全文获取类型
收费全文 | 34篇 |
免费 | 1篇 |
国内免费 | 799篇 |
专业分类
化学 | 828篇 |
物理学 | 6篇 |
出版年
2020年 | 2篇 |
2014年 | 2篇 |
2013年 | 7篇 |
2012年 | 2篇 |
2011年 | 1篇 |
2009年 | 1篇 |
2007年 | 5篇 |
2006年 | 2篇 |
2005年 | 3篇 |
2004年 | 7篇 |
2003年 | 31篇 |
2002年 | 16篇 |
2001年 | 51篇 |
2000年 | 20篇 |
1999年 | 8篇 |
1998年 | 11篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 30篇 |
1994年 | 33篇 |
1993年 | 51篇 |
1992年 | 31篇 |
1991年 | 40篇 |
1990年 | 45篇 |
1989年 | 57篇 |
1988年 | 65篇 |
1987年 | 64篇 |
1986年 | 55篇 |
1985年 | 50篇 |
1984年 | 49篇 |
1983年 | 41篇 |
排序方式: 共有834条查询结果,搜索用时 156 毫秒
641.
Perfluoroalkyl 3-oxaperfluoralkanesulfonates XCF2OCF2CF2SO3CF2OCF2X (1) (X=CF2I (1a), CF2Cl (1b), HCF2 (1c), Cl2CF (1d)) reacted readily with various mucleophiles leading to S--O seission exclusively, thus 1 -- XCF2OCF2CF2SO2Y+XCF2OCF2COZ In the presence of a catalytic amount of halide (F-, Cl-, Br-, I-) and thiecyanate in diglyme 1 decomposed to give the corresponding sulfonly fluoride 2 (X=F) and acyl fluoride 3(Z=F). At room temperature 1 did not react with excess ethanol, but under refluxing for 12.5h, 1 was converted to 2 (Y=F) and 3 (Z=OEt). More powerful nucleophile ethoxide ion reacted readily with 1 at-60 - -50`C yielding Et2O and 3 (Z=OEt) but no 2 (Z=F). When the reaction was carried out at 80`C the yields of the products varied with the order of mixing of the reactants i.e. when 1 was added to excess ethoxide in ethanol, products are 3(Z=OEt), Et2O and 2(Y=F), but with ethoxide adding to 1 the yield of 2 (Y=F) was increased and that of ether decreased whereas the yield of 3 (Z=OEt) remained constant. Carboxylates (CF3CO2-, CH3CO2-) also caused S--O cleavage of 1 to give acetyl fluoride, 2 (y=F) and 3 (Z=F) as a result of decomposition of the intermediary mixed anhydride by the fluoride ion. In the same manner R2NH, C6H5NH2 reacted with 1 giving the products of S--O cleavage. In contrast to the nucleophilic reactions of α, α-di-H-perfluoroalkyl perfluoroalkanesulfonates (mainly C--O cleavage) it has been found that all nucleophies attack the sulfur atom of 1 exclusively. A possible interpretation is that the SN2 attack at sp3 carbon atom in highly fluorinated system is made impossible by the shielding effect of the two fluorine atoms in the alcoholic moiety and leaving the attack on the sulfur as the only alternative. 相似文献
642.
硫酸盐水合物催化下的酯化反应 I: 反应级数,活化能及催化剂活性的测定 总被引:1,自引:0,他引:1
The order of esterification reaction of acetic acid and ethanol catalyzed by Fe2(SO4)3.7H2O and CusO4.5H2O has been determined. The reaction order was found to be 2 with respect to acid and ethanol with constant quantities of catalysts. The activity of catalysts can be expressed by Ym value, which is different from catalyst to catalyst. For Fe2(SO4)3.7H2O, Cr2(SO4)3.6H2O, CuSO4.kH2O, NiSO4.7H2O, ZnSO4.7H2O, CoSO4.7H2O, FeSO4.7H2O and MnSO4.5H2O the Ym values are 1.52, 1.21, 0.62, 0.42, 0.47, 0.25, 0.10 and 0.00 respectively. A linear relationship of Ym with the fraction of the second ionization potentiol (P) over radius (r) of metal ion of sulphate salts was found to be Ym=α(P/γ)+b. 相似文献
643.
