全文获取类型
收费全文 | 3137篇 |
免费 | 183篇 |
国内免费 | 12篇 |
专业分类
化学 | 2581篇 |
晶体学 | 44篇 |
力学 | 55篇 |
数学 | 341篇 |
物理学 | 311篇 |
出版年
2023年 | 21篇 |
2022年 | 34篇 |
2021年 | 41篇 |
2020年 | 44篇 |
2019年 | 58篇 |
2018年 | 40篇 |
2017年 | 25篇 |
2016年 | 90篇 |
2015年 | 111篇 |
2014年 | 113篇 |
2013年 | 190篇 |
2012年 | 233篇 |
2011年 | 257篇 |
2010年 | 119篇 |
2009年 | 141篇 |
2008年 | 215篇 |
2007年 | 210篇 |
2006年 | 208篇 |
2005年 | 184篇 |
2004年 | 148篇 |
2003年 | 123篇 |
2002年 | 135篇 |
2001年 | 51篇 |
2000年 | 31篇 |
1999年 | 43篇 |
1998年 | 24篇 |
1997年 | 31篇 |
1996年 | 39篇 |
1995年 | 29篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 23篇 |
1991年 | 20篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 13篇 |
1987年 | 14篇 |
1986年 | 19篇 |
1985年 | 17篇 |
1984年 | 12篇 |
1983年 | 16篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 15篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1974年 | 8篇 |
排序方式: 共有3332条查询结果,搜索用时 125 毫秒
901.
The synthesis of a library of N-alkylated O-arylated hydroxyproline derivatives has been achieved on solid phase. The choice of O-protection and the optimization of the Mitsunobu reaction involving a secondary alcohol were key to the success of this synthesis. First, acylation of resin-bound amines with N-Fmoc-O-THP-hydroxyproline was accomplished readily. Subsequent deprotection of the Fmoc and reductive amination with different aldehydes resulted in the tertiary amine intermediate. The deprotection of the THP group by p-toluenesulfonic acid was followed by a Mitsunobu reaction with a series of phenols. Finally, the products were cleaved from the resin using trifluoroacetic acid to produce a 10 200 member library. 相似文献
902.
Pennequin-Cardinal A Plaisance H Locoge N Ramalho O Kirchner S Galloo JC 《Talanta》2005,65(5):1233-1240
Radiello® diffusive samplers filled with a thermally desorbable adsorbent (graphitised charcoal Carbograph 4) have been tested for the monitoring of BTEX. The sampling rates have been estimated under various controlled atmospheres in order to evaluate the effects of two factors (exposure time, concentration levels and their interaction) on the performances of the Radiello® sampler. Experiments have been carried out under various atmospheres in exposure chamber. A total of 174 Radiello® samplers were exposed while varying two conditions: exposure time (1, 3, 7 and 14 days) and BTEX concentrations (low, medium and high levels). The results show that the sampling rates of benzene and toluene decrease for exposure of 14 days and especially for high concentration levels: decrease of 30% at 10 μg m−3 for benzene and 14% at 30 μg m−3 for toluene.
To try to explain the variations of these sampling rates, the breakthrough volumes (VB) of BTEX on Carbograph 4 have been determined at different temperature and concentration conditions in order to evaluate the Langmuir parameters and their adsorption enthalpy (−ΔHads) using the Van’t Hoff equation. With regard to these adsorption characteristics, the dependence of sampling rates with concentration level and exposure time were analysed and discussed. 相似文献
903.
New chiral Schiff base complexes have been obtained by condensation of 2,2'-diamino-1,1'-binaphthalene or 1,2-diaminocyclohexane and various salicylaldehydes and by subsequent metalation with manganese, iron, cobalt, nickel, copper, or zinc. The complete (1)H and (13)C NMR characterization of the ligands is reported, as are the X-ray crystal structures of (1R,2R)-(-)-N,N'-bis[3-(N,N-dimethylamino)salicylidene]-trans-1,2-cyclohexanediimine and [(1R,2R)-(-)-N,N'-bis(salicylidene)-trans-1,2-cyclohexanediiminato]copper(II). The new chiral manganese complexes have been evaluated in the oxygenation of prochiral olefins and sulfides using sodium hypochlorite, hydrogen peroxide, or N-methylmorpholine N-oxide/m-chloroperbenzoic acid as oxidant. 相似文献
904.
Karl Schnafinger Christine M. Yasenchak Anne Vollman Helen H. Ong 《Journal of heterocyclic chemistry》1988,25(2):535-537
A series of structurally novel 7,8,9,10-tetrahydropyrido[3′,4′:4,5]pyrrolo[2,3-c,]quinolines, 4a-c , were synthesized via a facile Fischer indole cyclization from the appropriately substituted hydrazinoquinolines 2a-c . Acetamides 4a,c were hydrolyzed to 5a,b and further converted to tertiary amines 6a-c . Potent antihypertensive activity has been observed with a number of the title compounds as well as the intermediate 3a . 相似文献
905.
