全文获取类型
收费全文 | 3815篇 |
免费 | 54篇 |
国内免费 | 26篇 |
专业分类
化学 | 2586篇 |
晶体学 | 59篇 |
力学 | 113篇 |
数学 | 725篇 |
物理学 | 412篇 |
出版年
2020年 | 28篇 |
2016年 | 41篇 |
2015年 | 37篇 |
2014年 | 42篇 |
2013年 | 165篇 |
2012年 | 129篇 |
2011年 | 139篇 |
2010年 | 81篇 |
2009年 | 75篇 |
2008年 | 148篇 |
2007年 | 149篇 |
2006年 | 177篇 |
2005年 | 164篇 |
2004年 | 132篇 |
2003年 | 144篇 |
2002年 | 111篇 |
2001年 | 53篇 |
2000年 | 60篇 |
1999年 | 44篇 |
1998年 | 33篇 |
1997年 | 56篇 |
1996年 | 58篇 |
1995年 | 61篇 |
1994年 | 62篇 |
1993年 | 72篇 |
1992年 | 44篇 |
1991年 | 67篇 |
1990年 | 52篇 |
1989年 | 49篇 |
1988年 | 45篇 |
1987年 | 40篇 |
1986年 | 41篇 |
1985年 | 73篇 |
1984年 | 78篇 |
1983年 | 50篇 |
1982年 | 80篇 |
1981年 | 73篇 |
1980年 | 65篇 |
1979年 | 78篇 |
1978年 | 70篇 |
1977年 | 73篇 |
1976年 | 60篇 |
1975年 | 56篇 |
1974年 | 67篇 |
1973年 | 80篇 |
1972年 | 39篇 |
1971年 | 39篇 |
1970年 | 35篇 |
1969年 | 29篇 |
1968年 | 30篇 |
排序方式: 共有3895条查询结果,搜索用时 31 毫秒
211.
Solvent‐Controlled Bifurcated Cascade Process for the Selective Preparation of Dihydrocarbazoles or Dihydropyridoindoles
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Dong‐Liang Mo Dr. Donald J. Wink Dr. Laura L. Anderson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(41):13217-13225
A solvent‐controlled cascade process has been identified for the dual purpose of the preparation of either dihydrocarbazoles or dihydropyridoindoles from identical N‐aryl‐α,β‐unsaturated nitrones and electron‐deficient allene starting materials. These reactions proceed smoothly under mild metal‐free conditions affording a range of two types of skeletally distinct indole‐based heterocycles in high yield and diastereoselectivity. These transformations demonstrate the use of a bifurcated cascade process that hinges on the ring‐opening event of a benzazepine intermediate for the synthesis of skeletally diverse heterocyclic products and rapid access to biologically‐significant, indole‐based structures. 相似文献
212.
Muhammad Imran ul‐haq Rajesh A. Shenoi Donald E. Brooks Jayachandran N. Kizhakkedathu 《Journal of polymer science. Part A, Polymer chemistry》2013,51(12):2614-2621
Hyperbranched polyglycerols (HPGs) are globular structures with a large number of functionalizable hydroxyl groups and have excellent in vitro and in vivo biocompatibility profiles comparable to polyethylene glycol. This work introduces a facile method for the synthesis of medium molecular weights (Mws) (50–300 kDa) HPGs, which has been difficult to synthesize with low polydispersity, with the assistance of solvents by ring opening polymerization. The influence of different solvents (1,4‐dioxane, tetrahydropyran (THP), ethylene glycol diethyl ether (EGDE) and decane), solvent to glycidol ratio, concentration of glycidol and the time of polymerization on Mw and polydispersity of HPGs has been studied. The Mw and polydispersity of HPGs are significantly affected by the nature of the polymerization phase (homogeneous or heterogeneous) and chemical structure of the solvent. The differences in the solvation of the potassium cations and change in the nucleophilicity of the alkoxide anion in various solvents may be responsible for the changes in Mw and PDI of the HPG. The Mw of the HPG decreases in the order 1,4‐dioxane > THP > EGDE >decane. The microstructure, solution and thermal properties of the HPG do not depend on the nature of solvent. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2614–2621 相似文献
213.
