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   1796篇
  免费   74篇
  国内免费   5篇
化学   1317篇
晶体学   2篇
力学   10篇
数学   265篇
物理学   281篇
  2023年   10篇
  2022年   25篇
  2021年   23篇
  2020年   31篇
  2019年   37篇
  2018年   15篇
  2017年   20篇
  2016年   42篇
  2015年   64篇
  2014年   57篇
  2013年   78篇
  2012年   104篇
  2011年   109篇
  2010年   78篇
  2009年   65篇
  2008年   91篇
  2007年   108篇
  2006年   87篇
  2005年   104篇
  2004年   64篇
  2003年   64篇
  2002年   71篇
  2001年   46篇
  2000年   32篇
  1999年   25篇
  1998年   32篇
  1997年   25篇
  1996年   28篇
  1995年   18篇
  1994年   31篇
  1993年   14篇
  1992年   17篇
  1991年   18篇
  1990年   16篇
  1989年   13篇
  1988年   5篇
  1987年   20篇
  1986年   11篇
  1985年   13篇
  1984年   13篇
  1983年   8篇
  1982年   21篇
  1981年   16篇
  1980年   13篇
  1979年   11篇
  1977年   6篇
  1973年   6篇
  1970年   6篇
  1927年   4篇
  1926年   4篇
排序方式: 共有1875条查询结果,搜索用时 0 毫秒
11.
12.
Real-space models of nondynamical correlation between electrons of opposite spin and of parallel spin in multicenter (molecular) systems are discussed. These models are designed to be partnered with Hartree-Fock or exact Kohn-Sham exchange. Thus the numerous and well-known problems of local density-functional exchange-correlation approximations, especially in stretched odd-electron systems, are circumvented.  相似文献   
13.
14.
15.
A new photoaldol route to alpha-amino-beta-hydroxy carboxylic acid esters is initiated by the photocycloaddition of aromatic or aliphatic aldehydes to 5-methoxyoxazoles. The 4-unsubstituted 5-methyloxazole 1 gave the cycloadducts 8a-f in high yields and excellent exo-diastereoselectivities. Hydrolysis of 8a-f gives the N-acetyl alpha-amino-beta-hydroxy esters 9a-f, which could be subsequently converted into the corresponding Z-didehydro alpha-amino acids 10a-f. Quartenary alpha-amino-beta-hydroxy esters 12, 14, 16, 18, and 20, which are stable against dehydration, were obtained from the 4-alkylated 5-methoxyoxazoles 2-6, in most cases highly erythro-selective due to the high degree of stereocontrol (exo) at the photocycloaddition (to give 11, 13, 15, 17, and 19) level. The relative configurations of the N-acetyl alpha-amino-beta-hydroxy esters were determined by NMR spectroscopy and comparison with chiral pool-derived compounds as well as by X-ray structure determination of the ester 23, formed by hydrolysis of the cycloadduct 22, derived from photocycloaddition of propionaldehyde to the isoleucine-derived oxazole 21.  相似文献   
16.
The reaction behavior of NaN3, AgN3, and Me3SiN3 towards FNO2, CINO2, NO2SbF6, and NO2BF4 was investigated. At -30°C or below in a solvent-free system sodium azide did not react with CINO2, NO2BF4, or NO2SbF6. Below -30°C silver azide did not react either with neat C1NO2. Treatment of Me3SiN3 with pure C1NO2 led to the formation of C1N3, N2O, and Me3SiOSiMe3. A mechanism for this reaction has been proposed. Pure chlorine azide was isolated by fractional condensation and identified by its low-temperature Raman spectrum (liquid state). The reaction of Cp2Ti(N3)2 with C1NO2 also yielded C1N3 as the only azide-containing reaction product. Treatment of FNO2 with NaN3 at temperatures as low as -78°C always ended in an explosion which was probably due to the formation of FN3 as one of the reaction products. The reaction of NO2SbF6 with NaN3 in liquid CO2 (-55°C· T· -35°C) as the solvent afforded a new azide species which was stable at low temperature in solution only and was investigated by means of low-temperature Raman spectroscopy. The obtained vibrational data give strong evidence for the presence of tetranitrogen dioxide, N4O2, which can be regarded as nitryl azide (NO2N3). The structure and vibrational frequencies of N4O2 were computed ab initio at correlated level (MP2/6-31 + G*). In liquid xenon (-100°C· T· -60°C) NaN3 did not react with NO2SbF6. A previous literature report on the preparation of N4O2 could not be established.  相似文献   
17.
Proton hyperfine data are reported for the radical anions of 1,8-di (propyn-1-yl)-naphthalene (I), 7,8,12,13-tetradehydro-10,11-dihydro-9H-cyclodeca[d,e]naphthalene (II) and 2,2′-di(propyn-1-yl)-biphenyl (III), as well as of 5,6,11,12-tetradehydro-7,8,9,10-tetrahydro-dibenzo[a,c]cyclodecene (IV) and its 8,8,9,9-tetradeuterio-derivative (IV-d4). The triple bonds in I and II can be regarded as roughly parallel, while those in IV (and IV-d4) may be considered as crossed. The π-spin distributions in I? to IV? are discussed in terms of simple MO models which suggest a weekly bonding interaction between the acetylenic fragments in IV?, in contrast to III? where such an effect appears to be negligible. The importance of an analogous interaction in I? and II? is difficult to deduce, since its inclusion into a MO model does not substantially affect the π-spin distribution in these radical anions.  相似文献   
18.
A characterisation of all real Δ≠0 is given, such that $$\overline {f(\sigma + i\Delta \mathbb{N})} = \overline {f(\sigma + i\mathbb{R})} $$ holds for all Dirichlet seriesf(s)=Σa n n ?s and all σ greater than the abscissa of absolute convergence off. For only countably many Δ relation (*) is not valid. By using transcendental results it is shown, that (*) holds in the special case, Δ=1.  相似文献   
19.
The development of the future French and European bioeconomies will involve developing new green chemical processes in which catalytic transformations are key. The VAALBIO team (valorization of alkanes and biomass) of the UCCS laboratory (Unité de Catalyse et Chimie du Solide) are working on various catalytic processes, either developing new catalysts and/or designing the whole catalytic processes. Our research is focused on both the fundamental and applied aspects of the processes. Through this review paper, we demonstrate the main topics developed by our team focusing mostly on oxygen- and hydrogen-related processes as well as on green hydrogen production and hybrid catalysis. The social impacts of the bioeconomy are also discussed applying the concept of the institutional compass.  相似文献   
20.
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