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   5155篇
  免费   112篇
  国内免费   11篇
化学   3615篇
晶体学   66篇
力学   113篇
数学   383篇
物理学   1101篇
  2022年   43篇
  2021年   54篇
  2020年   54篇
  2019年   64篇
  2016年   100篇
  2015年   92篇
  2014年   105篇
  2013年   199篇
  2012年   157篇
  2011年   223篇
  2010年   148篇
  2009年   119篇
  2008年   188篇
  2007年   182篇
  2006年   179篇
  2005年   168篇
  2004年   150篇
  2003年   131篇
  2002年   113篇
  2001年   88篇
  2000年   89篇
  1999年   88篇
  1998年   63篇
  1997年   78篇
  1996年   87篇
  1995年   86篇
  1994年   79篇
  1993年   104篇
  1992年   66篇
  1991年   56篇
  1990年   59篇
  1989年   67篇
  1988年   43篇
  1987年   61篇
  1986年   61篇
  1985年   61篇
  1984年   61篇
  1983年   48篇
  1982年   54篇
  1981年   63篇
  1980年   66篇
  1979年   51篇
  1978年   64篇
  1977年   41篇
  1976年   70篇
  1975年   63篇
  1974年   52篇
  1973年   65篇
  1972年   35篇
  1967年   44篇
排序方式: 共有5278条查询结果,搜索用时 15 毫秒
31.
In order to test the method of tritium determination and to determine the tritium activity, foods of known origin were investigated. The accuracy of the method over a prolonged period of time is confirmed by the statistical evaluation of the background values and of the counting efficiency. An average value of 600–800 pCi of tritium in one litre of water was found in milk, potatoes and apples. Drinking water contained very different activities of tritium, depending on its origin as ground or surface water. Food samples from the environment of nuclear reactors were not different in their tritium content when compared to those of other origin.  相似文献   
32.
The ground and excited state properties of two regions in the Δ15-configurational space of the phycoviolobilin chromophore in the α-subunit of phycoerythrocyanin are analyzed. Molecular dynamics calculations reveal that the chromophore geometry determines the active-site dynamics. The excited state torsional potential surface shows a negative barrier for isomerization and trapping of an activated complex. Strong coupling of excited states localized in the chromophore and charge transfer states from the surrounding polar residues provides favorable prerequisites for fast excited state surface crossing in competition with other deactivation processes. The formation of a photoreduced intermediate following the photoinduced charge transfer may trigger subsequent chemical reactions.  相似文献   
33.
Methylene Compounds of Non-Metals. VI. Amides of Methane-bis(phosphonous Acid), -bis(phosphonic Acid) and -bis(thiophosphonic Acid) Symmetrical- and asymmetrical-substituted amides of methane-bis(phosphonous acid), -bis(phosphonic acid) and -bis(thiophosphonic acid) have been obtained by stepwise aminolysis of the chlorides Cl2(Z)PCH2P(Z)Cl2 ( 1 : Z = electron pair; 7 : Z = S; 13 : Z = O) and the fluoride F2(S)PCH2P(S)F2 19 with dimethylaminotrimethylsilane. Oxidation reactions of trivalent phosphonous amides with sulfur and dimethylsulfoxide have also been investigated.  相似文献   
34.
The reaction of N,N′-dimethyloxamide with trialkyl derivatives of aluminium, gallium, and indium yields bis(dialkylmetal) compounds of structural formula (R2M)2[O2C2(NCH3)2] (M = Al, Ga, In; and R = CH3, C2H5). The M2O2C2N2 skeleton of these monomeric products forms an almost planar system of two fused five-membered rings, with S2 symmetry. For the dimethylgallium and dimethylindium derivatives, 1H and 13C NMR spectra show the presence of two conformational isomers which differ in the orientation of the N-methyl relative to the two metal-bound CH3 groups.  相似文献   
35.
Free radicals are generated in liquid solutions by harmonically modulated photolysis of suitable substrates. Harmonic analysis of the absorbance as functions of wavelength and modulation frequency yields the optical spectra and the decay kinetics of the transient species. The experimental technique and the analysis are described in detail. Results on t-butyl, 2-propyl and benzyl radicals generated by photolysis of the corresponding dialkyl resp. dibenzyl ketones are reported. They confirm previous spectral assignments and show that the termination reactions are diffusion controlled.  相似文献   
36.
Muonium-substituted organic free radicals are observed by muon spin rotation when positive muons are stopped in liquid unsaturated compounds. From muon precession frequencies in high external magnetic fields the isotropic muon-electron hyperfine coupling constants are determined. They lead to radical structure assignments. Results of a variety of projects are described which show thatSR can successfully complement conventional physical methods of free radical chemistry: A study of the temperature dependence of the coupling constants of isotopically substituted ethyl radicals yields information on structure and barriers to internal rotation. Rate constants for several radical reactions are extracted from the damping of theSR signals, in particular for unimolecular rearrangements and cis-trans-isomerizations. The theory for the analysis for the case of reversible site exchange is outlined.All results described here were obtained by E. Roduner, P. Burkhard and W. Strub of this laboratory, and B. Webster, M. Ramos and D. McKenna of the University of Glasgow. We all appreciate support from the Swiss National Foundation for Scientific Research, the National Institut for Scientific Investigations of Portugal, the Carnegie Trust for the Universities of Scotland, the Royal Society London and SIN.  相似文献   
37.
Thin layers of basic zinc carbonate can be used with good results for separation and multiple identification of carbamate and phenylurea pesticides. Five important substances could be separated by one-dimension TLC. The best results were obtained by the following developing solvents: a) benzene, b) a mixture of benzene/petroleum ether/chloroform (6∶1∶1). The substances are either recognisable in short-wave UV-light, when a fluorescence indicator (F 254 nm) is added, or in day-light, after spraying a solution of 0.1 N AgNO3 in 3 N HNO3, followed by an UV-exposure for about 4 min.  相似文献   
38.
A convenient method is described for the synthesis of functionalized spiro[cycloalkanono-2,3-thiophenes] by treatment of cyclic 3-oxoacid thioanilides with -nitrostyrenes. Reaction of the obtained products with acetic anhydride yielded the corresponding oxime acetates.  相似文献   
39.
The rates of substitution of the group X in 1-X-2-naphthol-6-sulfonic acids (X = H, Cl, Br, and I) by p-chlorobenzenediazonium ions in aqueous solution have been measured. The rates of the halogenated naphthols relative to that of the parent compound (X = H) are 0.0070:0.0089:0.149 for X = Cl, Br, and I respectively. The reaction of 1-bromo-2-naphthol-6-sulfonic acid is catalysed by thiosulfate ions; the relative rate observed for this compound does not, therefore, represent the ipso factor. It is postulated that in its substitution the release of the electrofugal leaving group (Br) is rate-limiting.  相似文献   
40.
A first infrared pulse at frequency ν1 interacts with vibrational states in S0 and a second visible pulse at ν2 promotes the excited molecules to the S1 state from where they fluoresce. Tuning the frequency ν2 over 600 cm?1 allows the observation of a detailed spectrum which gives information on vibrational states in S0 and on vibronic states in S1 together with corresponding Franck—Condon factors. The spectra differ drastically from the common broad and featureless absorption and fluorescence bands.  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