Christian A. Malapit Donald R. Caldwell Irungu K. Luvaga Dr. Jonathan T. Reeves Dr. Ivan Volchkov Dr. Nina C. Gonnella Dr. Zhengxu S. Han Dr. Carl A. Busacca Prof. Dr. Amy R. Howell Dr. Chris H. Senanayake 《Angewandte Chemie (International ed. in English)》2017,56(24):6999-7002
β-Ketonitriles bearing a quaternary carbon at the 2-position were prepared through Rh-catalyzed addition of aryl boronic acids to 2,2-disubstituted malononitriles. In contrast to the previously described transnitrilative cyanation of aryl boronic acids with dialkylmalononitriles, the present reaction avoids retro-Thorpe collapse of the intermediate addition product through the use of a milder base. The reaction was amenable to a variety of aryl boronic acids and disubstituted malononitriles, providing a diverse array of β-ketonitriles. The products could be further derivatized to valuable chiral α,α-disubstituted-β-aminonitriles through addition reactions to the corresponding N-tert-butanesulfinyl imines. 相似文献
214.
Huixin Wang Dr. Michael G. Leeming Dr. Junming Ho Dr. William A. Donald 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(3):823-834
Predicting the fragmentation patterns of proteins would be beneficial for the reliable identification of intact proteins by mass spectrometry. However, the ability to accurately make such predictions remains elusive. An approach to predict the specific cleavage sites in whole proteins resulting from collision-induced dissociation by use of an improved electrostatic model for calculating the proton configurations of highly-charged protein ions is reported. Using ubiquitin, cytochrome c, lysozyme and β-lactoglobulin as prototypical proteins, this approach can be used to predict the fragmentation patterns of intact proteins. For sufficiently highly charged proteins, specific cleavages occur near the first low-basicity amino acid residues that are protonated with increasing charge state. Hybrid QM/QM′ (QM=quantum mechanics) and molecular dynamics (MD) simulations and energy-resolved collision-induced dissociation measurements indicated that the barrier to the specific dissociation of the protonated amide backbone bond is significantly lower than competitive charge remote fragmentation. Unlike highly charged peptides, the protons at low-basicity sites in highly charged protein ions can be confined to a limited sequence of low-basicity amino acid residues by electrostatic repulsion, which results in highly specific fragmentation near the site of protonation. This research suggests that the optimal charge states to form specific sequence ions of intact proteins in higher abundances than the use of less specific ion dissociation methods can be predicted a priori. 相似文献
215.
Reflections from geometric discontinuities can be used with ultrasonic energy to predict the temperature of an interface where classical temperature measurement techniques are impractical because of physical access limitations or harsh environmental conditions. Additionally, these same ultrasonic measurements can be used with inversion methods commonly applied to ill-posed heat transfer problems to increase the accuracy of the measurement of surface temperature or heat flux at the surface of interest. Both methods for determining surface temperature are presented, along with a comparison of results both from a verification example and using data gathered in a field test of the methods. The results obtained with these two methods are shown to be in good agreement with an empirical relationship used in the design of large caliber guns. 相似文献
216.
217.
218.
Dr. Luis M. Mori‐Quiroz Kirk W. Shimkin Dr. Sina Rezazadeh Ryan A. Kozlowski Prof. Dr. Donald A. Watson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(44):15654-15658
The oxidative copper‐catalyzed cross‐coupling of functionalized alkyl boronic esters with primary amides is reported. Through the identification of appropriate diketimine ligands, conditions for efficient coupling of both primary and secondary alkyl boronic esters with diverse primary amides, including acetamide, have been developed. 相似文献
219.
In an attempt to gain further information on a stereo-structural model for activity of prostaglandins,2 we undertook the synthesis of a number of new analogues.3 In particular, 17,17-dimethyl-F2α was chosen as a good test of the “hairpin” conformation because the two 17-methyl groups would prevent side chain alignment in the particular sense of the “hairpin” model.4 相似文献
220.