首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   382788篇
  免费   8117篇
  国内免费   4108篇
化学   178930篇
晶体学   5742篇
力学   21281篇
综合类   147篇
数学   71680篇
物理学   117233篇
  2022年   2252篇
  2021年   2643篇
  2020年   3065篇
  2019年   3250篇
  2018年   19535篇
  2017年   20045篇
  2016年   13402篇
  2015年   5087篇
  2014年   5788篇
  2013年   11738篇
  2012年   16007篇
  2011年   30219篇
  2010年   18437篇
  2009年   18775篇
  2008年   23924篇
  2007年   28588篇
  2006年   8660篇
  2005年   14915篇
  2004年   10966篇
  2003年   10668篇
  2002年   7782篇
  2001年   7055篇
  2000年   5621篇
  1999年   4265篇
  1998年   3461篇
  1997年   3336篇
  1996年   3367篇
  1995年   3044篇
  1994年   2811篇
  1993年   2582篇
  1992年   3201篇
  1991年   3060篇
  1990年   2975篇
  1989年   2955篇
  1988年   2934篇
  1987年   2922篇
  1986年   2812篇
  1985年   3496篇
  1984年   3475篇
  1983年   2769篇
  1982年   2856篇
  1981年   2902篇
  1980年   2664篇
  1979年   3007篇
  1978年   3001篇
  1977年   3026篇
  1976年   2888篇
  1975年   2617篇
  1974年   2565篇
  1973年   2565篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
A simple and efficient nitrile-directed meta-C−H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U-shaped template to achieve a molecular U-turn and assemble the large-sized cyclophane transition state for the remote C−H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta-C−H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles. Notably, the remote meta-selectivity of biphenylnitriles could not be expected from previous results with a macrocyclophane nitrile template. DFT computational studies show that a ligand-containing Pd–Ag heterodimeric transition state (TS) favors the desired remote meta-selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non-directed meta-C−H activation. Substituted 2-pyridone ligands were found to be key in assisting the cleavage of the meta-C−H bond in the concerted metalation–deprotonation (CMD) process.  相似文献   
152.
Russian Chemical Bulletin - The study is concerned with structural rearrangements in the crystals of heterospin complexes Cu(hfac)2 with nitroxide radicals LR (hfac is hexafluoroacetylacetonate, LR...  相似文献   
153.
Functional, degradable polymers were synthesized via the copolymerization of vinyl acetate (VAc) and 2‐methylene‐1,3‐dioxepane (MDO) using a macro‐xanthate CTA, poly(N‐vinylpyrrolidone), resulting in the formation of amphiphilic block copolymers of poly(NVP)‐b‐poly(MDO‐co‐VAc). The behavior of the block copolymers in water was investigated and resulted in the formation of self‐assembled nanoparticles containing a hydrophobic core and a hydrophilic corona. The size of the resultant nanoparticles was able to be tuned with variation of the hydrophilic and hydrophobic segments of the core and corona by changing the incorporation of the macro‐CTA as well as the monomer composition in the copolymers, as observed by Dynamic Light Scattering, Static Light Scattering, and Transmission Electron Microscopy analyses. The concept was further applied to a VAc derivative monomer, vinyl bromobutanoate, to incorporate further functionalities such as fluorescent dithiomaleimide groups throughout the polymer backbone using azidation and “click” chemistry as postpolymerization tools to create fluorescently labeled nanoparticles. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2699–2710  相似文献   
154.
Journal of Experimental and Theoretical Physics - A systematic nonperturbative scheme for calculating the ground state energy is adapted for studying systems of strongly correlated electrons on a...  相似文献   
155.
156.
The intermolecular interaction determines the photophysical properties of the organic aggregates, which are critical to the performance of organic photovoltaics. Here, excitonic coupling, an important intermolecular interaction in organic aggregates, between the π-stacking graphene quantum dots is studied by using transient absorption spectroscopy. We find that the spectral evolution of the ground state bleach arises from the dynamic variation of the excitonic coupling in the excited π-stacks. According to the spectral simulations, we demonstrate that the kinetics of the vibronic peak can be exploited as a probe to measure the dynamics of excitonic coupling in the excited π-stacks.  相似文献   
157.
158.
159.
160.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号