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   4829篇
  免费   249篇
  国内免费   410篇
化学   3953篇
晶体学   82篇
力学   189篇
综合类   5篇
数学   12篇
物理学   1247篇
  2023年   21篇
  2022年   48篇
  2021年   56篇
  2020年   82篇
  2019年   67篇
  2018年   57篇
  2017年   54篇
  2016年   122篇
  2015年   130篇
  2014年   156篇
  2013年   358篇
  2012年   220篇
  2011年   200篇
  2010年   194篇
  2009年   252篇
  2008年   245篇
  2007年   322篇
  2006年   245篇
  2005年   276篇
  2004年   277篇
  2003年   279篇
  2002年   224篇
  2001年   138篇
  2000年   179篇
  1999年   164篇
  1998年   146篇
  1997年   118篇
  1996年   121篇
  1995年   138篇
  1994年   120篇
  1993年   75篇
  1992年   109篇
  1991年   40篇
  1990年   36篇
  1989年   35篇
  1988年   37篇
  1987年   22篇
  1986年   13篇
  1985年   12篇
  1984年   12篇
  1983年   7篇
  1981年   9篇
  1980年   15篇
  1979年   7篇
  1978年   9篇
  1977年   7篇
  1976年   8篇
  1975年   5篇
  1974年   5篇
  1973年   12篇
排序方式: 共有5488条查询结果,搜索用时 15 毫秒
41.
The root of Carlina acanthifolia All. (Asteraceae) contained 1.0% of essential oil (expressed in g per 100 g of dried plant material). Using GC and GC/MS, nine components were identified (100% of total oil). The structure of benzyl 2-furylacetylene (carlina oxide), which is the principal component of the oil (91.5%), was spectrometrically identified. __________ Published in Khimiya Prirodnykh Soedinenii, No. 4, pp. 331–332, July–August, 2005.  相似文献   
42.
The basicity of hydrazides of the highest aliphatic carboxylic acids RC(O)NHNH2 (R = CnH2n+1,n = 5-12) has been studied by potentiometric titration, and IR and1H NMR spectroscopy.Ab initio Hartree-Fork calculations using the 6–31G* basis set with full optimization of geometry were carried out on the simplest acy1hydrazines and their possible protonated forms. Based on these calculations, and the 1R and1H NMR spectra, the tautomerism of alkylhydrazides and the structures of their protonated forms are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2645–2649, November, 1996.  相似文献   
43.
Secondary phosphines 1-3 react readily with N-vinylpyrroles 4 and 5 under radical initiation to give regiospecifically anti-Markovnikov adducts, diorganyl-2-(1-pyrrolyl)ethylphosphines 6a-d, highly reactive building blocks for organic synthesis, in 88-91% yields.  相似文献   
44.
The 127I NQR, IR absorption and Raman spectra of impurity-doped and mixed lithium iodate Li1−xHxIO3 crystals grown from water solutions with different LiIO3/HIO3 ratios were investigated depending on the content of the impurity hydrogen x. The NQR results suggested that, at small concentration of doping iodic acid x<0.22, the lattice dynamics of the crystal grown from water solution changes significantly though the crystal retains hexagonal symmetry. Spectroscopic studies are compatible with average hexagonal symmetry of the grown doped crystals. From the results of Raman studies at room temperature and 100 K, the concentration range of hydrogen dopant 0.22<x<0.36 was found where disordered solid solution crystals Li1−xHxIO3 are formed.  相似文献   
45.
Ruthenium halides (Cl and Br) react with monotertiary arsines-Ph2RAs (R=Me, Et, Pr n ) in methoxyethanol, in the presence of aq. formaldehyde to give monocarbonyl complexes of ruthenium(II) of the type RuX2(CO) (Ph2RAs)3. Carbonylation of an ethanolic solution containing ruthenium trichloride and the arsine at room temperature yieldtrans dicarbonyl compounds of the formula RuCl2(CO)2 (Ph2RAs)2. The osmium monocarbonyls OsX2(CO) (Ph2RAs)3 (X=Cl, Br; R=Me, Et) react with NaBH4 in methanol to yield complexes of the composition OsHX(CO) (Ph2RAs)3. The ruthenium analogues RuHCl(CO) (Ph2RAs)3 have also been made. Structures have been assigned to all these compounds on the basis of IR and NMR spectral results.  相似文献   
46.
天然和人工培植红景天挥发油的对比研究   总被引:2,自引:0,他引:2  
应用GC-MS和GC-IR两种联机法对比研究了天然和人工培植红景天挥发油、分别鉴定出27个和24个化合物。MS给出结构信息和IR给出官能团信息相结合,提高了分析测试复杂样品的可靠性。  相似文献   
47.
以红外光谱法为主要手段,原位“跟踪”测定铁(Ⅲ)与乙二酸形成的配合物Na3Fe(C2O4)3.5H2O的热解过程,分析了气、固相热解产物。并以色谱分析、差热-热重分析和X-射线粉末衍射法定性、定量地验证其结果。测定了该配合物快原子轰击质谱,提出了断裂规律。铁的二元羧酸配合物的质谱尚属首见。  相似文献   
48.
The rare earth element 2,4,6-trimethylbenzoates were prepared as solids with the general formula Ln(C10 H11 O2 )3 ×n H2 O, where n =2 for Ln =Y, La–Nd, and n =1 for Ln =Sm–Lu. The IR spectra of the complexes prepared were recorded and their solubilities in water and thermal decomposition in the air were investigated. During heating the hydrated complexes lose all the crystallization water molecules in one (Y, Ce–Lu) or two steps (La) and then the anhydrous complexes decompose either directly to oxides (Y, Ce, Pr, Sm–Lu) or with intermediate formation oxocarbonates Ln2 O2 CO3 (La, Nd). The carboxylate groups in the complexes prepared act probably as mono- and bidentate. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
49.
用时间分辨富里叶红外发射谱研究了高振动激发态CO向C2H2的传能,得到了CO(v=1-3)各振动态布居及其随时间的变化。利用微分法解出弛豫微分方程组,获得CO(v=1-3)向C2H2的传能速率常数分别为:2.0±0.1,6.0±0.2和9.4±0.8(10^-13cm^3·molecule^-1·s^-1)。传能速率随着振动量子数的增加而迅速增加。CO的振动能应向C2H2的对称伸缩模v2近共振V-  相似文献   
50.
A new ammonium indium phosphate (NH4)In(OH)PO4 was prepared by hydrothermal reaction in the In2O3-NH4H2PO4-NH3/OH system (T=200°C, autogenous pressure, 7 days). The formula (NH4)In(OH)PO4 was determined on the basis of chemical and thermal analysis (TG/DSC), X-ray powder diffraction and IR-spectroscopy. (NH4)In(OH)PO4 crystallizes in the tetragonal system with space group P43212 (No. 96); a=9.4232(1) Å, c=11.1766(1) Å, V=992.45(2) Å3; Z=8. The crystal structure was refined by the Rietveld method (Rw=6.35%, Rp=5.10%). The second-harmonic generation study confirmed that structure of (NH4)In(OH)PO4 does not have a center of symmetry. The cis-InO4(OH)2 octahedra form helical chains, parallel to the c-axis. The In-O-In bonds are nearly equidistant. The chains are interconnected by phosphate tetrahedra and create tunnels containing the NH4+ ions along the c-axis. (NH4)In(OH)PO4 is isostructural with RbIn(OH)PO4.  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