全文获取类型
收费全文 | 5503篇 |
免费 | 102篇 |
国内免费 | 16篇 |
专业分类
化学 | 3773篇 |
晶体学 | 26篇 |
力学 | 77篇 |
数学 | 979篇 |
物理学 | 766篇 |
出版年
2020年 | 43篇 |
2019年 | 54篇 |
2018年 | 42篇 |
2016年 | 81篇 |
2015年 | 91篇 |
2014年 | 84篇 |
2013年 | 224篇 |
2012年 | 168篇 |
2011年 | 198篇 |
2010年 | 127篇 |
2009年 | 165篇 |
2008年 | 199篇 |
2007年 | 186篇 |
2006年 | 167篇 |
2005年 | 192篇 |
2004年 | 185篇 |
2003年 | 142篇 |
2002年 | 150篇 |
2001年 | 112篇 |
2000年 | 112篇 |
1999年 | 97篇 |
1998年 | 99篇 |
1997年 | 107篇 |
1996年 | 98篇 |
1995年 | 71篇 |
1994年 | 101篇 |
1993年 | 99篇 |
1992年 | 90篇 |
1991年 | 71篇 |
1990年 | 70篇 |
1989年 | 89篇 |
1988年 | 98篇 |
1987年 | 103篇 |
1986年 | 82篇 |
1985年 | 95篇 |
1984年 | 96篇 |
1983年 | 64篇 |
1982年 | 102篇 |
1981年 | 83篇 |
1980年 | 118篇 |
1979年 | 87篇 |
1978年 | 87篇 |
1977年 | 55篇 |
1976年 | 75篇 |
1975年 | 71篇 |
1974年 | 57篇 |
1973年 | 57篇 |
1972年 | 50篇 |
1971年 | 50篇 |
1967年 | 37篇 |
排序方式: 共有5621条查询结果,搜索用时 15 毫秒
991.
Noah Z. Burns Phil S. Baran Prof. Dr. Reinhard W. Hoffmann Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(16):2854-2867
Stimulus package for organic synthesis : The economy of step and atom usage has been widely reviewed and acknowledged as being useful frameworks to evaluate the efficiency of synthesis. This Review brings to attention another form of economy which should be considered in the planning and evaluation of a multistep synthesis: redox economy. Several guidelines and examples are included to illustrate the logic of this approach and to stimulate the design of syntheses.
992.
Reinhard Oertel Norman Arenz Jörg Pietsch Wilhelm Kirch 《Journal of separation science》2009,32(2):238-243
A specific and automated method was developed to quantify the anticonvulsants gabapentin, pregabalin and vigabatrin simultaneously in human serum. Samples were prepared with a protein precipitation. The hydrophilic interaction chromatography (HILIC) with a mobile phase gradient was used to divide off ions of the matrix and for separation of the analytes. Four different HILIC‐columns and two different column temperatures were tested. The Tosoh‐Amid column gave the best results: single small peaks. The anticonvulsants were detected in the multiple reaction monitoring mode (MRM) with ESI‐MS‐MS. Using a volume of 100 μL biological sample the lowest point of the standard curve, i.e. the lower LOQs were 312 ng/mL. The described HILIC‐MS‐MS method is suitable for therapeutic drug monitoring and for clinical and pharmcokinetical investigations of the anticonvulsives. 相似文献
993.
Horst Osswald 《Journal of Theoretical Probability》2009,22(2):474-481
The smooth approach to Malliavin calculus for Lévy processes in (Osswald in J. Theor. Probab., 2008) is used to study time-anticipative Girsanov transformations for a large class of Lévy processes by means of the substitution
rule in finite-dimensional analysis.
Dedicated to Wolfram Pohlers on the occasion of his 65th birthday. 相似文献
994.
995.
Howell GP Fletcher SP Geurts K ter Horst B Feringa BL 《Journal of the American Chemical Society》2006,128(46):14977-14985
Herein, we report efficient acyclic stereocontrol in tandem 1,4-addition-aldol reactions triggered by catalytic asymmetric organometallic addition. Grignard reagents add to alpha,beta-unsaturated thioesters in a 1,4-fashion and the resulting magnesium enolates are trapped with aromatic or aliphatic aldehydes. The process provides a range of tandem products bearing three contiguous stereocenters with excellent control of relative and absolute stereochemistry. The various diastereomeric products have been fully characterized using single-crystal X-ray analysis and the origins of stereocontrol in this tandem protocol are discussed. The versatility and efficiency of this methodology are demonstrated in the first catalytic asymmetric synthesis of (-)-phaseolinic acid with 54% overall yield via a short and concise route. 相似文献
996.
We investigate the importance of finite-size effects in simulations of nucleation processes. Most molecular dynamics simulations of first order phase transitions, such as vapor-liquid nucleation, are performed in the canonical NVT ensemble where, owing to the fixed total number of molecules N, the growth of the new phase causes the depletion of the metastable phase. This effect may lead to significant errors in the simulation and even to the impossibility of observing nucleation in a small finite system. We present a theory to estimate the system size beyond which these finite-size effects are expected to be negligible. This optimization saves valuable calculation time and can extend the range of supersaturations and rates attainable by simulations by several orders of magnitude. Our results are applicable to diverse situations, such as crystallization, capillary condensation, or the melting of nanoclusters. 相似文献
997.
