全文获取类型
收费全文 | 13693篇 |
免费 | 380篇 |
国内免费 | 48篇 |
专业分类
化学 | 10874篇 |
晶体学 | 73篇 |
力学 | 212篇 |
数学 | 1736篇 |
物理学 | 1226篇 |
出版年
2023年 | 109篇 |
2022年 | 454篇 |
2021年 | 520篇 |
2020年 | 294篇 |
2019年 | 335篇 |
2018年 | 259篇 |
2017年 | 216篇 |
2016年 | 446篇 |
2015年 | 450篇 |
2014年 | 424篇 |
2013年 | 839篇 |
2012年 | 868篇 |
2011年 | 986篇 |
2010年 | 596篇 |
2009年 | 585篇 |
2008年 | 834篇 |
2007年 | 814篇 |
2006年 | 694篇 |
2005年 | 631篇 |
2004年 | 548篇 |
2003年 | 457篇 |
2002年 | 448篇 |
2001年 | 167篇 |
2000年 | 137篇 |
1999年 | 116篇 |
1998年 | 118篇 |
1997年 | 103篇 |
1996年 | 134篇 |
1995年 | 87篇 |
1994年 | 94篇 |
1993年 | 77篇 |
1992年 | 63篇 |
1991年 | 80篇 |
1990年 | 56篇 |
1989年 | 75篇 |
1988年 | 66篇 |
1987年 | 52篇 |
1986年 | 55篇 |
1985年 | 67篇 |
1984年 | 85篇 |
1983年 | 44篇 |
1982年 | 66篇 |
1981年 | 59篇 |
1980年 | 55篇 |
1979年 | 46篇 |
1978年 | 55篇 |
1977年 | 38篇 |
1976年 | 34篇 |
1975年 | 24篇 |
1963年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Herbert Weisz Siegbert Pantel Carmen Maria Dilger Ulrike Glatz 《Mikrochimica acta》1984,82(1-2):69-83
Summary The ring oven segment technique already described earlier has been applied to the determination of ng- tog-amounts of organic and inorganic substances using fluorescence reactions. The methods proposed here are based either on self-fluorescence or on appearance or disappearance of fluorescence following a chemical reaction.
Anwendung von Fluoreszenzreaktionen in der Ringofen-Segmenttechnik
Zusammenfassung Die bereits früher beschriebene Ringofen-Segmenttechnik wurde auf die Bestimmung organischer und anorganischer Substanzen im ng- bisg-Bereich mit Hilfe von Fluoreszenzreaktionen angewendet. Die hier vorgeschlagenen Methoden beruhen entweder auf Eigenfluoreszenz oder aber auf der Bildung bzw. dem Verschwinden von Fluoreszenz nach einer chemischen Reaktion.相似文献
92.
Carlo Bonini Lucia Chiummiento Maria Funicello Gerardina Suanno Pietro Campaner 《Tetrahedron》2005,61(27):6580-6589
An efficient method has been developed for the synthesis of a versatile intermediate bearing azido, hydroxyl and ester functions, a useful precursor for peptidomimetic compounds. The two main features for this synthesis were the use of the Sharpless asymmetric dihydroxylation on thiophene acrylate and the subsequent regioselective ring opening by sodium azide of the cyclic sulfite. Highly chemoselective reduction of the azido alcohol led to a key compound which was utilized for the synthesis of two analogues of commercial anti HIV PR such as nelfinavir and saquinavir. The biological activity and molecular modelling study on these two new potential drugs have been evaluated. 相似文献
93.
Lyubomir T. Vlaev Maria M. Nikolova Georgy G. Gospodinov 《Monatshefte für Chemie / Chemical Monthly》2005,83(2):1553-1566
The dehydration kinetics of crystallohydrates of beryllium, magnesium and calcium selenites were studied under non-isothermal conditions. The values of the activation energy of dehydration, the pre-exponential factor in Arrhenius equation and the change of entropy for the formation of the activated complex were calculated using the calculation procedure of Coats and Redfern. Thermal stability and activation energy of dehydration of the crystallohydrates were found to increase by the same order. The relationships observed were interpreted using Klopman’s generalized perturbation theory of chemical reactivity. The same theory was applied to explain the differences in the IR spectra of the selenite crystallohydrates studied. 相似文献
94.
