全文获取类型
收费全文 | 5058篇 |
免费 | 106篇 |
国内免费 | 28篇 |
专业分类
化学 | 3239篇 |
晶体学 | 32篇 |
力学 | 180篇 |
数学 | 758篇 |
物理学 | 983篇 |
出版年
2021年 | 29篇 |
2020年 | 43篇 |
2019年 | 41篇 |
2018年 | 43篇 |
2017年 | 39篇 |
2016年 | 76篇 |
2015年 | 89篇 |
2014年 | 95篇 |
2013年 | 245篇 |
2012年 | 198篇 |
2011年 | 228篇 |
2010年 | 150篇 |
2009年 | 144篇 |
2008年 | 196篇 |
2007年 | 199篇 |
2006年 | 218篇 |
2005年 | 238篇 |
2004年 | 191篇 |
2003年 | 165篇 |
2002年 | 174篇 |
2001年 | 85篇 |
2000年 | 81篇 |
1999年 | 82篇 |
1998年 | 63篇 |
1997年 | 89篇 |
1996年 | 86篇 |
1995年 | 76篇 |
1994年 | 69篇 |
1993年 | 70篇 |
1992年 | 61篇 |
1991年 | 62篇 |
1990年 | 45篇 |
1989年 | 54篇 |
1988年 | 41篇 |
1987年 | 41篇 |
1985年 | 88篇 |
1984年 | 70篇 |
1983年 | 64篇 |
1982年 | 80篇 |
1981年 | 90篇 |
1980年 | 66篇 |
1979年 | 71篇 |
1978年 | 78篇 |
1977年 | 80篇 |
1976年 | 60篇 |
1975年 | 62篇 |
1974年 | 59篇 |
1973年 | 45篇 |
1971年 | 41篇 |
1968年 | 27篇 |
排序方式: 共有5192条查询结果,搜索用时 343 毫秒
81.
Under Lewis acid activation, hemiaminals of trifluoroacetaldehyde and related (fluoroalkyl)aldehydes generate iminium species that can react with various nucleophiles to provide fluorinated amines. 相似文献
82.
Activation of CO2 at Transition Metal Centres: The Route of the CO2 Reduction at Nikel(0) Moieties A competing reaction in the catalytic cyclooligomerization of hex-3-yne and CO2 at the (TMED)Ni(0)-fragment (TMED = N,N,N′,N′-tetramethylethylendiamine) is the formation of carbon monoxide and (TMED)Ni(CO3). So it is possible to explain the generation of II (TMED)Ni(diethylmalicacidanhydride) and III (a nickel trimer with two (TMED)Ni(CO3) units). Both complexes are characterized by X-ray analysis. The reduction of CO2 to CO most likely proceeds via an intermediate in which two molecules of carbon dioxide are coupled head-to-tail to form a metallacycle. An ab initio scf geometry optimization supports the existence of such an intermediate. 相似文献
83.
Greenblatt HM Guillou C Guénard D Argaman A Botti S Badet B Thal C Silman I Sussman JL 《Journal of the American Chemical Society》2004,126(47):15405-15411
Bifunctional derivatives of the alkaloid galanthamine, designed to interact with both the active site of the enzyme acetylcholinesterase (AChE) and its peripheral cation binding site, have been assayed with Torpedo californica AChE (TcAChE), and the three-dimensional structures of their complexes with the enzyme have been solved by X-ray crystallography. Differences were noted between the IC(50) values obtained for TcAChE and those for Electrophorus electricus AChE. These differences are ascribed to sequence differences in one or two residues lining the active-site gorge of the enzyme. The binding of one of the inhibitors disrupts the native conformation of one wall of the gorge, formed by the loop Trp279-Phe290. It is proposed that flexibility of this loop may permit the binding of inhibitors such as galanthamine, which are too bulky to penetrate the narrow neck of the gorge formed by Tyr121 and Phe330 as seen in the crystal structure. 相似文献
84.
Polarography and chronopotentiometry are used to study the primary processes of the reduction of 2,2′-bipyridine at mercury electrodes in aqueous solutions (12 < pH < 14). Bipyridine and its first reduction product are strongly adsorbed. The reduction is preceded by a complex equilibrium between bipyridine and the cation of the supporting electrolyte (Li, Na, K, Ba). Due to the great excess of the cations, the equilibrium is shifted completely to the side of the complex, which exists as cis- and trans-isomer. The cis-form is more stable, forming a chelate, the trans-form however is more easily reducible. The kinetic and thermodynamic data of the isomerization are determined. The first reduction product reacts irreversibly giving a substance which can be reoxidized to bipyridine via a radical intermediate. 相似文献
85.
