全文获取类型
收费全文 | 1500篇 |
免费 | 78篇 |
国内免费 | 204篇 |
专业分类
化学 | 1540篇 |
晶体学 | 28篇 |
力学 | 3篇 |
综合类 | 6篇 |
数学 | 7篇 |
物理学 | 198篇 |
出版年
2023年 | 9篇 |
2022年 | 17篇 |
2021年 | 31篇 |
2020年 | 31篇 |
2019年 | 44篇 |
2018年 | 51篇 |
2017年 | 52篇 |
2016年 | 42篇 |
2015年 | 40篇 |
2014年 | 77篇 |
2013年 | 147篇 |
2012年 | 108篇 |
2011年 | 87篇 |
2010年 | 79篇 |
2009年 | 85篇 |
2008年 | 94篇 |
2007年 | 104篇 |
2006年 | 73篇 |
2005年 | 81篇 |
2004年 | 72篇 |
2003年 | 72篇 |
2002年 | 52篇 |
2001年 | 42篇 |
2000年 | 36篇 |
1999年 | 30篇 |
1998年 | 30篇 |
1997年 | 23篇 |
1996年 | 24篇 |
1995年 | 14篇 |
1994年 | 18篇 |
1993年 | 19篇 |
1992年 | 16篇 |
1991年 | 16篇 |
1990年 | 7篇 |
1989年 | 16篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
排序方式: 共有1782条查询结果,搜索用时 15 毫秒
61.
Salama Tarek A. El-Ahl Abdel-Aziz S. Khalil Abdel-Galil M. Girges Margret M. Lackner Bernd Steindl Christian Elmorsy Saad S. 《Monatshefte für Chemie / Chemical Monthly》2003,134(9):1241-1252
Summary. Several new 1-aryl-, aralkyl-, and heteroaryl-5-(4-phenylbuta-1,3-dienyl)tetrazole derivatives and annulated tetrazole derivatives were efficiently and regiospecifically prepared in nearly quantitative yield via a facile one step reaction of dienones with a combination of tetrachlorosilane and sodium azide in acetonitrile under mild conditions. A complete structure assignment of three representative examples of the tetrazoles was achieved by advanced 2D NMR measurements including COSY, TOCSY, HSQC, HMBC, NOESY, and ROESY experiments.Received March 17, 2003; accepted March 18, 2003
Published online July 28, 2003 相似文献
62.
The reaction scope of trifluoromethanesulfonyl azide in diazo transfer reactions was extended to include the preparation of α-cyano-α-diazo-carbonyls, phenyl sulfonyl diazoacetophenone and diethyl diazomalonate in high yields. The effect of the bases used in the diazo transfer reactions were found to have a dramatic influence on the success of the reaction with pyridine being the base of choice. 相似文献
63.
Rate constants for the reaction of ethyl bromoacetate with three series of substituted naphthoate ions have been measured
in an acetone-water mixture (90% v/v). Using σ
p
values rate constants at 30o correlate well with the Hammett equation yielding ρ=−0.54, −0.19 and −0.25 for (4,1−), (6,1−) and (6,2−) series, respectively.
Comparison of these ρ values with those of the reaction of phenacyl bromide reveals the failure of the reactivity-selectivity
principle RSP in these reactions. Failure of RSP has been explained in terms of isoselective temperature. 相似文献
64.
Neutral hydroxylamine extracts of wheat contained a product that was colourless at pH<5 (λmax 340 nm) and yellow at pH>9 (λmax 400 nm). ESI-MS showed a major ion m/z 184.0 and a possible parent ion m/z 367.2 (MH+) suggesting that the product resulted from the reaction of 2,6-dimethoxy-p-quinone with hydroxylamine. However, mass spectral and other spectroscopic data indicated that the compound was neither of the 2,6-dimethoxy-p-quinone oximes. A product with identical absorbance, mass spectrum, electrophoretic mobility and HPLC retention time as the pigment from hydroxylamine extracts of flour was observed amongst the reaction products of hydroxylamine and 1,4-dihydroxy-2,6-dimethoxybenzene. The structure of this product was identified by NMR, 2D NMR and IR as 4,4′-dihydroxy-3,3′,5,5′-tetramethoxyazodioxybenzene. 相似文献
65.
