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   237篇
  免费   13篇
  国内免费   39篇
化学   252篇
晶体学   7篇
综合类   3篇
物理学   27篇
  2023年   2篇
  2022年   5篇
  2021年   7篇
  2020年   6篇
  2019年   4篇
  2018年   3篇
  2016年   6篇
  2015年   8篇
  2014年   12篇
  2013年   17篇
  2012年   15篇
  2011年   8篇
  2010年   6篇
  2009年   12篇
  2008年   11篇
  2007年   11篇
  2006年   15篇
  2005年   13篇
  2004年   13篇
  2003年   10篇
  2002年   11篇
  2001年   10篇
  2000年   9篇
  1999年   9篇
  1998年   11篇
  1997年   6篇
  1996年   5篇
  1995年   3篇
  1994年   5篇
  1993年   7篇
  1992年   8篇
  1991年   3篇
  1989年   3篇
  1988年   1篇
  1987年   1篇
  1986年   3篇
  1985年   1篇
  1982年   3篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1978年   1篇
  1977年   2篇
排序方式: 共有289条查询结果,搜索用时 15 毫秒
81.
The reactions of N-methylmorpholinium 4-aryl(hetaryl)-5-cyano-2-oxo-1,2,3,4-tetrahydropyridine-6-thiolates with malononitrile and acetone in ethanol afforded substituted tetrahydropyridothienopyridinones. In the absence of acetone, tetrahydropyridothiopyranopyridinones were isolated as the major reaction products. The latter were also synthesized independently by the reactions of the above-mentioned thiolates with 2-amino-1,1,3-tricyanopropene. The structure of 2,4-diamino-10-(2-chlorophenyl)-3-cyano-5-imino-8-oxo-7,8,9,10-tetrahydro-5H-pyrido[2",3":2,3]thiopyrano[4,5-b]pyridine was established by X-ray diffraction analysis.  相似文献   
82.
王秀荣  南照东  谭志诚 《中国化学》2006,24(10):1301-1304
Molar heat capacities of the pure samples of acetone,methanol and the azeotropic mixture composed of acetone,cyclohexane and methanol were measured by an adiabatic calorimeter from 78 to 320 K.The solid-solid andsolid-liquid phase transitions of the pure samples and the mixture were determined based on the curve of the heatcapacity with respect to temperature.The phase transitions took place at(126.16±0.68)and(178.96±1.47)K forthe sample of acetone,(157.79±0.95)and(175.93±0.95)K for methanol,which were corresponding to thesolid-solid and the solid-liquid phase transitions of the acetone and the methanol,respectively.And the phase tran-sitions occurred in the temperature ranges of 120 to 190 K and 278 to 280 K corresponding to the solid-solid andthe solid-liquid phase transitions of mixture of acetone,cyclohexane and methanol,respectively.The thermody-namic functions and the excess thermodynamic functions of the mixture relative to standard temperature of 298.15K were derived based on the relationships of the thermodynamic functions and the function of the measured heatcapacity with respect to temperature.  相似文献   
83.
A mild and efficient ligand‐free Suzuki‐type cross‐coupling reaction of benzoyl chlorides and arylboronic acids catalyzed by heterogeneous Pd/C was developed. Benzoyl chlorides undergo cross‐coupling with electronically diverse arylboronic acids to give biaryl ketones in excellent yield, under aqueous media and optimum temperature. The application of 3 mol% of 10 wt% Pd/C to the cross‐coupling delivers utmost efficiency, and could be reused up to many consecutive cycles without any loss in activity. This method proceeds under aqueous media and a recyclable catalytic system, offering an environmentally benign alternative to the existing protocols. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
84.