A new relationship based on the Hammett equation is derived for the esterification reaction of substituted carboxylic acid in the presence of hydrated sulphate salts and is called a catalyzing linear free energy equation. The equation can be expressed as log(KR(M)/K0Mn))=\s\*\T\^M + Ym where KR(M) is the rate constant of the esterification reaction of substituted acetic acid catalyzed by hydrated sulphate salts of M matal, K0(Mn) is the rate constant of that of acetic acid catalyzed by MnSO4.H2O, Ym is active index of catalyst. For reactions catalyzed by certain catalysts the rate constant of substituted acetic acid can be estimated from the equation. 相似文献
644.
碳氟表面活性剂与碳氢表面活性剂混合水溶液的表面吸附和胶团形成 I: 全氟辛酸钠-溴化辛基三甲铵体系 总被引:2,自引:0,他引:2
The surface adsorption and micelle formation of the mixed aqueous solutions of sodium perfluorooctanoate (7CFNa) and n-octyltrimethylamonium bromide (C8NBr) have been investigated by studying the surface tension-concentration relations of the solutions. It has been found that (1) The surface activity of the mixed system is much higher than that of 7CFNa and C8NBr. C8NBr is a very effective synergist for 7CFNa in surface activity, and vice versa. (2) By applying Gibbs adsorption equation, the total amount of adsorption (\I\t), the individual adsorption amount of the single surfactants (\I\7CF-, \I\C8N+) and the average molecular areas of them have been calculated. The adsorption value (saturated, \I\T) increases by 54% as the molal ratio of 7CFNa-C8NBr varies from 100:1 to 1:300. (3) From the adsorption data, it appears that the structure of surface layer of the 7CFNa-C8NBr solutions is peculiar and quite different from that of the cationic-anionic hydrocarbon surfactants. Firstly, the molal ratio of the two components in the surface layer of the 1:1 mixed solution is not 1:1 (7CFNa is adsorbed perferentially). Secondly, the molecular area at the maximum adsorption is very small (22.7A2). This could be attributed to the balancing of the electrical attraction between positive and negative ions and the mutual phobicity between the fluorocarbon and hydrocarbon chains. It has been concluded that the saturated adsorption film would possibly have a double-layer structure: the upper layer consists of the oriented 7CF- with the fluorocarbon chain toward the gaseous phase and the lower layer consists of randomly oriented C8N+ with the positive ionic head attached to the negative ionic head of 7CF- -- thus the 7CF- adsorption layer would behave as a platform for the adsorption of C8N+. (4) The limiting surface tension of the mixed solutions is very low (-15mNm-1) even if the molal ratio of 7CFNa-C8NBr is small, and so is the oil-water interfacial tension. Therefore it is capable of spreading the dilute aqu 相似文献
645.
Much work has been recorded in the literature concerning chlorination of photographic couplers. However, no detailed report can be found with the coupler of the structure 1. In this paper, the chlorination of 1 using SO2Cl2 in CHCl3 was shown to give the chlorination product 2. UV, IR, 1H NMR and MS of the chlorination product are compared with that of 1, and with those reported in the literature for benzoylacetanilide. Thermolysis of 2 gives a liquid fraction, HPLC shows one peak, which was identifield by 1H NMR and MS to be o-CH3COC6H4OCH2CHClC14H29. Elemental analyses as well as MS of its 2, 4-dinitrophenylhydrazone agree well with the structure assigned. 相似文献
647.
648.
The intramolecular Wittig Reaction has been applied to the synthesis of sec-butyl perfluorobutynoate. the saponification of the product with aqueous sodium hudroxide gave 3-sec-butoxy-4, 4, 4-trifluoro-(2) butenoic acid and 4, 4, 4-trifluoro-3-oxobutyric acid in 1:5 ration. 相似文献
649.
650.