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   28949篇
  免费   5472篇
  国内免费   4558篇
化学   21339篇
晶体学   470篇
力学   1831篇
综合类   402篇
数学   3348篇
物理学   11589篇
  2024年   54篇
  2023年   470篇
  2022年   988篇
  2021年   1007篇
  2020年   1226篇
  2019年   1168篇
  2018年   1034篇
  2017年   1003篇
  2016年   1360篇
  2015年   1469篇
  2014年   1842篇
  2013年   2247篇
  2012年   2588篇
  2011年   2787篇
  2010年   2075篇
  2009年   2056篇
  2008年   2236篇
  2007年   1989篇
  2006年   1845篇
  2005年   1510篇
  2004年   1232篇
  2003年   945篇
  2002年   958篇
  2001年   837篇
  2000年   631篇
  1999年   544篇
  1998年   409篇
  1997年   413篇
  1996年   364篇
  1995年   295篇
  1994年   257篇
  1993年   201篇
  1992年   135篇
  1991年   153篇
  1990年   132篇
  1989年   88篇
  1988年   84篇
  1987年   74篇
  1986年   55篇
  1985年   44篇
  1984年   33篇
  1983年   31篇
  1982年   29篇
  1981年   18篇
  1980年   16篇
  1979年   7篇
  1976年   6篇
  1975年   8篇
  1959年   5篇
  1957年   5篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
952.
The mechanism‐oriented reaction design for the divergent synthesis of chiral molecules from simple starting materials is highly desirable. In this work, aromatic amide‐derived nonbiarylatropisomer/silver (silver/Xing‐Phos) complex was used to catalyze the Michael addition of glycine aldimino esters to chalcones and successfully applied to the subsequent cyclocondensation to afford substituted cis‐Δ(1)‐pyrroline derivatives with up to 98 % ee. Besides the inherent performance of the chiral Ag/Xing‐Phos catalyst system, it was found that the workup of such reactions played an important role for the stereoselective construction of stereodivergent Δ(1)‐pyrrolines, in which an epimerization of the cis‐Δ(1)‐pyrrolines to the trans‐isomers during was revealed.  相似文献   
953.
This letter presents a new method for continuous signal modeling. Firstly, the continuous signal can be represented as a function of the trigonometric functional extension (Fourier series). Fourier series of the signal are parameterized by the fundamental frequency and unknown parameters. Then, the gradient-based iterative identification algorithm is derived, for estimating parameters of the signal model with known and unknown frequencies, separately. Finally, the simulation results indicate that the proposed algorithm is effective.  相似文献   
954.
A facile and simple catalyst‐free protocol has been developed for the condensation of 1,2‐diketones with aromatic 1,2‐diamines in polyethylene glycol (PEG), providing quinoxaline derivatives in good yields. The important features of the methodology are broad substrates scope, simple workup, catalyst free, environmentally benign, and no requirement for metal catalysts. It is noteworthy that the cyclization reaction of 1,2‐diketones with aliphatic 1,2‐diamines is also conducted smoothly to afford pyrazines in good yields under the standard conditions. In addition, PEG could be recovered easily and was reused without evident loss in activity  相似文献   
955.
956.
957.
The study of the catalytic activity and activation mechanism of asymmetric uranyl‐salophens with α, β‐unsaturated aldehydes or α, β‐unsaturated ketones, is a research hotspot. In this paper, the complexes of the uranyl–salophen(U‐S) modified by unilateral benzene, coordinated with cyclohexenone, cyclopentenone and acrolein, were investigated using density functional theory calculations at the level of B3LYP/6‐311G(d, p) basis set. The results showed that the uranyl‐salophen(U‐S) weakened the large π bond between C = C and C = O of the α, β‐unsaturated aldehydes and ketones, making the unsaturated aldehydes and ketones activated. In addition, the molecular‐recognition selectivity of the asymmetrical uranyl‐salophen for cyclohexenone and cyclopentenone were much higher than for acrolein.  相似文献   
958.
959.
960.
The salt effects on molecular orientation at air/liquid methanol interface were investigated by the polarization-dependent sum frequency generation vibrational spectroscopy(SFG-VS). We clarified that the average tilting angle of the methyl group to be u = 308 58 at the air/pure methanol surface assuming a d-function orientational distribution. Upon the addition of 3 mol/L Na I, the methyl group tilts further away from the surface normal with a new u = 418 38. This orientational change does not explain the enhancement of the SFG-VS intensities when adding Na I, implying the number density of the methanol molecules with a net polar ordering in the surface region also changed with the Na I concentrations. These spectroscopic findings shed new light on the salt effects on the surfaces structures of the polar organic solutions. It was also shown that the accurate determination of the bulk refractive indices and Raman depolarization ratios for different salt concentrations is crucial to quantitatively interpret the SFG-VS data.  相似文献   
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