丙烯水合醚化反应过程中丙酮生成机理的研究   总被引:1,自引:0,他引:1  
 在与色-质联用仪相连的微反装置上,考察了Hβ沸石及改性的\r\nHβ沸石催化剂上丙烯水合醚化及异丙醇脱水的反应行为.丙烯水合醚\r\n化反应中有副产物丙酮生成,而异丙醇单独进料时产物中没有发现丙酮\r\n的生成.这表明丙酮并不是异丙醇氧化脱氢的结果,而是一个应予重视\r\n的不利于丙烯水合醚化的副反应.用同位素示踪反应对丙酮的生成机理\r\n进行了探讨,为研制抑制这一副反应的新催化剂提供了参考依据.  相似文献   
85.
2-(E-2-苯乙烯基)喹啉的合成   总被引:4,自引:1,他引:4  
杨定乔  曾和平  彭柳  吕芬 《合成化学》2002,10(2):154-155,160
以苯甲醛为原料在碱性条件下与丙酮缩合得苄叉丙酮(1),1与邻氨基苯甲醛发生Friedlaender缩合反应得到新的目标产物2-(E-2-苯乙烯基)喹啉(2),产率81%。  相似文献   
86.
本文提出了一种气相色谱-傅里叶变换红外光谱联用技术分析工业丙酮杂质成份的方法,该方法采用FFAP毛细管柱,柱长25m,内径0.25mm,膜厚0.33μm,占总面积1.16%的12种杂能够被分离和鉴定。  相似文献   
87.
助剂对L-脯氨酸催化直接不对称Aldol反应的影响   总被引:1,自引:0,他引:1  
在L-脯氨酸催化的芳香醛与丙酮直接不对称aldol反应体系中添加二醇或二酚类助剂可有效提高反应活性.助剂的使用降低了该反应体系中L-脯氨酸的用量.特别是以邻苯二酚为助剂时,最佳反应条件下,仅用5mol%L-脯氨酸与1mol%助剂催化2-硝基苯甲醛与丙酮直接不对称aldol反应,aldol产物产率高达90%,对映选择性为...  相似文献   
88.
1 INTRODUCTIONPolymolybdateanionshavebeenstudiedformanyyearsbecauseoftheirapplica tionincatalysischemistry〔1〕.Inthesynthesisoforganicpolymolybdatederivativesusingacetoneassolvent,wefoundthatthecondensationreactionofacetoneoccurredandthecondensationprod…  相似文献   
89.
A universal photoassisted pathway to functionalize polymeric surfaces is presented by transferring the inert surface sp3 C? H bonds into reactive groups, such as ? SO3H, ? NH2, ? SH, and ? COOH. The proposed method uses acetone as photoinitiator and different phenols with a para substituent XR as the reactants. Acetone excited by UV irradiation acts as a pair of scissors cutting both the surface C? H bonds of the polymer substrate and the O? H bonds of phenol, leading to the formation of carbon‐centered surface chain free radicals and oxygen‐centered phenoxy free radicals. By coupling of these two radicals, a variety of functional X groups with an R spacer from XR species of different phenol reactants were readily bonded to the polymeric surfaces, where phenol reactants included 4‐hydroxylbenzene sulfonic acid for ? SO3H, p‐aminophenol and tyramine for ? NH2, 4‐hydroxythiophenol for ? SH, and tyrosine for ? COOH. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011  相似文献   
90.
The visible absorption spectra of 4-(2-benzothiazolylazo)resorcinol (BTAR) and 4-(2-benzothiazolylazo)salicylic acid (BTAS) have been recorded in water-organic solvent mixtures in the pH range 0.5–12.0. The organic solvents used are methanol, ethanol,n-propanol, iso-propanol, acetone, dioxane and dimethyl formamide. The spectral changes have been explained in terms of shifts in equilibria among different molecular and ionic species of BTAR and BTAS existing in solution. The pK a values corresponding to the different ionization steps have been determined at 25°C and I=0.1M (KNO3) by graphic analysis of the absorbance-pH curves. The results obtained are discussed in terms of the molecular structure of the reagents and the nature of the organic co-solvent used.  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