全文获取类型
收费全文 | 529篇 |
免费 | 23篇 |
国内免费 | 4篇 |
专业分类
化学 | 377篇 |
晶体学 | 6篇 |
力学 | 7篇 |
数学 | 84篇 |
物理学 | 82篇 |
出版年
2023年 | 5篇 |
2022年 | 10篇 |
2021年 | 22篇 |
2020年 | 15篇 |
2019年 | 15篇 |
2018年 | 13篇 |
2017年 | 6篇 |
2016年 | 13篇 |
2015年 | 12篇 |
2014年 | 21篇 |
2013年 | 44篇 |
2012年 | 37篇 |
2011年 | 26篇 |
2010年 | 24篇 |
2009年 | 27篇 |
2008年 | 32篇 |
2007年 | 28篇 |
2006年 | 33篇 |
2005年 | 30篇 |
2004年 | 15篇 |
2003年 | 17篇 |
2002年 | 16篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 5篇 |
1998年 | 5篇 |
1996年 | 10篇 |
1995年 | 3篇 |
1993年 | 6篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1964年 | 2篇 |
1951年 | 2篇 |
1931年 | 2篇 |
1927年 | 2篇 |
1926年 | 2篇 |
1925年 | 1篇 |
1924年 | 1篇 |
1923年 | 1篇 |
1912年 | 1篇 |
1910年 | 1篇 |
1909年 | 1篇 |
1868年 | 1篇 |
排序方式: 共有556条查询结果,搜索用时 15 毫秒
1.
Wen Zhang Hui Huang Yu Ding Margherita Gavagnin Ernesto Mollo Guido Cimino Yue‐Wei Guo 《Helvetica chimica acta》2006,89(4):813-820
Three new polyoxygenated steroids, muricesteroid ( 1 ), and menellsteroids A ( 2 ) and B ( 3 ), were isolated from two species of the South China Sea gorgonian Muricella flexuosa and Menella verrucosa Brundin , respectively. The structures of these new compounds were elucidated on the basis of extensive spectroscopic analysis, chemical methods and comparison with known related compounds. 相似文献
2.
We investigate the dependence of the relaxation time of the current flowing in a nematic cell submitted to an external dc voltage on the physical properties of the substrate. We show that previously presented analyses of the same problem are not very useful for practical applications. We compare our theoretical predictions with experimental data, and show that the agreement is rather good. The influence of the adsorption-desorption phenomenon on the relaxation time is also discussed. 相似文献
3.
Grumelli DE Forzani ES Morales GM Miras MC Barbero CA Calvo EJ 《Langmuir : the ACS journal of surfaces and colloids》2004,20(6):2349-2355
The sulfonation of polyaniline (PANI) films by nucleophilic addition of sulfite ion has been controlled through the polymer oxidation state under electrochemical control. The process was monitored by in situ electrochemical quartz crystal microbalance (EQCM), and the polymer oxidation was accomplished by electrode potential steps in sulfite aqueous solutions. The nucleophilic addition of sulfite to PANI only takes place on the oxidized polymer. From the ratio of added mass to the injected charge, the degree of sulfonation has been obtained with a yield as high as 50%. It has been observed that the ion-exchange mechanism during the oxidation-reduction process in the resulting sulfonated polymer is analogous to the polymer produced by electrophilic sulfonation of polyaniline or by copolymerization of aniline with aminosulfonic acids, unlike the ionic exchange observed for unmodified PANI. 相似文献
4.
Environmental certification is becoming the main tool for application of sustainable development principles. The European Regulation Emas and the international standard ISO 14001 both require for certification, to perform an environmental management system to prevent environmental impacts and to continuously improve environmental performance. For a good environmental performance evaluation (EPE), certification needs to use scientific methodologies and to interface with scientific research; here we proposed emergy analysis as a valid method for EPE and emergetic environmental performance and condition indicators (EPIs, ECIs) to monitor a territorial system: Ravenna province (Italy). Together with emergy indicators were selected other indicators for a deeper EPE: emitted/adsorbed CO2, energy consumptions, air and water pollution measures. The paper showed that Ravenna system has a good environmental performance and demonstrated how different indicators from the most advanced chemical research (chemical-physical, analytical, etc.) contribute to a complete EPE of a complex territorial system and are useful for environmental certification and sustainable development. 相似文献
5.
6.
Summary. The reaction of 2-chloroisobutyrophenones and nitromethanide anion gives stereoselectively (E)-3-nitro allylic alcohols. The Gibbs free enthalpies of reaction in DMSO for carbanion addition, epoxide formation, and rearrangement to 3-nitro allylic alcohol, as elementary steps for the reaction, were estimated from corresponding neutral gas reactions and using a thermodynamical approach to the transfer of gaseous compounds to DMSO. A criterion for assigning the sign of affinity of liquid compounds to DMSO was developed on the basis of the Gibbs enthalpies of liquefaction. The information obtained on reaction rate and thermodynamic viability of the steps indicates that carbanion addition is the rate-determining step.In memory of Prof. Dr. M. Ballester, deceased on April 6, 2005 相似文献
7.