An enzyme inhibition biosensor, developed in our laboratory and previously used for the analysis of compounds with anticholinesterase activity (e.g. physostigmine, neostigmine, pyridostigmine nicotine and organophosphorus compounds) has now been tested for the analysis of another recently synthesized cholinesterase inhibitor, i.e. eptastigmine. In addition nicotinic acid and nicotinamide, although displaying weaker inhibition properties, were also tested in pharmaceutical products using the same inhibition enzyme sensor. The biosensor consisted of a hydrogen peroxide amperometric electrode coupled to a functionalised nylon membrane chemically bonding both the enzymes butyrylcholinesterase and choline oxidase; a butyrylcholine standard solution in glycine buffer acted as substrate. The response of the system to all the inhibitors considered was characterised completely and the analysis of several pharmaceutical formulations containing nicotinamide or nicotinic acid was also performed. 相似文献
95.
Maria L. Foresti Francesco Pergola Rolando Guidelli 《Journal of Electroanalytical Chemistry》1980,107(2):307-321
Both the oxidation of Cu0 at dropping amalgam electrodes immersed in solutions of benzotriazole (BTA) and the reduction of CuII at a dropping mercury electrode from BTA solutions have been investigated by the single potential-step chronocoulometric technique. The dependence of the charge Q(t) flowing as a consequence of a given potential jump Ei→Ef upon the initial and final potentials Ei and Ef, as well as upon the time t elapsed from the instant of the potential jump provides direct evidence for the presence of a single adsorbed monolayer of a CuI compound on a mercury electrode immersed in a CuII solution containing BTA, at applied potentials positive to ≈?0.4 V/SCE. Analogous measurements carried out at dropping amalgam electrodes reveal the presence of a single adsorbed monolayer of a CuI compound, or else of an adsorbed multilayer, depending on the potential range investigated. The results of the chronocoulometric measurements are in agreement with those of the polarographic measurements of Part I. 相似文献
96.
The application of contact transformation theory to the perturbed harmonic oscillator is reexamined in the light of the harmonic oscillator tensors previously presented. It is found that the recasting of the formalism of this problem in terms of harmonic oscillator tensors results in great simplifications, most of which stem from the introduction of the additional algebraic quantum numbers (l, m). The order of magnitude of each fragment of the Hamiltonian is easily recognizable, and the diagonal and nondiagonal parts contained therein are readily identifiable. The determination of the contact transformation operator is reduced to a simple formula. First, an analysis is made for a single mode of vibration, and it is subsequently extended to a multimode case. The perturbed diatomic vibrator is presented as an example. 相似文献
97.
Summary Copper(II) complexes derived from substituted cinnamic acids 3, 4-dimethoxycinnamic acid (3, 4-DMCH) and 3, 5-dimethoxycinnamic acid (3, 5-DMCH), of the formula [Cu(3, 4-DMC)2]·H2O (1), [Cu(3, 5-DMC)2]·H2O (2) were prepared. The magnetic properties of the complexes suggest dimeric structures typical of copper(II) acetate monohydrate-like complexes. X-band e.s.r. spectra of polycrystalline samples at low temperature are typical of triplet state systems S=1. Their ability to catalyze the aerial oxidation of 3, 5-di-t-butylcatechol was measured spectrophotometrically at 30°C. The complexes are models for oxidases. 相似文献
98.
A function is presented to test equality between lists. In any case the function requires at most one single traversal of the lists and behaves as follows:
- if there are no cycles, it works as standardEQU AL functions;
- if there are cycles, it does not give information about equality, but it can detect cycles, i.e. it signals which lists are cyclic.
99.
100.
Friedrich Boberg Maria Ruhr Karl-Heinz Garburg Alfons Garming 《Journal of heterocyclic chemistry》1986,23(3):759-766
Dihydro-1,2-oxazines 8 and 12-oxazines 12 are formed by the reaction of the nitronic acids of the adducts of α-nitroolefins and β-dicarbonyl compounds ( 6 ) by two ways: A ) The nitronic acids 6 are heated in a boiling solution of urea in ethanol or methanol. B ) The nitronic acids 6 are reduced with an aqueous solution of ammoniachloride and sodium sulfide. 1H-nmr and 13C-nmr investigations prove the constitutions. The mechanism is discussed. 相似文献