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   852篇
  免费   63篇
  国内免费   98篇
化学   841篇
晶体学   29篇
综合类   5篇
数学   1篇
物理学   83篇
无线电   54篇
  2024年   4篇
  2023年   8篇
  2022年   15篇
  2021年   20篇
  2020年   17篇
  2019年   29篇
  2018年   25篇
  2017年   35篇
  2016年   28篇
  2015年   25篇
  2014年   38篇
  2013年   102篇
  2012年   49篇
  2011年   44篇
  2010年   40篇
  2009年   31篇
  2008年   40篇
  2007年   58篇
  2006年   42篇
  2005年   52篇
  2004年   50篇
  2003年   47篇
  2002年   24篇
  2001年   16篇
  2000年   12篇
  1999年   16篇
  1998年   16篇
  1997年   15篇
  1996年   19篇
  1995年   17篇
  1994年   8篇
  1993年   14篇
  1992年   9篇
  1991年   9篇
  1990年   4篇
  1989年   10篇
  1988年   5篇
  1987年   7篇
  1986年   4篇
  1985年   1篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
  1978年   1篇
  1976年   3篇
  1975年   1篇
排序方式: 共有1013条查询结果,搜索用时 15 毫秒
991.
Various substituted benzaldehydes were converted into benzoate esters with household bleach and sodium iodide in an alcohol solvent. The reaction works best with methanol. Only 4-chlorobenzaldehyde, 3-nitrobenzaldehyde, and 4-nitrobenzaldehyde reacted completely with 1-propanol under these conditions.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]  相似文献   

992.
N,N′‐Dibenzylethylenediamine is presented as a new, efficient, and versatile bidentate ligand suitable for the copper catalyzed formation of the C‐N bond. This bidentate ligand has been demonstrated to facilitate the copper catalyzed cross‐coupling reactions of aryl iodides with amides to afford the desired products in good to excellent yields.  相似文献   
993.
The polyfluoroalkyl-lactonization of 5-hexenoic acids with the polyfluoroalkyl iodides initiated by sodium dithionite was carried out in aqueous acetonitrile, providing an efficient method for the synthesis of fluorine-containing δ-valerolactones. Formation of RfCH2CHCH(CH2)2COOH appeared to be unexpected minor product in sulfinatodehalogenation reagents.  相似文献   
994.
Recent developments in dynamic kinetic resolution   总被引:1,自引:0,他引:1  
  相似文献   
995.
An efficient and selective method for esterification of alcohols using N-(p-toluenesulfonyl)imidazole (TsIm) is described. In this method, alcohols are refluxed with a mixture of RCO2Na (R: alkyl and aryl), TsIm, and triethylamine in the presence of catalytic amounts of tetra-n-butylammonium iodide (TBAI) in DMF to afford the corresponding esters in good yields. This methodology is highly efficient for various structurally diverse alcohols with selectivity for ROH: 1° > 2° > 3°.  相似文献   
996.
This review provides a comprehensive overview of strategies and methodologies for reducing C−O and heteroatomic−oxygen bonds (N−O, S−O, P−O) using I2/I, as well as other synthetically relevant bonds such as C−C, N−N, C−N, C−X, C−S. It highlights and discusses most of the mechanistic details provided by the original authors. Selected examples of other halides (Br and Cl) as reductants are also covered.  相似文献   
997.
Here, we investigate in detail the impact of the size of the methylammonium iodide (MAI) reactants in the mechanochemical powder synthesis of the halide perovskite methylammonium lead iodide (MAPbI3). Morphology and structural characterizations by scanning electron microscopy and X-ray diffraction reveal that with increasing MAI reactant size, the particle size of the perovskite powder increases, while its defect density decreases, as suggested by nuclear quadrupole resonance spectroscopy and photoluminescence investigations. The reason for this behavior seems to be associated to the sensitive influence of the MAI size on the time durations of MAPbI3 synthesis and delayed MAPbI3 crushing stage during ball milling. Thus, our results emphasize the high importance the reactant properties have on the mechanochemical synthesis of halide perovskites and will contribute to enhance the reproducibility and control of the fabrication of halide perovskites in powder form.  相似文献   
998.
《Analytical letters》2012,45(15):2763-2774
Abstract

The oxidation of iodide to iodine by nitrous acid in aqueous acidic medium takes place catalytically in the presence of dissolved oxygen and can be followed spectrophotometrically at 288 and 352 nm. An indirect molar absorptivity for nitrite on the basis of I3 ? formation can be as high as 8.5×105 L mol?1 cm?1 at 288 nm. Analytical curves were established. The iodine released in the catalytic cycle can also be titrated with thiosulfate. The reaction is pseudo–second‐order in oxygen consumed, with t1/2=15.7 min at 25°C. A rate determining step could be the NO · O2 as the activated species. Measurements of the iodine formed at catalytic conditions was used to determine nitrite in meat extracts and NOx in car exhausts.  相似文献   
999.
Abstract

A direct method of obtaining PbEnI2.DMSO is reported. The crystal structure of the compound was determined by X-ray techniques. PbEnI2.DMSO is triclinic, space group P 1, C4H14I2N2PbS, a=10.225(3), b=10.132(3), c=6.900(2) Å; α=90.83(2), β=88.30(2), γ=106.35(4)°; V=685.6(4) Å3; z=2, calculated density 2.92 gcm?3. Neutral PbEnI2 complexes are associated with DMSO molecules via H-bonds. The lead(II) ion is covalently linked with a chelated En molecule (Pb-N 2.46–2.48 Å) and I? anions (Pb-I 3.087–3.343 Å). Covalently bonded atoms form an umbrella-like coordination Pb(II) polyhedron. The side containing the lone electron pair of the lead(II) ion has coordination completed by two I? anions of neighbouring molecules with the Pb-I 3.621–3.627 Å.  相似文献   
1000.
《Analytical letters》2012,45(9):2051-2058
Abstract

The present paper reports a new chemiluminescence system, i.e, acetone-H2O2?C10?, which can be catalyzed by iodide ion (I?). Based on this catalysis, a new chemiluminescence method for the determination of trace iodide ion is proposed. the optimum conditions are reported in this note. the detection limit is 2 × 10?11 g/ml I?, the linear dynamic range is 4 × 10?10 g/ml to 3 × 10?7 g/ml I?, and the variation coefficient at an iodide concentration of 5 × 10?9 g/ml I? (n=10) is 4.6%. the method has been satisfactorily applied to the determination of trace iodide ion in water.  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