Ashton PR Balzani V Clemente-León M Colonna B Credi A Jayaraman N Raymo FM Stoddart JF Venturi M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(3):673-684
Aliphatic amines, incorporating one or three (branched) acylated beta-D-glucopyranosyl residues, were coupled with the acid chloride of ferrocenecarboxylic acid and with the diacid chloride of 1,1'-ferrocenedicarboxylic acid to afford four dendrimer-type, carbohydrate-coated ferrocene derivatives in good yields (54-92%). Deprotection of the peracylated beta-D-glucopyranosyl residues was achieved quantitatively by using Zemplén conditions, affording four water-soluble ferrocene derivatives. When only one of the two cyclopentadienyl rings of the ferrocene unit is substituted, strong complexes are formed with beta-cyclodextrin in H2O, as demonstrated by liquid secondary ion mass spectrometry (LSIMS), 1H NMR spectroscopy, electrochemical measurements, and circular dichroism spectroscopy. Molecular dynamics calculations showed that the unsubstituted cyclopentadienyl ring is inserted through the cavity of the toroidal host in these complexes. The electrochemical behavior of the protected and deprotected ferrocene-containing dendrimers was investigated in acetonitrile and water, respectively. The diffusion coefficient decreases with increasing molecular weight of the compound. The potential for oxidation of the ferrocene core, the rate constant of heterogeneous electron transfer, and the rate constant for the energy-transfer reaction with the luminescent excited state of the [Ru(bpy)3]2+ complex (bpy = 2,2'-bipyridine) are strongly affected by the number (one or two) of substituents and by the number (one or three) of carbohydrate branches present in the substituents. These effects are assigned to shielding of the ferrocene core by the dendritic branches. Electrochemical evidence for the existence of different conformers for one of the dendrimers in aqueous solution was obtained. 相似文献
8.
The mean square tilt angle of a nematic slab with finite anchoring energy and periodic boundary conditions has been theoretically investigated, as a function of the slab geometry and of the reduced extrapolation length. If the anchoring strength is free-surfacelike, the contrast is affected by a loss 10% at room temperature if the ratio between the anchoring pitch and the cell thickness is 0.5.Glossary
anchoring pitch
-
h
cell thickness
-
/h
- ( = x/, = y/h)
reduced coordinates
-
(, )
local tilt angle
-
elastic constant
- wa
anchoring energy anisotropy
-
b=/w
a
de Gennes-Kleman extrapolation length
-
B=b/h
reduced extrapolation length
-
T
NI
nematic-isotropic transition temperature
-
:=(T/T
NI
) – 1
reduced temperature
-
easy axis direction
-
MAX
-
±
2
mean square tilt angle along the boundary
-
()
absorbance coefficients of the p-dye
-
r
/: dichroic ratio
-
c
contrast
-
G
contrast gain
-
S
order parameter 相似文献
9.
Sommovigo M Ferretti A Venturi M Ceroni P Giardi C Denti G 《Inorganic chemistry》2002,41(5):1263-1271
A study of absorption spectra in the near-infrared (NIR) and visible (vis) regions of trinuclear Ru complexes containing pyrazine (pyz) as bridging ligand, trans-[(Ru(NH(3))(5)pyz)(2)Ru(NH(3))(4)](m+)(m = 6-9), is reported. The spectra were recorded on aqueous solutions containing the described species formed in situ by stoichiometric additions of a standard solution of Ce(SO(4))(2). They were interpreted in terms of a simple 5-orbital-3-parameter model which includes the effects of d-pi interaction and electronic correlation. The model is shown to account for the observed NIR-vis spectra of the complex ions. The 6+ parent species was synthesized by an improved literature method and fully characterized. The novel 8+ complex was also prepared and characterized. The 9+ ion was established to be slowly reduced by water, with dioxygen formation. Electrochemical (CV and DPV) studies were performed on the trinuclear 6+ complex, as well as on its constituent fragments [Ru(NH(3))(5)(pyz)](2+) and trans-[Ru(NH(3))(4)(pyz)(2)](2+). 相似文献
10.
Zusammenfassung Die allgemeine Verwendbarkeit von modifizierten Bentone-Kolonnen zur Trennung von aromatischen Kohlenwasserstoffen, insbesondere Polyphenylen, wird gezeigt. Trennresultate auf festen Caesiumchlorid-Phasen, die bis 500° C verwendbar und praktisch unbegrenzt haltbar sind, werden gegenübergestellt.Als Beispiele für die Trennung substituierter Aromaten werden bromierte Bi- und Terphenyle angeführt. Es wurden nur gepackte Kolonnen verwendet.
Die vorliegende Arbeit wurde im Rahmen des ReaktorforschungsprogrammsOrgel ausgeführt.
Die Fa. Progil, Lyon, steuerte unter Euratom-Kontrakt wesentliche Ergebnisse und dank langjähriger Erfahrung auf dem Gebiet der Polyphenyle wertvolle fachliche Diskussionen bei.
Die Autoren danken auch Herrn Dr. W.Keess, der als Euratom-Forschungspraktikant insbesondere mit der Ausarbeitung von Analysenmethoden für die leichtersiedenden Anteile der Polyphenylgemische beauftragt war. 相似文献
Summary The general utility of modified Bentone-columns for the separation of aromatic hydrocarbons, especially of polyphenyls, is shown. The separation results are compared with those obtained on solid cesium chloride phases which guarantee an almost unlimited duration of life, even up to 500° C. Mixtures of brominated bi- and terphenyls figure as examples for the separation of substituted aromatics. Packed columns have been used exclusively.
Die vorliegende Arbeit wurde im Rahmen des ReaktorforschungsprogrammsOrgel ausgeführt.
Die Fa. Progil, Lyon, steuerte unter Euratom-Kontrakt wesentliche Ergebnisse und dank langjähriger Erfahrung auf dem Gebiet der Polyphenyle wertvolle fachliche Diskussionen bei.
Die Autoren danken auch Herrn Dr. W.Keess, der als Euratom-Forschungspraktikant insbesondere mit der Ausarbeitung von Analysenmethoden für die leichtersiedenden Anteile der Polyphenylgemische beauftragt war. 相似文献