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   1985篇
  免费   63篇
  国内免费   11篇
化学   1484篇
晶体学   29篇
力学   67篇
数学   127篇
物理学   352篇
  2023年   8篇
  2022年   29篇
  2021年   54篇
  2020年   41篇
  2019年   47篇
  2018年   46篇
  2017年   48篇
  2016年   89篇
  2015年   65篇
  2014年   118篇
  2013年   165篇
  2012年   143篇
  2011年   162篇
  2010年   90篇
  2009年   67篇
  2008年   117篇
  2007年   121篇
  2006年   95篇
  2005年   90篇
  2004年   54篇
  2003年   49篇
  2002年   47篇
  2001年   16篇
  2000年   18篇
  1999年   11篇
  1998年   20篇
  1997年   13篇
  1996年   17篇
  1995年   16篇
  1994年   13篇
  1993年   9篇
  1992年   7篇
  1991年   13篇
  1990年   6篇
  1989年   5篇
  1988年   6篇
  1987年   9篇
  1985年   13篇
  1984年   16篇
  1983年   4篇
  1982年   7篇
  1981年   8篇
  1980年   9篇
  1979年   16篇
  1978年   8篇
  1977年   5篇
  1976年   8篇
  1975年   6篇
  1973年   6篇
  1964年   5篇
排序方式: 共有2059条查询结果,搜索用时 15 毫秒
91.
Abstract

A simple, efficient, and environmentally benign methodology has been accomplished for the synthesis of α-aminophosphonates by one-pot three-component reaction of ethyl-2-amino-α-(methoxy-imino)-4-thiazoleacetate, aldehydes, and diethylphosphite by using Amberlyst-15 as catalyst at room temperature under solvent-free conditions. Their chemical structures were characterized by infrared (IR), NMR (1H, 13C & 31P), mass spectral, and elemental analysis. All the title compounds were screened for radical-scavenging activity by 1,1-diphenyl-2-picrylhydrazyl (DPPH), nitic oxide (NO), and hydrogen peroxide (H2O2) methods.

Supplementary materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements for the following free supplemental files: Additional text and figures.  相似文献   
92.
Pralidoxime chloride known as 2-PAM chloride is used as antidote for nerve agent’s poisoning. This study was undertaken to establish the compatibility of 2-PAM chloride with a number of commonly used excipients by using thermoanalytical technique viz., differential scanning calorimetry (DSC) and thermogravimetry/differential thermogravimetry (TG/DTG) used in pharmaceutical formulation. The TG and DSC both results demonstrated that polyvinyl alcohol, polyacrylamide, microcrestline cellulose, hydroxypropyl cellulose, cellulose acetate, ethyl cellulose found to be compatible with 2-PAM chloride and chosen for the preparation of antidote against chemical warfare agents.  相似文献   
93.
In green chemistry, biocatalysis using microwaves is a very attractive tool for various regioselective syntheses. L-Ascorbyl fatty acid esters were synthesized by an immobilized lipase from Bulkholderia multivorans using microwaves, with a dynamically enhanced rate of reaction and appreciable yield of around 80%. Microwave radiation had no effect on enzymic inactivation, however, synergism between microwaves and biocatalyst was observed.  相似文献   
94.
A facile method for the synthesis of (2R,3R)‐1,4‐dimethoxy‐1,1,4,4‐tetrasubstituted‐2,3‐butanediols involving oxidative cleavage of benzylidene acetal as a key step is described. These sterically hindered diols unusually formed cyclic sulfites as the major product under methanesulfonylation reaction conditions.  相似文献   
95.
The present work introduces for the first time the catalytic utility of micron particulate aluminium nitride (AlN/Al) as a novel and reusable heterogeneous catalyst for the synthesis of bis(indolyl)methanes involving the electrophilic substitution of indoles on aldehydes. The catalyst is recyclable and works without any significant change in its catalytic activity.  相似文献   
96.
A mild and efficient synthesis of bis(indolyl) methanes by the reaction of indoles with various aldehydes at room temperature in the presence of a catalytic amount of a La(NO3)3 · 6H2O afforded the corresponding bis(indolyl) methanes in excellent yields under solvent‐free conditions.  相似文献   
97.
One-pot three component synthesis of novel pyrido[1,4]-oxazepine fused spiropyrrolidines has been accomplished in good yields by 1,3-dipolar cycloaddition reaction of azomethine ylide, derived from paraformaldehyde and sarcosine with (E)-4-benzylidene-hexahydro-1H-pyrido[2,1-c][1,4]-oxazepine-3(7H)-one as dipolarophiles derived from Baylis-Hillman adducts. The effect of solvent on the [3 + 2]-cycloaddition reaction is also studied.  相似文献   
98.
A rapid method for selective cleavage of aryl methylethers can be achieved in the presence of a protic acid and a catalytic amount of phase-transfer catalyst (Aliquat-336). Aliquat-336 accelerates the rate of reaction and affords the corresponding phenols in excellent to good yields on a wide variety of substrates.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]  相似文献   
99.
An efficient superparamagnetic Pd–ZnFe2O4 solid catalyst has been synthesized by loading Pd(0) species on zinc ferrite nanoparticles. Sonogashira cross couplings between terminal alkynes and aryl halides were achieved in the absence of any Cu co-catalyst. A Heck–Matsuda coupling reaction of structurally different aryldiazonium tetrafluoroborate substrates was preceded at 40 °C in water. Cyanation of aryl halides was successfully done using K4[Fe(CN)6] as the cyanide source over Pd–ZnFe2O4. The catalyst was also employed for Ullmann type cross coupling reactions. Excellent yield of the products, reusability, and uncomplicated work-up make this catalyst efficient for C–C and C–O coupling reactions. Good yield of products, easy separation, and negligible leaching of Pd from the catalyst surface confirm the true heterogeneity in these catalytic reactions.  相似文献   
100.
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