Reif B Van Rossum BJ Castellani F Rehbein K Diehl A Oschkinat H 《Journal of the American Chemical Society》2003,125(6):1488-1489
In this communication, we demonstrate the feasibility of obtaining long-range (1)H-(1)H distance information by MAS solid-state NMR for a microcrystalline, uniformly (2)H,(15)N-labeled sample of a SH3 domain of chicken alpha-spectrin. The experiments yield NOESY-type spectra and rely on the favorable dispersion of the (15)N chemical shifts of the protein backbone. Perdeuteration of nonexchangeable sites is employed to simplify proton spin systems and to obtain multiple structural information. Two mixing schemes, (1)H-(1)H double quantum filtered Post-C7 and (1)H spin diffusion, are implemented to obtain quantitative (1)H-(1)H distance information. Post-C7 and spin diffusion cross-peak buildup rates are discussed for initial-rate fitting and in the framework of n = 0 rotational resonance (rotor driven spin diffusion), respectively. Different deuteration schemes were tested to find conditions where short-range (1)H-(1)H interactions are truncated (e.g., between H(N) and H(alpha)), but long-range interactions are retained (e.g., between H(N) and H(N)). 相似文献
906.
[reaction: see text] Palladium-catalyzed cross-coupling of vinyl- and arylstannanes with pi-electron-deficient heteroaromatics was performed in good yields. This Stille-type reaction was carried out with a methylthioether function as an electrophile in the presence of a copper(I) bromide-dimethyl sulfide complex. 相似文献
907.
This paper describes a simple plasma-based dry etching method that enables patterned cell culture inside microfluidic devices by allowing patterning, fluidic bonding and sterilization steps to be carried out in a single step. This plasma-based dry etching method was used to pattern cell-adhesive and non-adhesive areas on the glass and polystyrene substrates. The patterned substrate was used for selective attachment and growth of human umbilical vein endothelial cells, MDA-MB-231 human breast cancer cells, NIH 3T3 mouse fibroblasts, and primary rat cortical neurons. Finally, we have successfully combined the dry-patterned substrate with a microfluidic device. Patterned primary rat neurons were maintained for up to 6 days inside the microfluidic devices and the neurons' somas and processes were confined to the cell-adhesive region. The method developed in this work offers a convenient way of micropatterning biomaterials for selective attachment of cells on the substrates, and enables culturing of patterned cells inside microfluidic devices for a number of biological research applications where cells need to be exposed to well-controlled fluidic microenvironment. 相似文献
908.
Aggregation of anionic surfactants was investigated by frontal analysis continuous capillary electrophoresis (FACCE), a method involving the continuous electrokinetic introduction of the surfactant sample into the separation capillary. This process results in a partial separation of the monomeric and aggregated forms without perturbing the monomer-aggregate equilibrium. The critical micelle concentration (CMC) can then be easily derived from the height of the firstly detected migration front, corresponding to the monomeric form. This approach is exemplified with octyl and dodecylbenzenesulfonates and compared with conductimetry and surface tension measurements. FACCE turns out to be an effective method for the determination of CMC and intermediate aggregation phenomena with very small sample and short time requirements. 相似文献
909.
We have explored the dienone-phenol rearrangement of substrates where: only the p-cresol pathway is possible and relative migratory aptitudes should play no role in determining the regiochemistry of the reaction. For these substrates the selectivity of the rearrangement was found to depend on the stereochemistry of the spirocyclic intermediate formed during the course of the rearrangement. Rearrangement of one of these substrates gave—surprisingly—a single regioisomeric product. Selectivity in this case can be correlated with the relative stability of cationic intermediates, which lie on the pathway between spirocycle and final product. 相似文献
910.
Azametallacyclobutene Cp(2)ZrN-t-BuCEt=CEt (1) underwent an insertion reaction with CO to form the acyl complex 2 (Cp(2)Zr(N-t-BuCEtCEtCO), 67% yield). The addition of acetone to azametallacyclobutene 3 (Cp(2)Zr(NArCMeCPh), Ar = 2,6-dimethylphenyl) yielded the N-bonded enamine and O-bonded enolate complex of zirconocene 4 (Cp(2)Zr(NArCMeCPhH)(OCMeCH(2)), 76% yield). The addition of aldehydes RCOH to metallacycle 3 resulted in the insertion of the aldehyde into the Zr-C bond to form complexes Cp(2)Zr(NArCMeCPhCRHO) (8a) and Cp(2)Zr(NArCMeCPhC(i-Pr)HO (9) in 85% (R = Ph) and 73% yields, respectively. Similarly, treatment of metallacycle 10 (Cp(2)Zr(NArCEtCEt)) with benzaldehyde yielded the insertion product 11 (Cp(2)Zr(NArCEtCEtCPhHO)) in 56% isolated yield. The structure of complex 11 was confirmed by an X-ray crystallographic study. Heating the insertion products 8a and 9 led to elimination of the alpha,beta-unsaturated imines 13 and 14a (ArN=CMeCPh=CRH) in 53% and 72% yields, respectively, and the formation of oxozirconocene oligomer (Cp(2)ZrO)(n)(). The oxozirconocene monomer was trapped by dimethylzirconocene, preventing the formation of oligomer and resulting in the isolation of product 15. A kinetic study of this retrocycloaddition produced the following activation parameters: DeltaH() = 26.5 kcal/mol, DeltaS() = 3.48 eu. A Hammett sigma/rho study showed that electron-donating groups alpha to the metallacycle oxygen accelerate the retrocycloaddition (rho = -0.8). 相似文献