Wahl F Weiler A Landenberger P Sackers E Voss T Haas A Lieb M Hunkler D Wörth J Knothe L Prinzbach H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(24):6255-6267
“One‐pot” substitution of the twenty hydrogen atoms in pentagonal dodecahedrane (C20H20) by OH, F, Cl, and Br atoms is explored. Electrophilic insertion of oxygen atoms with DMDO and TFMDO as oxidizing reagents ended, far off the desired C20(OH)20, in complex polyol mixtures (up to C20H10(OH)10 decols, a trace of C20H(OH)19?). Perfluorination was successful in a NaF matrix but (nearly pure) C20F20 could be secured only in very low yield. “Brute‐force” photochlorination (heat, light, pressure, time) provided a mixture of hydrogen‐free, barely soluble C20Cl16 dienes in high yield and C20Cl20 as a trace component. Upon electron‐impact ionization of the C20Cl16 material sequential loss of the chlorine atoms was the major fragmentation pathway furnishing, however, only minor amounts of chlorine‐free C20+ ions. “Brute‐force” photobrominations delivered an extremely complex mixture of polybromides with C20HBr13 trienes as the highest masses. The MS spectra exhibited exclusive loss of the Br substituents ending in rather intense singly, doubly, and triply charged C20H4–0+(2+)(3+) ions. The insoluble ~C20HBr13 fraction (C20Br14 trienes as highest masses) obtained along a modified bromination protocol, ultimately allowed the neat mass selection of C20? ions. The C20Cl16 dienes and C20H0–3Br14–12 tri‐/tetraenes, in spite of their very high olefinic pyramidalization, proved resistant to oxygen and dimerization (polymerization) but added CH2N2 smoothly. Dehalogenation of the respective cycloaddition products through electron‐impact ionization resulted in C22–24H4–8+(2+) ions possibly constituting bis‐/tris‐/tetrakis‐methano‐C20 fullerenes or partly hydrogenated C22, C23, and C24 cages. 相似文献
998.
Dziadek S Griesinger C Kunz H Reinscheid UM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(19):4981-4993
To study the effect of O-glycosylation on the conformational propensities of a peptide backbone, a 20-residue peptide (GSTAPPAHGVTSAPDTRPAP) representing the full length tandem repeat sequence of the human mucin MUC1 and its analogue glycosylated with the (2,6)-sialyl-T antigen on Thr11, were prepared and investigated by NMR and molecular modeling. The peptides contain both the GVTSAP sequence, which is an effective substrate for GalNAc transferases, and the PDTRP fragment, a known epitope recognized by several anti-MUC1 monoclonal antibodies. It has been shown that glycosylation of threonine in the GVTSAP sequence is a prerequisite for subsequent glycosylation of the serine at GVTSAP. Furthermore, carbohydrates serve as additional epitopes for MUC1 antibodies. Investigation of the solution structure of the sialyl-T glycoeicosapeptide in a H(2)O/D(2)O mixture (9:1) under physiological conditions (25 degrees C and pH 6.5) revealed that the attachment of the saccharide side-chain affects the conformational equilibrium of the peptide backbone near the glycosylated Thr11 residue. For the GVTSA region, an extended, rod-like secondary structure was found by restrained molecular dynamics simulation. The APDTR region formed a turn structure which is more flexibly organized. Taken together, the joined sequence GVTSAPDTR represents the largest structural model of MUC1 derived glycopeptides analyzed so far. 相似文献
999.
Scifoni E Gianturco FA Grebenshchikov SY Schinke R 《The Journal of chemical physics》2006,125(16):164304
Ab initio quantum calculations have been carried out on the helium ionic trimer. The potential energy surface is accurately fitted, especially in the vicinity of the three equivalent minima. The spectrum of bound states for the zero angular momentum is computed and analyzed in detail. Energies and wave functions reveal several interesting features related to the fact that He3+ represents one of the few homonuclear ionic trimers that are linear in their ground vibrational state. At low energies, the triply degenerate eigenfunctions are localized at the potential minimum. With growing excitation energy, however, the wave functions exhibit stronger spatial delocalization. 相似文献
1000.
The study of interfacial properties in the marine environment is important for the understanding of air-sea gas exchange processes, especially with respect to the behaviour of entrained air bubbles. Seawater contains surfactant material, much of which is thought to origin from the exudation of dissolved organic material (DOM) by phytoplankton. This study aims at investigating the influence of different phytoplankton species on the surface shear viscosity of an air-water interface. Measurements of surface shear viscosity were carried out with the ISR1 interfacial shear rheometer. Surface shear viscosities of stock cultures of Phaeocystis sp., Thalassiosira rotula, Thalassiosira punctigera and Nitzschia closterium as well as of F/2 nutrient medium and seawater were measured. The surface shear viscosity of N. closterium was investigated during different stages of its growth as well as for an unfiltered stock culture sample and its filtrate. Results reveal that the influence of phytoplankton on the surface shear viscosity is species specific. An increase in surface shear viscosity occurred for the N. closterium stock culture only. The remaining cultures showed similar behaviour to F/2 nutrient medium. The increase of surface shear viscosity during the growth of N. closterium occurred mainly during the exponential growth phase. The increases in surface shear viscosity depend on the presence of phytoplankton cells in the sample. The formation of compact mechanical structures at the air-water interface originating from the aggregation of DOM released by N. closterium as a cause for the observed increases in surface shear viscosity is discussed. 相似文献