenation: Chiral Phosphoric Acid Catalyzed Dynamic Kinetic Resolution 下载免费PDF全文
Prof. Dr. Keiji Mori Tsubasa Itakura Prof. Dr. Takahiko Akiyama 《Angewandte Chemie (International ed. in English)》2016,55(38):11642-11646
Reported herein is an enantiodivergent synthesis of chiral biaryls by a chiral phosphoric acid catalyzed asymmetric transfer hydrogenation reaction. Upon treatment of biaryl lactols with aromatic amines and a Hantzsch ester in the presence of chiral phosphoric acid, dynamic kinetic resolution (DKR) involving a reductive amination reaction proceeded smoothly to furnish both R and S isomers of chiral biaryls with excellent enantioselectivities by proper choice of hydroxyaniline derivative. This trend was observed in wide variety of substrates, and various chiral biphenyl and phenyl naphthyl adducts were synthesized with satisfactory enantioselectivities in enantiodivergent fashion. The enantiodivergent synthesis of synthetically challenging, chiral o‐tetrasubstituted biaryls were also accomplished, and suggests high synthetic potential of the present method. 相似文献
1000.