全文获取类型
收费全文 | 1682篇 |
免费 | 20篇 |
国内免费 | 22篇 |
专业分类
化学 | 1094篇 |
晶体学 | 15篇 |
力学 | 51篇 |
数学 | 312篇 |
物理学 | 252篇 |
出版年
2022年 | 17篇 |
2021年 | 24篇 |
2020年 | 15篇 |
2019年 | 19篇 |
2018年 | 19篇 |
2017年 | 21篇 |
2016年 | 38篇 |
2015年 | 20篇 |
2014年 | 44篇 |
2013年 | 81篇 |
2012年 | 80篇 |
2011年 | 76篇 |
2010年 | 60篇 |
2009年 | 69篇 |
2008年 | 90篇 |
2007年 | 111篇 |
2006年 | 102篇 |
2005年 | 86篇 |
2004年 | 81篇 |
2003年 | 60篇 |
2002年 | 50篇 |
2001年 | 34篇 |
2000年 | 25篇 |
1999年 | 20篇 |
1998年 | 20篇 |
1997年 | 23篇 |
1996年 | 34篇 |
1995年 | 24篇 |
1994年 | 22篇 |
1993年 | 23篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1990年 | 21篇 |
1989年 | 15篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 15篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 14篇 |
1982年 | 25篇 |
1981年 | 14篇 |
1980年 | 15篇 |
1979年 | 9篇 |
1978年 | 13篇 |
1977年 | 19篇 |
1976年 | 10篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1965年 | 8篇 |
排序方式: 共有1724条查询结果,搜索用时 15 毫秒
61.
Racemic phosphocarnitine 3 has been synthesized starting from diethyl 3-chloro-2-oxopropanephosphonate 4 in three steps involving reduction of 4 to the corresponding 2-hydroxyphosphonate 5, conversion of the latter to phosphonic acid 6, and final reaction with trimethylamine, affording the trimethylammonium salt of 3. Baker's yeast reduction of 4 and enzymatic kinetic resolution of (+/-)-5 afforded the enantiomerically pure precursors of phosphocarnitine, (R)-(+)-5 and (S)-(-)-5, which were converted to (S)-(-)- and (R)-(+)-phosphocarnitine 3, respectively. 相似文献
62.
63.
The synthesis and characterization of CuX2L; (X = CH3CH2COO?, ClCH2CH2COO?, CH3CHClCOO? or CH3CCl2COO?; and L = 4-acetylpyridine N-oxide) is reported. The characterization of these compounds was based on electronic, infrared and EPR spectra, as well as magnetic measurements over the temperature range. Several correlations between the spectral and magnetic data as well as pKa values of the respective acids were found and discussed. On the basis of spectral and magnetic data the structures of the compounds are discussed. 相似文献
64.
Jerzy Jurczyk 《Fresenius' Journal of Analytical Chemistry》1965,210(5):324-334
Zusammenfassung Es wurden Versuche zur Aufklärung der Anwendbarkeit des Benzoatverfahrens in Verbindung mit der chelometrischen Titration zur Aluminiumbestimmung in Rohstoffen und Erzeugnissen der Eisenhüttenindustrie durchgeführt. Die Resultate bestätigen, daß die Benzoatmethode bei Anwendung von Thioglykolsäure als Chelierungs- und Reduktionsmittel und die folgende chelometrische Titration die Aluminiumbestimmung in Erzen, Schlacken, legierten und unlegierten Stählen ungemein vereinfacht und außerordentlich beschleunigt. Nach den angegebenen Methoden ist es möglich, das Aluminium mit einer Genauigkeit zu bestimmen, die den in den Hüttenlaboratorien üblicherweise verwendeten Methoden nicht nachsteht. Legierte Stähle können auch in Gegenwart von Cr (bis 30%), Ti, Co, Mo, V, Cu, Ni, Mn u. a. binnen 90 min (vom Auflösen der Probe an gerechnet) analysiert werden.
Nachtrag bei der Korrektur. 10 Jurczyk, J.: Hutnik (Polen) (im Druck). — 11 Jurczyk, J.: Chem. analit. (Warszawa) (im Druck). 相似文献
Summary Investigations were performed on the use of the benzoate method in combination with chelometric titration for the determination of aluminium in metallurgical materials. It has been shown that the benzoate method in the presence of thioglycollic acid as chelating and reducing agent and the subsequent chelometric titration permit a speedy and easy determination of aluminium in ores, slags, alloy and nonalloy steels. It is possible to determine aluminium in alloy steels in the presence of Cr (up to 30%) Ti, Co, Mo, V, Cu, Ni, Mn and others within 90 min (calculated from the moment after dissolving the sample) with an accuracy not less than that obtained by methods usually employed in metallurgical laboratories.
Nachtrag bei der Korrektur. 10 Jurczyk, J.: Hutnik (Polen) (im Druck). — 11 Jurczyk, J.: Chem. analit. (Warszawa) (im Druck). 相似文献
65.
The selenenylation of racemic and optically active α-phosphoryl sulfoxides is a key step leading efficiently to α-phosphorylvinyl sulfoxides or α-phosphorylvinyl selenides depending on the reaction conditions. Oxidation of α-phosphorylvinyl selenides and subsequent thermolysis of selenoxides afford alkynylphosphonates. Studies of the stereochemical course of nucleophilic addition to α-phosphoryl sulfoxides show high facial stereoselectivity of the reaction, however, epimerisation at the α-carbon atom leads to mixtures of diastereomers. 相似文献
66.