W. Ketterle P. Graßhoff H. Figger H. Walther 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1988,9(4):325-329
The emission of the ND4 Schüler band was observed after neutralization of a mass-selected ND 4 + ion beam. Thus the assignment of this band to the ammonium radical was confirmed. The lifetime of the upper state (3p 2 F 2) was determined to be 4.2 ns, which is much shorter than ab initio values of the radiative lifetime, showing that this state decays predominantly by predissociation. The Schuster band was not observed, neither after neutralization of ND 4 + , nor of ND 3 + . 相似文献
86.
In the presence of AlCl3, [1-β-chloroethyl-3,4-dimethylphosphole]pentacarbonyltungsten() rearranges into [1-chloro-3,4-dimethyl-6,7-dihydrophosphepin]pentacarbonyltungsten (). 相似文献
87.
After a review of 1-perfluoroalkene RFCFCF2 synthesis, with RFC4F9, C5F11, C6F13, we have studied their fine structures by 19F N.M.R. This study has led us to first generalize the rules set for the chemical shifts and coupling constants of olefinic fluorine atoms of fluoropropene and their derivatives. Then, we have been able to determinate unambiguously the chemical shifts of the difluoromethylene groups of the perfluorinated chains. We have thus shown, by irradiation, that inversions may take place in the chemical shifts of the CF2 groups at the β and γ position of the double bond. 相似文献
88.
Abstract— This paper reports the present conclusions of an extended investigation on photo-sensitization of silver bromide. A general molecular packing structure for dye aggregates on the AgBr (111) surface is proposed. This structure, along with the observed spectral absorption displacements of small aggregates, is used to elucidate the phenomena of antisensitization and supersensitization. Supersensitization is seen as a partitioning of large dye aggregates into small aggregates by a deliberately added second component or by gaps between the aggregates. This partitioning isolates antisensitizing dye in a small fraction of the aggregates and minimizes its effect. The smallest aggregates are least likely to contain an antisensitizer, and show the highest quantum efficiency for photoconductivity and photographic action. Photoconductivity measurements establish that supersensitization occurs before the electron appears in the AgBr phase.
The question of direct electron injection vs. energy transfer as mechanisms for AgBr photosensitization is examined in terms of radiationless transfer to surface Ag2 S on AgBr. The absorption of surface Ag2 S is shown to be adequate for acceptance of Förster transfer from the dye, and surface Ag2 S is known to be photographically active. However, this mechanism is inefficient, and inadequate to account for observed high efficiency infrared sensitization. Direct electron injection is seen as the more probable mechanism for efficient dye sensitization of AgBr. 相似文献
The question of direct electron injection vs. energy transfer as mechanisms for AgBr photosensitization is examined in terms of radiationless transfer to surface Ag
89.
Summary This is a systematic X-ray diffraction study of the structure of the mesomorphic phases occurring with potassium soaps ranging from C8 to C22.Four types of structure have been identified. The first is lamellar with both polar groups and paraffin chains in the crystalline state. The second is also lamellar, but with both polar groups and paraffin chains in the liquid state. The third corresponds to the localization of indefinitely long ribbons on a two-dimensional oblique lattice; polar groups are crystalline and paraffin chains desorganized. The fourth corresponds to the localization of discs on a three-dimensionnalB-faces centered orthorhombic lattice; polar groups are crystalline and paraffin chains disorganized.The structural parameters and the polymorphism of the polar groups have been discussed.
Zusammenfassung Wir beschreiben hier eine systematische Röntgenstrahlendiffraktionsforschung der Struktur der mesomorphen Phasen von Kalium Seifen von C8 bis C22.Vier Strukturtypen wurden gefunden. Die erste Struktur ist lamellar; beides, die polaren Gruppen und die Paraffinketten sind kristallisiert. Die zweite Struktur ist auch lamellar; aber in diesem Falle sind beides, die polaren Gruppen und Paraffinketten geschmolzen. Die dritte Struktur entspricht einer Lokalisation von Seifenbändern in einem zweidimensionalen monoklinen Gitter; die polaren Gruppen sind kristallisiert und die Paraffinketten geschmolzen. Die vierte Struktur entspricht einer Lokalisation von Seifenscheiben in einemB-flächenzentrierten orthorhombischen Raumgitter; die Polargruppen sind kristallisiert und die Paraffinketten geschmolzen.Wir haben aus den experimentellen Resultaten einige Schlußfolgerungen gezogen.相似文献
90.
Supercritical fluid chromatography of carbohydrates 总被引:1,自引:0,他引:1
Michel Lafosse Bernard Herbreteau Luc Morin-Allory 《Journal of chromatography. A》1996,720(1-2):61-73
Supercritical fluid chromatography (SFC) is a relatively new technique applied to polar solutes such as carbohydrates about 10 years ago. The developments in the SFC of carbohydrates are summarized and a comparison between capillary and packed column SFC is presented. High-efficiency capillary columns are suitable only for derivatized carbohydrates since various packed columns are well adapted for non derivatized mono-, di- and trisaccharides and provide complementary selectivities. 相似文献