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   44282篇
  免费   11220篇
  国内免费   2700篇
化学   47444篇
晶体学   498篇
力学   1076篇
综合类   147篇
数学   2923篇
物理学   6114篇
  2024年   57篇
  2023年   267篇
  2022年   454篇
  2021年   608篇
  2020年   1740篇
  2019年   3000篇
  2018年   1430篇
  2017年   1026篇
  2016年   3853篇
  2015年   4021篇
  2014年   4088篇
  2013年   4811篇
  2012年   3773篇
  2011年   2991篇
  2010年   3446篇
  2009年   3319篇
  2008年   2928篇
  2007年   2237篇
  2006年   1856篇
  2005年   1942篇
  2004年   1618篇
  2003年   1519篇
  2002年   2160篇
  2001年   1498篇
  2000年   1434篇
  1999年   530篇
  1998年   179篇
  1997年   169篇
  1996年   163篇
  1995年   133篇
  1994年   122篇
  1993年   109篇
  1992年   94篇
  1991年   76篇
  1990年   86篇
  1989年   56篇
  1988年   58篇
  1987年   46篇
  1986年   40篇
  1985年   34篇
  1984年   25篇
  1983年   19篇
  1982年   24篇
  1981年   16篇
  1980年   11篇
  1979年   12篇
  1975年   14篇
  1974年   13篇
  1972年   14篇
  1970年   17篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The Raman spectra of the p-polyphenyls: benzene, biphenyl, p-terphenyl, and p-quaterphenyl, are compared with their electronic spectra. The relative intensity of the double bond stretching mode around 1600 Kaisers to that of the breathing mode around 1000K increases steadily when the phenyl chain is lengthened. This intensity ratio can be calculated using our theory proposed previously.1 The results are in very good agreement with the experimental observations  相似文献   
992.
The Raman spectra of powdered biphenyl, p-terphenyl, and p-quaterphenyl are compared with each other and also with the Raman spectrum of benzene. Most Raman lines in polyphenyls can be considered as derived from normal vibrations in benzene. The intense lines are thus assigned and the correlations between them are established.  相似文献   
993.
3-[1′(1′H)-Substituent-pyrazol-5′-yl]benzo[5,6]coumarins and 3-(1′,2′-oxazol-5′-yl)benzo[5,6]coumarin were prepared via condensation of 3-(2′-formyl-1′-chlorovinyl)benzo[5,6] coumarin with hydrazine derivatives or hydroxylamine.Reaction of 3-[1′(1′H)-pyrazol-5′-yl]benzo[5,6]coumarin with alkyl halides,olefinic compunds or acid chlorides are described.  相似文献   
994.
The title compound, [5,10,15,20‐meso‐tetrakis(4‐hydroxy­phenyl)­porphyrinato‐κ4N]­zinc(II) tris(dibenzo‐24‐crown‐8) methanol tetrasolvate, [Zn(C44H28N4O4)]·3C24H32O8·4CH4O, was synthesized and its molecular structure precisely charac­terized by low‐temperature single‐crystal analysis. All the components are involved in hydrogen bonding with each other, thus forming an extensively hydrogen‐bonded supramolecular lattice. The functionalized porphyrin moiety coordinates both equatorially and axially to the neighboring species.  相似文献   
995.
A single-walled carbon nanotube (SWNT)-modified electrode was fabricated and characterized by SEM and ac impedance techniques. The direct electrochemistry of cytochrome c (Cyt c), which was adsorbed on the surface of the SWNT, was studied by cyclic voltammetry. The results from cyclic voltammetry and infrared spectroscopy indicated that Cyt c remained in its original structure and did not undergo structural change after its immobilization on the SWNT. Further results demonstrated that the SWNT had promotional effects on the direct electron transfer of Cyt c and also indicated that the immobilized Cyt c retained its electrocatalytic activity to the reduction of H2O2. This modified electrode might be used in development of new biosensors and the biofuel cells.  相似文献   
996.
997.
Synthesis of poly(thiocarbonate)s from the copolymerization of epoxides and carbon disulfide (CS2) remains a tough challenge, due to inevitable oxygen‐sulfur atom scrambling process. In this work, we utilized the oxygen‐sulfur exchange reaction (O‐S ER) to synthesize a random copolymer with monothiocarbonate and trithiocarbonate units from CS2 and phenyl glycidyl ether (PGE) via metal‐free Lewis pairs. The copolymers contained monothiocarbonate and trithiocarbonate units of which the molar fraction could be tuned by varying either the types of Lewis pairs or the reaction temperature. Keeping track on the intermediate provided an insight in the process of O‐S ER and thus gave a hint to control the product structure. Production and consumption of phenyl thioglycidyl ether was the key process to regulate the chain structure. Remarkably, the oxygen atoms of PGE could be excluded out of the chain, resulting in the nearly complete production of poly(trithiocarbonate)s. Correspondingly, the refractive index of this kind of copolymer could be regulated in a wide range of 1.73—1.79 (at 590 nm).  相似文献   
998.
研究了几种小孔分子筛ZSM-5,HY,NaY及β担载ZrO2对C加氢反应的催化作用,并就产物的分布情况与担载ZrO2的硅铝中孔分子筛进行了对比研究。其中中孔分子筛分别用季铵盐和一级胺为模板剂,在水热条件和常温常压下合成;并用BET,XRD,SEM等手段表征了它们的孔道和骨架结构。另外,考察了碱金属K^2+,Na^2+对催化剂催化性能的影响。  相似文献   
999.
Three new lanostane‐type triterpenoids, inonotsulides A, B, and C ( 1 – 3 , resp.) were isolated from the sclerotia of Inonotus obliquus (Pers .: Fr.) (Japanese name: Kabanoanatake; Russian name: Chaga). Their structures were determined to be (20R,24S)‐3β,25‐dihydroxylanost‐8‐en‐20,24‐olide ( 1 ), (20R,24R)‐3β,25‐dihydroxylanost‐8‐en‐20,24‐olide ( 2 ), and (20R,24S)‐3β,25‐dihydroxylanosta‐7,9(11)‐dien‐20,24‐olide ( 3 ) on the basis of chemical transformation, NMR spectroscopy including 1D and 2D (1H,1H‐COSY, NOESY, HMQC, HMBC), EI‐MS, and single‐crystal X‐ray analysis.  相似文献   
1000.
A novel type of phosphated puerarin derivatives were synthesized through a simplified Atheron‐Todd reaction for the first time. The structure of these compounds were elucidated by IR, ESI‐MS and NMR. Moreover, the reason the dialkylphophite reagent had different chemselectivities toward different hydroxys on the puerarin was discussed.  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