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   9509篇
  免费   368篇
  国内免费   1232篇
化学   10259篇
晶体学   108篇
力学   16篇
综合类   33篇
数学   141篇
物理学   552篇
  2024年   9篇
  2023年   126篇
  2022年   89篇
  2021年   219篇
  2020年   314篇
  2019年   266篇
  2018年   232篇
  2017年   296篇
  2016年   255篇
  2015年   221篇
  2014年   445篇
  2013年   1112篇
  2012年   447篇
  2011年   468篇
  2010年   395篇
  2009年   477篇
  2008年   574篇
  2007年   649篇
  2006年   604篇
  2005年   619篇
  2004年   575篇
  2003年   446篇
  2002年   301篇
  2001年   270篇
  2000年   238篇
  1999年   233篇
  1998年   181篇
  1997年   114篇
  1996年   128篇
  1995年   118篇
  1994年   155篇
  1993年   119篇
  1992年   94篇
  1991年   66篇
  1990年   29篇
  1989年   35篇
  1988年   35篇
  1987年   27篇
  1986年   18篇
  1985年   17篇
  1984年   23篇
  1983年   5篇
  1982年   18篇
  1981年   15篇
  1980年   6篇
  1979年   13篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
  1973年   3篇
排序方式: 共有10000条查询结果,搜索用时 183 毫秒
101.
The reaction of 2,3-dihydro-2,3-benzo[b]furandione with the triphenylphosphazines of diazo compounds leads to substituted 2-methylenehydrazono-2,3-dihydro-3-benzo[b]furanones, which are hydrolyzed in an acidic medium to the substituted methylenehydrazides of o-hydroxyphenylglyoxalic acid and react with amines to form the products of addition at the activated CH=N bond of the side chain or 2-hydrazono-2,3-dihydro-3-benzo[b]furanone respectively. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 817–822, June, 2007.  相似文献   
102.
Summary.  Reactions of cyanomethanesulfonamides with aromatic aldehydes in the presence of AcOH and piperidine produced the addition products, the 1-cyano-2-arylethenesulfonamides, whereas reactions with benzonitrile yielded the 2-amino-1-cyano-2-phenylethenesulfonamides only when done in THF with BuLi. No addition products were isolated from the analogue reactions with 2-hydroxybenzaldehyde (salicylaldehyde). Instead, we obtained 2-imino-2H-chromene-3-sulfonamides with good to excellent yields. These 2H-chromene derivatives allowed a number of transformations, from which the reactions with orthoformates opened an approach to the hitherto unknown benzopyrano[3,2-e] [1,2,4]thiadiazine ring system.  相似文献   
103.
8-R-1-Aryl-4,4,6-trimethyl-4H-pyrrolo[3,2,1-ij]quinolines have been synthesized by the Bischler-Melau reaction in the series of 6-R-2,2,4-trimethylhydroquinolines.  相似文献   
104.
N-[2(3)-Hydroxyalkylj-4-thiazolidinones have been synthesized by the reaction of 2(3)-vinyloxyalkylamines with mercaptoacetic acid in 24–69 % yield. The structure of the compounds obtained was supported by (R and1H NMR spectroscopic data.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 677–679, March, 1996.  相似文献   
105.
Single crystals of Y5Re2O12 have been grown, and the crystal structure has been determined by X-ray diffraction. This compound crystallizes in space group C2/m with cell dimensions of a=12.4081(10) Å b=5.6604(5)Å, c=7.4951(6) Å, β=107.837(3)°, Z=2. The final refinement led to R1=0.0238, WR2=0.0459 for 1053 observed reflections with F>4σ(F0). Edge-sharing ReO6 octahedra form infinite linear [ReO2O4/2]n chains along the b direction with alternating short and long Re-Re distances. Three crystallographically independent yttrium atoms surround O2 to form OY4 tetrahedra, which share edges and corners in the ab plane to form a two-dimensional Y5O4 network which separates the [ReO2O4/2]n magnetic chains. This compound is therefore isostructural with the series Ln5Re2O12Ln=Gd-Lu, which have been known since 1969. The average Re oxidation state is +4.5 in the chains and a reasonable, if qualitative MO scheme results in one unpaired electron per Re dimer. Consistent with this, magnetic susceptibility data can be fitted to the one-dimensional antiferromagnetic Heisenberg model with S=1/2 and parameters Jintra/k=−89(1)K, g=2.15(4) and χ(TIP)=5(1)×10−4 emu/mol. There is no sign of long-range magnetic order down to 2 K. These results are contrasted with those for the isostructural Y5Mo2O12.  相似文献   
106.
在离子液体中一步法合成吡唑[5,4-b]-γ-吡喃衍生物   总被引:1,自引:0,他引:1  
芳醛、丙二腈与4,5-二氢-3-甲基-5-氧代-1-苯基吡唑在离子液体[bmim][BF4]中反应, 合成了2-氨基-4-芳基-3-氰基-5-甲基-7-苯基吡唑[5,4-b]-γ-吡喃衍生物, 该法快速、高效, 是一种洁净的合成方法. 产物的结构通过红外光谱、核磁共振氢谱和元素分析进行表征.  相似文献   
107.
The title compound (C40H42N2) has been synthesized by the reaction of 1-(1-naphthyl)-1-phenyl-3-chloropropylene and bis(4-(diethylamino) phenyl)methanone, and characterized by IR, 1H NMR, MS and X-ray diffraction analysis. The crystal belongs to the monoclinic system, space group P21/c with a = 17.047(3), b = 10.807(2), c = 1&494(4) (A), β = 105.727(4)°, V=3279.4(11) A3, Mr = 550.76, Z = 4, Dc = 1.115 g/cm3, μ(MoKα) = 0.085 mm-1, F(000) = 1184, the final R = 0.0625 and wR = 0.1384 for 2276 observed reflections (I > 2σ(Ⅰ)). X-ray analysis reveals that the butadiene fragment adopts a planar cisiod conformation and makes a dihedral angel of 69.4(2)° with the naphthalene ring.  相似文献   
108.
将取代色满酮(1)与芳肼反应生成的腙与HNCO发生[3+2]环加成反应,加成产物(2)经氧化得到偕偶氮异氰酸酯(3).化合物3在HBF4的催化下发生环化-重排反应,得到新颖的三环系2-氧代-1,2,4-三唑并[3,2-d][1,5]苯并氧氮杂化合物5a~5g.  相似文献   
109.
A concise synthesis of highly functionalized 5,6-dihydrobenzo[h]quinoline-3-carbonitriles is delineated through base induced ring transformation of 4-sec-amino-2-oxo-5,6-dihydro-2H-benzo[h]chromene-3-carbonitriles with S-methylisothiourea sulfate and 1-carboxamidinepyrazole hydrochloride, separately, in DMF. Under analogous reaction conditions the ring transformation of 4-sec-amino-2-oxo-5,6-dihydro-2H-benzo[h]chromene-3-carbonitriles by formamidine acetate provided 4-sec-amino-benzo[h]quinoline-3-carbonitriles in moderate yields, while with benzamidine hydrochloride, the reaction followed the same mechanism to yield 2-phenyl-4-sec-amino-5,6-dihydrobenzo[h]quinoline-3-carbonitriles.  相似文献   
110.
A facile and short synthesis of 2-oxo-4-sec-amino-2,3,5,6-tetrahydrobenzo[f]isoquinoline-1-carbonitriles has been delineated through base catalyzed ring transformation of 5,6-dihydro-2-oxo-4-sec-amino-2H-benzo[h]chromene-3-carbonitrile by cyanoacetamide in excellent yields.  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