全文获取类型
收费全文 | 1894篇 |
免费 | 83篇 |
国内免费 | 2篇 |
专业分类
化学 | 1533篇 |
晶体学 | 8篇 |
力学 | 13篇 |
数学 | 132篇 |
物理学 | 293篇 |
出版年
2023年 | 9篇 |
2022年 | 18篇 |
2021年 | 28篇 |
2020年 | 38篇 |
2019年 | 33篇 |
2018年 | 21篇 |
2017年 | 10篇 |
2016年 | 63篇 |
2015年 | 54篇 |
2014年 | 51篇 |
2013年 | 76篇 |
2012年 | 102篇 |
2011年 | 150篇 |
2010年 | 77篇 |
2009年 | 88篇 |
2008年 | 102篇 |
2007年 | 114篇 |
2006年 | 94篇 |
2005年 | 89篇 |
2004年 | 94篇 |
2003年 | 70篇 |
2002年 | 77篇 |
2001年 | 47篇 |
2000年 | 27篇 |
1999年 | 38篇 |
1998年 | 32篇 |
1997年 | 30篇 |
1996年 | 30篇 |
1995年 | 26篇 |
1994年 | 18篇 |
1993年 | 17篇 |
1992年 | 20篇 |
1991年 | 17篇 |
1990年 | 16篇 |
1989年 | 16篇 |
1988年 | 13篇 |
1987年 | 8篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1982年 | 14篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 15篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1974年 | 6篇 |
1973年 | 8篇 |
1970年 | 6篇 |
1969年 | 6篇 |
1968年 | 6篇 |
排序方式: 共有1979条查询结果,搜索用时 15 毫秒
31.
Irradiation (λ > 340 nm) of the title compound 2a in t-BuOH affords 3,3,10,10-tetramethyl-2,8-dioxatricyclo-[4.3.2.01,6]undecane-5,9-dione (3) via photoextrusion of 2-methylpropene and subsequent photocycloaddition of 2a to the alkene. The same regioisomer 3 is formed selectively and in much higher yields on irradiating 2a in the presence of excess 2-methylpropene. Irradiation of 3 (λ = 300 nm) in t-BuOH in the presence of the same alkene gives a 6:1 mixture of spirooxetanes 7a and 7b but not α-cleavage products. In 2-propanol, 2a is photoreduced to a 2:1 mixture of diastereoisomeric hydrodimers 9 . 相似文献
32.
A new redox indicator reaction between Cu(2+), Mn(2+) and S(2)O(3)(2-) in the presence of 2,2'-bicinchoninic acid is investigated. Optimum reaction conditions are determined. The influence of nonaqueous solvents on the reaction rate is studied. It is shown that in the presence of acetone the S(2)O(3)(2-) determination limit is lowered 0.005 mug ml(-1). Random errors lie within the limits allowable at the determination of microconcentrations. To raise the sensitivity of the determination of S(2)O(3)(2-) in some inorganic salts (KCl, KNO(3), CsNO(3)), this impurity was concentrated by low-temperature directed crystallisation. Metrologic characteristics of the developed technique are presented. 相似文献
33.
Jagoda M Warzeska S Pritzkow H Wadepohl H Imhof P Smith JC Krämer R 《Journal of the American Chemical Society》2005,127(43):15061-15070
For a number of phosphoryltransfer enzymes, including the exonuclease subunit of DNA polymerase I, a mechanism involving two-metal ions and double Lewis-acid activation of the substrate, combined with leaving group stabilization, has been proposed. Inspired by the active site structure of this enzyme, we have designed as a synthetic phosphoryl transfer catalyst the dicopper(II) macrocyclic complex LCu(2). Crystal structures of complexes [(L)Cu(2)(mu-NO(3))(NO(3))](NO(3))(2) (1), [(L)Cu(2)(mu-CO(3))(CH(3)OH)](BF(4))(2) (2), and [(L)Cu(2)(mu-O(2)P(OCH(3))(2))(NO(3))](NO(3))(2) (3) illustrate various possibilities for the interaction of oxoanions with the dicopper(II) site. 1 efficiently promotes the transesterification of dimethyl phosphate (DMP) in CD(3)OD, k(cat) = 2 x 10(-)(4) s(-)(1) at 55 degrees C. 1 is the only available catalyst for the smooth transesterification of highly inert simple dialkyl phosphates. From photometric titrations and the pH dependence of reactivity, we conclude that a complex [(L)Cu(2)(DMP)(OCH(3))](2+) is the reactive species. Steric bulk at the -OR substituents of phosphodiester substrates O(2)P(OR)(2)(-) drastically reduces the reactivity of 1. This is explained with -OR leaving group stabilization by Cu coordination, an interaction which is sensitive to steric crowding at the alpha-C-atom of substituent R. A proposed reaction mechanism related to that of the exonuclease unit of DNA polymerase I is supported by DFT calculations on reaction intermediates. The complex [(L)Cu(3)(mu(3)-OH)(mu-CH(3)O)(2)(CH(3)CN)(2)](ClO(4))(3) (4) incorporates a [Cu(OH)(OCH(3))(2)(CH(3)CN)(2)](-) complex anion, which might be considered as an analogue of the [PO(2)(OCH(3))(2)(OCD(3))](2)(-) transition state (or intermediate) of DMP transesterification catalyzed by LCu(2). 相似文献
34.
