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   2326篇
  免费   170篇
  国内免费   76篇
化学   632篇
晶体学   43篇
力学   30篇
综合类   17篇
数学   417篇
物理学   1053篇
无线电   380篇
  2023年   11篇
  2022年   16篇
  2021年   24篇
  2020年   16篇
  2019年   20篇
  2018年   19篇
  2017年   29篇
  2016年   28篇
  2015年   44篇
  2014年   67篇
  2013年   96篇
  2012年   62篇
  2011年   79篇
  2010年   64篇
  2009年   192篇
  2008年   196篇
  2007年   176篇
  2006年   152篇
  2005年   116篇
  2004年   147篇
  2003年   127篇
  2002年   94篇
  2001年   82篇
  2000年   94篇
  1999年   114篇
  1998年   47篇
  1997年   35篇
  1996年   44篇
  1995年   30篇
  1994年   54篇
  1993年   38篇
  1992年   46篇
  1991年   31篇
  1990年   20篇
  1989年   16篇
  1988年   17篇
  1987年   15篇
  1986年   14篇
  1985年   13篇
  1984年   13篇
  1983年   6篇
  1982年   10篇
  1981年   11篇
  1980年   12篇
  1979年   8篇
  1978年   7篇
  1977年   7篇
  1976年   6篇
  1975年   2篇
  1973年   3篇
排序方式: 共有2572条查询结果,搜索用时 656 毫秒
111.
    
  相似文献   
112.
Synthesis of volatile complexes based on -ketoimine pivalyltrifluoroacetone, C(CH3)3C(NH)CH2COCF3, is described. The general formula of the complexes is M(L)2, where M = Cu, Ni, Pd. Complexes of this kind with Ni and Pd were obtained for the first time. The Cu and Pd complexes were found to be isostructural. A comprehensive crystal-chemical study showed that all structures are molecular and built of trans-complexes. The central atom has a square plane environment. The average M-O and M-N distances are nearly equal in all compounds: 1.84 , 1.92 , and 1.98 for Ni, Cu, and Pd complexes, respectively; the mean values of the O-M-N chelate angles are 93.4°, 91.9°, and 92.7°, respectively.Original Russian Text Copyright © 2004 by I. A. Baidina, G. I. Zharkova, N. V. Pervukhina, S. A. Gromilov, and I. K. IgumenovTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 4, pp. 713–722, July–August, 2004.This revised version was published online in April 2005 with a corrected cover date.  相似文献   
113.
Benzophenone analog 3 has been synthesized and characterized by the X-ray diffraction (XRD) method. The compound crystallizes in a monoclinic space group P21/c with cell parameters a = 7.701(8) Å, b = 7.151(5) Å, c= 28.323(3) Å, = 104.639(4)°, Z = 4. The structure exhibits intra- and intermolecular hydrogen bonding of the type N–HO, C–HO, and N–HN. The molecules are interlinked through hydrogen bonds forming an infinite chain. This polymeric-like structure may play an important role in biological activity.  相似文献   
114.
X-ray photoelectron spectroscopy provides evidence that when [Cu(phen)2]2+(phen = 1,10-phenanthroline) is diffused between the layers of -zirconium phosphate, the complex species does not remain intact after intercalation, but some Cu-N bonds are broken, replaced with Cu-O bonds and the released nitrogen atoms can now interact with the PO3-OH groups of the host. XPS also provides evidence for coordination of the Cu2+ ions when they are diffused by ion exchange in the phenanthroline--zirconium phosphate intercalation compound. Although Cu2+ and phen are in a 1 : 1 molar ratio in the interlayer region of the host, so that a 1 : 1 coordination could be expected between the two species, the characteristic peaks of the uncoordinated phenanthroline, even though at a low intensity, are still present. The differences between the two Cu(II)-intercalation compounds are discussed.  相似文献   
115.
A seven-step synthesis ofS-(+)-hydroprene (S-1) in 20 % overall yield starting fromS-(+)-3,7-dimethyl-1,6-octadiene (2) of 55+-10 % optical purity is described. The introduction of an optical enhancement step in the synthetic sequence at the stage ofS-(–)-3,7-dimethyl-1-octanol (9) raises the optical purity ofS-1 from 50 % to 80 %.For part 13, see. ref.1 Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 342–348, February, 1993.  相似文献   
116.
An improved procedure for the synthesis of N-bromoacetyl--glycopyranosylamines from the corresponding -glycosylamines was developed. The procedure is applicable to a wide range of derivatives of monosaccharides (hexoses, 2-acetamido-2-deoxyhexoses, hexuronamides, and 6-deoxyhexoses) and some disaccharides. For the derivatives of pentoses and 2-deoxyhexoses, the use of the corresponding -glycosylammonium carbamates was found to be more convenient. N-Bromoacetyl--glycopyranosylamines derived from D-mannose, L-rhamnose, D-glucuronamide, 2-deoxy-D-arabino-hexose, 2-deoxy-D-lyxo-hexose, and melibiose were obtained for the first time.  相似文献   
117.
The thermal reactions of Ru3(CO)12 with RCOCH=CHPh (R=Me, p-MeC6H4) in hydrocarbon solvents lead to the formation of a series of complexes, several of which have been isolated as individual compounds by chromatography. The dinuclear complex Ru2(-H)(CO)6(-MeCOCH=CPh) and the tetranuclear complex Ru4(-H)(-CO)(CO)7(p-MeC 6H4 COCH=CPh)(-p-MeC6H4COCH=CPh)(4-p-MeC6H3COCH=CHPh) are characterized by an X-ray structural study. The structures of other reaction products are discussed on the basis of spectral data. The reactions are accompanied by reduction of the starting enones to the corresponding unsaturated ketones.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1285–1293, July, 1993.  相似文献   
118.
Preparation of the Iminium Salts CF3? NX?CF2+MF6? (X = CH3, F and M = As, Sb) and CF3? NCl?CF2+ AsF6? The preparation of the iminiumsalts CF3? NX?CF2+ MF6? (X = CH3, F and M = As, Sb) and CF3? NCl?CF2+ AsF6? is reported. The salts were characterized by NMR and infrared spectroscopy. CF3? NCH3?CF2+MF6? decompose into MF5 and (CF3)2NCH3.  相似文献   
119.
120.
A new turn on fluorescent probe for ferric ion based on poly(m‐phenyleneethynylene salicylaldimine) ( PPE‐IM ) has been developed. The preparation of PPE‐IM involves post‐polymerization functionalization of the corresponding polymeric amine, PPE‐AM , via the condensation with salicylaldehyde. The degree of polymerization of both PPE‐IM and PPE‐IM is 17 with polydispersity index of 1.5. In aqueous solution, the polymeric PPE‐IM is highly stable unlike its small molecule analog which is gradually hydrolyzed. The weak fluorescence of initial PPE ‐ IM (λem = 470) is greatly enhanced by 300 folds upon the addition of Fe3+. The 1H NMR reveals that the fluorescence enhancement is caused by Fe3+‐induced hydrolysis of the imine group. The sensing system shows a detection limit of 0.14 μM of Fe3+. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1155–1161  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