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   4154篇
  免费   585篇
  国内免费   481篇
化学   2702篇
晶体学   51篇
力学   77篇
综合类   13篇
数学   14篇
物理学   2363篇
  2024年   5篇
  2023年   65篇
  2022年   98篇
  2021年   138篇
  2020年   133篇
  2019年   77篇
  2018年   154篇
  2017年   192篇
  2016年   198篇
  2015年   110篇
  2014年   271篇
  2013年   708篇
  2012年   276篇
  2011年   248篇
  2010年   207篇
  2009年   251篇
  2008年   267篇
  2007年   301篇
  2006年   224篇
  2005年   207篇
  2004年   184篇
  2003年   166篇
  2002年   149篇
  2001年   78篇
  2000年   89篇
  1999年   72篇
  1998年   97篇
  1997年   44篇
  1996年   32篇
  1995年   40篇
  1994年   23篇
  1993年   38篇
  1992年   11篇
  1991年   12篇
  1990年   13篇
  1989年   8篇
  1988年   5篇
  1987年   3篇
  1986年   3篇
  1985年   4篇
  1984年   5篇
  1983年   3篇
  1982年   1篇
  1981年   2篇
  1980年   1篇
  1979年   2篇
  1978年   1篇
  1977年   2篇
  1974年   1篇
  1973年   1篇
排序方式: 共有5220条查询结果,搜索用时 31 毫秒
71.
1 INTRODUCTION 1,4-Dihydropyridines (1,4-DHPs) are well-knowncompounds because of their pharmacological pro-perty as calcium channel modulators[1, . The che- 2]mical modifications, such as the introduction of dif-ferent substituents[3, or heteroatoms[5], of the DHP 4]ring have allowed the expansion of researches onthe structure-activity relationship to gain new in-sights into the molecular interactions at the receptorlevel. In fact, it is well established th…  相似文献   
72.
Low-temperature synthesis of allyl dimethylamine (ADA) by selective heating under microwave irradiation (MI) used for water treatment is investigated. The effect of MI, ultrasound irradiation (UI) and conventional heating on yield of ADA, reaction time and the flocculation efficiency of polydiallyl dimethylammunion chloride (PDADMAC) prepared form ADA were studied. The results show that by selective heating at low temperature, MI not only increases yield of ADA and reduces reaction time, but also greatly enhances the flocculation efficiency of PDADMAC.  相似文献   
73.
微波干法催化芳香醛与绕丹宁及N-苯基绕丹宁的缩合反应   总被引:1,自引:0,他引:1  
利用微波辐射和固体碱性载体试剂催化绕丹宁及N-苯基绕丹宁和芳香醛的缩合反应合成了14个绕丹宁衍生物并研究了影响反应的因素。  相似文献   
74.
在微波促进无溶剂条件下, 由N-甲基-2-甲基吡啶碘盐与取代芳香醛在哌啶作用下合成了一系列2-取代的吡啶半菁染料, 产物结构由元素分析, 1H NMR, MS, IR, UV得到确认. 该方法操作简便, 收率高. 在此基础上, 对N-甲 基-2-(p-N,N-二甲氨基-苯乙烯基)吡啶半菁染料的光谱性质进行了理论计算.  相似文献   
75.
手性芳酰胺类分子钳的设计与微波合成   总被引:3,自引:0,他引:3  
以间苯二甲酸为隔离基, 酰胺键桥联L-氨基酸甲酯构成手臂, 在微波辐射下合成了具有不同手性中心和裂穴的新型手性芳酰胺类分子钳. 结构均经1H NMR, IR及元素分析确证, 并考察了其对芳香胺类化合物和D/L-氨基酸甲酯的识别性能. 初步研究表明, 这类分子钳受体不仅对中性小分子具有优良的识别性能, 其结合常数(Ka)可达2.66×103 L•mol-1, 而且对D/L-氨基酸甲酯亦具有良好的手性识别能力.  相似文献   
76.
The development of analytical methods for routine simultaneous identification and quantification of carboxylic fatty acids (CFAs) are required in different fields, such as, pharmaceutical cosmetics, food products and formulations of water–microemulsion–oil systems. Determination of CFAs has been developed mainly by gas chromatography (GC). As an alternative to GC, liquid chromatography (LC) has better sensitivity and selectivity. However, most CFAs show no useful absorption in ultraviolet–violet (UV–Vis) region, one of the more used detection technique in high-performance liquid chromatography (HPLC). In order to allow the use of UV–Vis detection, the use of pre-column derivatization has been reported to increase sensitivity and selectivity. Therefore, establishment of a simpler and faster on-line method with complete separation is needed for the screening of large numbers of samples. 2,4-Dinitrophenylhydrazine (2,4-DNPH.), benzoil chloride (BC), and phenylhydrazine (PH) were used for derivatization of different FAs by microwaves radiation (MW). After the on-line derivatization, products were separated and quantified by HPLC. Reactor coil was placed inside of microwaves oven at 450 W. Parameters as flow, amount of reagents, irradiation time, and chromatographic conditions were optimized. The continuous analysis using the MW–HPLC–UV system provided high sensitivity and reduced both the amount of reagent used and the analysis times. This proposed method can be used for the routine analysis of FAs contained in water–microemulsion–oil systems, to quantify the total acid fraction in each phase.  相似文献   
77.
讨论了微波辐照下 ,以丙酮 水为分散介质 ,利用阳离子型自由基引发剂偶氮二异丁基脒盐酸盐(AIBA)引发苯乙烯 (St)和甲基丙烯酸甲酯 (MMA)共聚 ,合成出表面带正电荷的P(St MMA)共聚物纳米粒子 ,考察了丙酮用量、单体和引发剂浓度对纳米粒子粒径、粒径分布和乳液稳定性的影响 .结果表明 ,丙酮 水的体积比由 0增加到 1 2 6∶1时 ,粒子的平均水化半径从 12 2 2 1nm降低到 2 4 6 8nm ,粒径分布变宽 ,乳液抗电解质稳定性逐渐增强 ;增加引发剂和共聚单体MMA的浓度 ,粒子的水化半径逐渐减小 ,粒径分散系数增大 .  相似文献   
78.
1 INTRODUCTION aryl or methyl group to the nitrogen atom of these compounds leads to enhanced activities of fluo- A lot of natural and synthetic compounds con- rescence[7, . Since the pyridine scaffold is included 8] taining acridine skeletons display interesting biolo- in decahydroacridine-1,8-dione, a question is whether gical and physical activities[1]. Acridinedione, for the same modification on the nitrogen atom of de- example, has been identified as antimalarial and cahydroacridine-1…  相似文献   
79.
A rapid and easy route to formamides by microwave assisted N-formylation of primary and secondary amines is described. Using an insoluble polymer or an inorganic solid-supported reagent as a formylating agent, microwave irradiation furnished the corresponding formamides in high yields, with reduced reaction time and solvent volume over the conventional approach.  相似文献   
80.
A series of new diethers were obtained by alkylation of furoin under microwave irradiation (MWI) in phase transfer catalysis (PTC) conditions. The products of alkyl halides were synthesized in good yields (>75%) within a few minutes, and the products of dihalides were synthesized in fair yields (about 45%). The yields are dramatically improved compared to conventional heating under the same conditions, in spite of similar profiles of rising in temperature.  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