35.
A. B. Blank 《Journal of Analytical Chemistry》2005,60(12):1173-1174
Standard documents introducing the notion of uncertainty in measurements into the common practice of assessing the results of chemical analysis are considered and criticized. 相似文献
36.
Previously studied reactions of aluminum(III) with Xylenol Orange and of cobalt(III) with nitroso-R salt in weakly acidic solutions were proposed for the simultaneous catalymetric determination of carbonate and borate ions and carbonate and phosphate ions, respectively. It was demonstrated that the mathematical apparatus of two-component spectrophotometry is suitable for processing the results of analysis.Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 1, 2005, pp. 7–10.Original Russian Text Copyright © 2005 by Mikhailova, Belikov, Blank. 相似文献
37.
An enantioselective synthesis of mitosane core (-)-1 has been achieved. Key steps include a rapid assembly of a key eight-membered-ring intermediate employing ring-closing metathesis. Kinetic resolution of an advanced secondary alcohol was then accomplished by using a peptide-based asymmetric acyl transfer catalyst that was discovered from a parallel screen of catalyst candidates. Optically pure material was then converted to the mitosane core, which was the subject of additional studies on the selective modification to produce several substituted compounds containing a mitosane ring system. 相似文献
38.
We demonstrate that silicon AFM tips can be modified by etching with 2% HF solution followed by reaction with an alpha,omega-oligo(ethylene glycol)alkene. Tips properly modified by this technique maintain a small tip size and effectively reduce the nonspecific interaction with fibrinogen and bovine serum albumin, resulting in greatly improved image resolution and contrast for high-coverage fibrinogen films. 相似文献
39.
The reactivity of a Cr‐Te multilayer film consisting of 326 Cr/Te double‐layers with about 88(3) at% Te was investigated with in‐situ X‐ray diffraction. The occurrence of a superstructure reflection caused by the multilayer system is observed. During the annealing procedure between 80 and 90 °C the layer‐by‐layer arrangement is destroyed and elemental Te crystallizes. After 6 h isothermal annealing at 140 °C CrTe3 starts to crystallize. The amount and the crystal size of CrTe3 increase with raising temperature and Te is consumed during the growth process. At 220 °C the formation of crystalline Cr2Te3 is observed and between 220 and 260 °C CrTe3 is completely decomposed into Cr2Te3. The two Cr tellurides grow as highly textured materials. The size of CrTe3 crystals is significantly larger than that of Cr2Te3. For a co‐deposited Cr‐Te film with composition 77(3) at% Te the tri‐telluride and elemental Te crystallize simultaneously. The texture of CrTe3 is less pronounced and different reflections of the tri‐telluride occur besides the (h00)‐reflections. Up to 160 °C a pronounced increase of the crystallite size is observed and the crystals are significantly larger than those obtained with the multifilm sample. At 130 °C the crystallization kinetics seem to be controlled by nucleation. 相似文献
40.
Sabine Rentsch Gudrun Hermann Mark Bischoff Dietmar Strehlow Manfred Rentsch 《Photochemistry and photobiology》1997,66(5):585-590
Abstract— The excited state behavior of the red light-absorbing form of phytochrome (Pr) was studied on the femtosecond time scale. After excitation of Pr with 75 fs laser pulses at 616 nm the kinetics of the transient absorption changes was recorded at selected wavelengths probing mainly the bleaching of the Pr ground-state absorption and the stimulated emission. The kinetic data obtained indicate the population of an excited state with a 3 ps lifetime immediately after excitation. This state precedes the formation of another excited state with a 32 ps lifetime. The decay of the latter state is followed by the appearance of a first product state that is assumed to represent lunii-R. In addition, 2,3-dihydrobiliverdin, which is considered to be an adequate model of the Pr chro-mophore, was included in the femtosecond studies. The absorption difference spectra recorded at various delay times show an immediate bleaching of the ground-state absorption. Simultaneously with bleaching a broad transient absorption appears between 410 and 525 nm. The data analysis yields similar kinetic components as they were observed in the decay of Pr. It is suggested from this finding that within the first tens of picoseconds after excitation the excited-state properties of Pr are mainly determined by the properties of the chromophore itself. 相似文献