Miyuki Narita Junya Itoh Tohru Kikuchi Fumio Hamada 《Journal of inclusion phenomena and macrocyclic chemistry》2002,42(1-2):107-114
-Cyclodextrin dimer linked with ethylenediamine at the upper rim of the cyclodextrin has been synthesized and then modified with two dansyl moieties inthe presence of N,N'-dicyclohexylcarbodiimide. The sensing ability and bindingproperty of the title compound were investigated for steroids and terpenoids. Thefluorescence intensity of this dimer was decreased when a host–guest complex was formed. The value I/I0, where I0 and I are fluorescence intensitiesin the absence and presence of a guest and I is I0- I, was used as a parameter of sensitivity. This host exhibited a much higher sensitivity and selective molecular recognition ability for bile acids such as ursodeoxycholic acid andchenodeoxycholic acid and terpenoids such as (-)-borneol than the dansyl-modifiedcyclodextrins reported previously including -cyclodextrin dimer. The behaviors of the appended moieties of the host during the formation of host–guest complexes were studied using induced circular dichroism (ICD) and fluorescence spectra. The ICD intensityof this dimer was decreased on accommodation of a guest and this spectral pattern of the title dimer was opposite to that of bis dansyl-modified -cyclodextrin monomer. Theguest-induced variations in the fluorescence and ICD intensities suggest that this dimer formed a 1 : 1 host–guest complex and the appended moieties act as a hydrophobic cap. 相似文献
66.
《Journal of separation science》2003,26(11):1063-1066
The application of a modified iodine‐azide procedure for the detection of proline, arginine, and lysine is described. Phenyl isothiocyanate was used to transform amino acids into phenyl thiocarbamyl derivatives (derivatization in situ). The developed plates were sprayed with a mixture of sodium azide and starch solution, adjusted to pH 5.5, and exposed to iodine vapour. Due to the catalytic effect of the C? S bond, the spots appeared white on a violet‐grey background and were stable for 20 minutes. The detection limits were found to lie in the pmole range. The iodine‐azide test is compared with other procedures (iodine, UV, ninhydrin). 相似文献
67.
Qing LI Hui LI Bin SU Xiang Bao MENG Meng Shen CAI Zhong Jun LI* Department of Chemical Biology School of Pharmaceutical Sciences National Research Laboratory of Natural Biomimetic Drugs Peking University Beijing 《中国化学快报》2002,13(4):303-305
Our previous paper1,2reported the results of selective synthesis of mannose type glycosides and their dimers in our attempt to synthesize derivatives of N-acetyllacto- samine, which could have potential activity of anti-metastasis3. Further studies on the glycosylation of the accepters exploiting glycosyl nitrate instead of the acetate as donor showed interesting chemoselectivity between the two kinds of donor, namely, the glucose type and the mannose type. The reactivity difference of the t… 相似文献
68.
Christophe Hoarau Alexis Du Fou de Kerdaniel Pierre Grandclaudon Francis Marsais 《Tetrahedron letters》2005,46(49):8573-8577
The efficient and regioselective palladium-catalyzed C-2 arylation of ethyl 4-oxazolecarboxylate 1 with phenyliodide is described. The different parameters (solvent, base, ligand and catalyst) for the optimal conditions of this arylation process have been screened. 相似文献
69.
Copolymers of ethyl cellulose (EC) with polystyrene (PSt) were synthesized through atom transfer radical polymerization (ATRP). The molecular weight of graft copolymers increased without any trace of the EC macro-initiator, and the polydispersity of the side chains was low. The molecular weight of the side chains increased with the monomer conversion. Kinetic study indicated that the polymerization was first order. The micelle characteristics of the graft copolymer in acetone were investigated using dynamic light scattering (DLS), atom force microscopy (AFM) and transmission electron microscopy (TEM). With increasing the concentration, micelles were gradually formed from the solution. The TEM and AFM images indicated that the micelles had spherical shape and showed core-shell structure. 相似文献
70.
从TGEV 3CL蛋白酶二聚体结构出发,研究了TGEV 3CL蛋白酶二聚体单体之间的静电和疏水相互作用.蛋白质的静电相互作用通过有限差分方法求解Poisson-Boltzmann方程得到,疏水相互作用通过分析溶剂可及性表面模型得到.考察了不同pH值对TGEV 3CL蛋白酶二聚体静电和疏水相互作用的影响,在pH值为5.5~8.5时,二聚体静电相互作用能、静电去溶剂化能和疏水自由能都较小,表明在该条件下静电和疏水相互作用有利于二聚体的稳定存在,这符合实验结晶所需条件.pH值对静电去溶剂化能的影响大于疏水自由能,表明静电作用是造成强酸或强碱条件下二聚体不能稳定存在的主要原因. 相似文献