[reaction: see text] An expedient solid-phase synthetic approach to secondary and tertiary amines was developed. The protocol employs conversion of resin-bound amino alcohols to the corresponding iodides, followed by iodide displacement with primary or secondary amines or with unprotected amino alcohols. This two-step procedure, affording products in good to excellent yields, is suitable for solid-phase synthesis of polyamines. 相似文献
67.
Dyguda E Grembecka J Sokalski WA Leszczyński J 《Journal of the American Chemical Society》2005,127(6):1658-1659
Interaction energies of phenylalanine ammonia-lyase (PAL) active site residues with a series of PAL inhibitors have been partitioned into electrostatic, exchange, delocalization, and correlation components and compared with analogous results obtained previously for leucine aminopeptidase (LAP). In the latter metalloenzyme, either of the two charged residues controls entirely relative inhibitor binding energies, while at least four residues are required to determine ligand relative stabilization in neutral PAL. Significant correlation with experimental inhibitory activity was found between the stabilization energy at gradually decreasing levels of theory (MP2, SCF) down to the first-order Heitler-London term. Contrary to the LAP case, where the electrostatic term was sufficient to reproduce experimentally observed trends, in the case of PAL, exchange repulsion effects also have to be considered. Computational protocol presented herein constitutes a promising way to incorporate the first principle calculation's accuracy into the process of rational binding affinity prediction, revealing the physical nature of the interactions, where successive approximations can be introduced in a systematic and justifiable manner. 相似文献
68.
Jerzy Mroziński Jiří Kohout Mária Hvastijová Helmut Köhler 《Transition Metal Chemistry》1986,11(12):481-484
Summary New coligand isomers of composition Cu{N(CN)2}2(pz)2 and Cu{pz · N(CN)2}2 (pz = pyrazole) were prepared and studied by measuring their magnetic susceptibilities up to 4.2K and by aid of their e.s.r., ligand field and i.r. spectra. The susceptibility data have been analysed with various models for the exchange-coupled copper(II) polymers. It is shown that the resultant exchange coupling is ferromagnetic for Cu{N(CN)2}2(pz)2 (J 1.1 - 1.4 cm–1) but antiferromagnetic for Cu{pz · N(CN)2}2 (J –0.4 cm–1). A polymeric chain structure is proposed for Cu{N(CN)2}2(pz)2 havingpseudo-octahedrally coordinated copper(II) and CN-bridging dicyanamide ligands. Its coligand isomer contains anionic chelate ligands, formed by nucleophilic addition between N(CN)2 and pz in the copper(II) coordination sphere, and giving with this central atom a square-planar system. Definite, but slight axial interaction takes place between these structure units. 相似文献
69.
A method for direct de termination of total in organic arsenic (III+V), arsenic (III) and dimethylarsinate (DMA) in sea water was developed by combining continuous‐flow selective hydride generation and inductively coupled plasma mass spectrometry (ICP‐MS) is presented. The principle underlying selective hydride generation is based on proper control of the reaction conditions for achieving separation of the respective arsenic species. The effects of pH and composition of reaction media on mutual interference between the arsenic species were investigated in detail. The results indicate that the appropriate media for the selective determination of total in organic arsenic, DMA and As(III) are 6 M HNO3, acetate buffer at pH = 4.63 and citrate buffer at pH = 6.54, respectively. The concentrations of total inorganic arsenic species, As(III+V), and As(III) were respectively deter mined and that of As(V) was obtained by the difference between them. As to the concentration of DMA, it was obtained after correction from the interference caused by As(III) and As(V). By following the established procedure, the detection lim its (as based on 3‐sigma criterion) for As(III+V), As(III) and DMA were 0.050, 0.009, and 0.002 ng/mL, respectively. There liability of the pro posed method was evaluated in terms of precision and spike addition. The results indicated that the precision of better than 3% and spike recovery of 95 to 105% for all the arsenic species tested in the natural sea water samples can be obtained. 相似文献
70.
Mária Hvastijová Jiri Kohout Marián Okruhlica Jerzy Mroziński Lothar Jäger 《Transition Metal Chemistry》1993,18(6):579-582
Summary Dicyanamide complexes of CuII, NiII and CoII of the type M[N(CN)2]2L2, where L = benzimidazole, 2-methyl- or 2-ethylbenzimidazole, have been prepared and studied by spectroscopy and magnetochemistry. The complexes, except for Co[N(CN)2]2 (benzimidazole)2, are six-coordinate, involving bidentate bridging dicyanamide groups. While the NiII complexes have practically octahedral structures, the CuII complexes are pseudooctahedral with similar tetragonal distortion. The ligand field strength in these complexes depends mainly on the steric effect of the benzimidazole ligands. The CoII complex of benzimidazole is monomeric tetrahedral, but that of 2-ethylbenzimidazole is tetragonal octahedral. The oridging function of dicyanamide in the six-coordinate complexes is realized either through both cyanide or through amide and cyanide nitrogens. The complex Cu[N(CN)2]2 (2-methylbenzimidazole)2 is a weak antiferromagnet (J = -0.1 cm–1), exhibiting under ca. 15 K a long-range antiferromagnetic ordering. 相似文献