全文获取类型
收费全文 | 4156篇 |
免费 | 74篇 |
国内免费 | 14篇 |
专业分类
化学 | 2799篇 |
晶体学 | 25篇 |
力学 | 64篇 |
数学 | 855篇 |
物理学 | 501篇 |
出版年
2019年 | 36篇 |
2016年 | 53篇 |
2015年 | 49篇 |
2014年 | 54篇 |
2013年 | 137篇 |
2012年 | 104篇 |
2011年 | 111篇 |
2010年 | 81篇 |
2009年 | 112篇 |
2008年 | 141篇 |
2007年 | 115篇 |
2006年 | 113篇 |
2005年 | 131篇 |
2004年 | 121篇 |
2003年 | 102篇 |
2002年 | 95篇 |
2001年 | 81篇 |
2000年 | 72篇 |
1999年 | 45篇 |
1998年 | 47篇 |
1997年 | 64篇 |
1996年 | 61篇 |
1995年 | 47篇 |
1994年 | 65篇 |
1993年 | 59篇 |
1992年 | 67篇 |
1991年 | 50篇 |
1990年 | 54篇 |
1989年 | 70篇 |
1988年 | 77篇 |
1987年 | 81篇 |
1986年 | 41篇 |
1985年 | 69篇 |
1984年 | 65篇 |
1983年 | 38篇 |
1982年 | 55篇 |
1981年 | 60篇 |
1980年 | 82篇 |
1979年 | 60篇 |
1978年 | 63篇 |
1977年 | 40篇 |
1976年 | 39篇 |
1975年 | 55篇 |
1974年 | 46篇 |
1973年 | 55篇 |
1972年 | 36篇 |
1971年 | 39篇 |
1970年 | 38篇 |
1967年 | 32篇 |
1932年 | 34篇 |
排序方式: 共有4244条查询结果,搜索用时 31 毫秒
1.
Enantiomeric Cyclic Peptides with Different Caco‐2 Permeability Suggest Carrier‐Mediated Transport
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
B. Sc. Eduard Puig Prof. Dr. Beat Ernst Prof. Dr. Horst Kessler 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):8023-8027
Recently, oral absorption of cyclic hexapeptides was improved by N‐methylation of their backbone amides. However, the number and position of N‐methylations or of solvent exposed NHs did not correlate to intestinal permeability, measured in a Caco‐2 model. In this study, we investigate enantiomeric pairs of three polar and two lipophilic peptides to demonstrate the participation of carrier‐mediated transporters. As expected, all the enantiomeric peptides exhibited identical lipophilicity (logD7.4) and passive transcellular permeability determined by the parallel artificial membrane permeability assay (PAMPA). However, the enantiomeric polar peptides exhibited different Caco‐2 permeability (Papp) in both directions a–b and b–a. The same trend was observed for one of the lipophilic peptide, whereas the second lipophilic enantiomer pair showed identical Caco‐2 permeability (within the errors). These findings provide the first evidence for the involvement of carrier‐mediated transport for peptides, especially for those of polar nature. 相似文献
2.
Gary W. Breton Lauren A. Hahn Kenneth L. Martin 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(9):1208-1212
Tetrahydrotetrazoles are five‐membered‐ring heterocycles containing four contiguous saturated nitrogen atoms. Very few examples of such compounds have been reported in the literature. Our previous attempt at the synthesis of a member of this class of compound suggested that the N—N bonds may be more labile than expected. This finding raised the question as to whether the structures of any of the previously reported tetrahydrotetrazoles had been properly assigned. We have reproduced the synthesis of a reported tetrahydrotetrazole, namely 1,2‐di‐tert‐butyl 3‐phenyl‐1H,2H,3H,10bH‐[1,2,3,4]tetrazolo[5,1‐a]isoquinoline‐1,2‐dicarboxylate, C25H30N4O4, and have now confidently confirmed its structure via X‐ray crystallography. However, while sufficiently stable in the crystal phase, we discovered that it remains very labile in solution (having a half‐life of only 15 min at 20 °C in CDCl3). A tentative reaction pathway for its dissociation based on 1H NMR spectral evidence is provided. 相似文献
3.
The European Physical Journal E - The isothermal gas adsorption of two hexane isomers (n-hexane and cyclohexane) in the mesopores of MCM-41 silica have been investigated by small angle neutron... 相似文献
4.
Horst Guido Müller 《Colloid and polymer science》1941,96(1):11-19
Zusammenfassung Einige charakteristische ?nderungen physikalischer Werkstoffeigenschaften beim heterogenen Zerfall übers?ttigter Mischkristalle
werden am Beispiel der fl?chenzentrierten Eisen-Nickel-Kupfer-Legierungen beschrieben. Bei der Behandlung des Einflusses der
Ausscheidungsvorg?nge auf die Rekristallisationserscheinungen tritt die Bedeutung des übers?ttigungsgrades der zerfallenden
festen L?sung für den Ablauf dieser Zustands?nderungen besonders klar zutage. Kritische Stadien der Ausscheidung und der Koagulation
der heterogenen Phasen sind bei diesen Legierungen für die Erzeugung besonders hochwertiger magnetischer Eigenschaften verantwortlich.
Der vorliegende Bericht wurde von Herrn Prof., Dr. Wo. Ostwald angeregt. Für das diesem Arbeitsgebiet entgegengebrachte Interesse
m?chte ich Herrn Prof. Ostwald meinen besten Dank aussprechen. 相似文献
5.
In this paper, the special construction of a parallel robot, called spatial servopneumatic multi-axis test facility, will be discussed. The investigations include the following aspects: (i) the laboratory set-up of the robot, (ii) various results obtained in laboratory experiments, taking into account quite different control algorithms and command-input signals, (iii) a comparison of the laboratory experiments with the computer simulations of Part I of this paper, and ({vi}) a quality check of the results compared with the cost of the different controller realizations. The results of both the computer simulations and the laboratory experiments show: (i) The dynamic behavior of the parallel structure can be tremendously improved by using sophisticated nonlinear control algorithms. (ii) This improvement has to be paid by a drastically increased amount of work for deriving the model equations and control algorithms, and by augmented hardware cost of the sensing elements and controller electronics. (iii) Carefully developed model equations and identified model parameters provide theoretical models of the complex parallel structure that are very close to reality. This enables the design engineer to systematically investigate constructive alternatives of the design parameters, sensor and actuator concepts, and control strategies of the MAP prior to their hardware realization.This work has been supported by the German Science Foundation (DFG) under Contract No. Ha 1666/6-3. 相似文献
6.
7.
Kerstin Paech Adrian Dumitru Jürgen Schaffner-Bielich Stefan Schramm Horst Stöcker Gebhard Zeeb Detlef Zschiesche 《Acta Physica Hungarica A》2004,21(2-4):151-156
The measured particle ratios in central heavy-ion collisions are investigated within a chemical and thermal equilibrium chiral SU(3) σ?ω approach. Contrary to the commonly adopted non-interacting gas calculations, the chiral SU(3) model predicts modified effective hadron masses and effective chemical potentials in the medium and a transition to a chirally restored phase at high temperatures or chemical potentials. the influence of three different types of phase transitions is investigated. We show that the deduced freeze-out values considerably depend on the underlying model while the quality of the fit is approximately the same. 相似文献
8.
The association of complex formation with static quenching in CT systems was investigated. Evaluation of the data made evident
that the inner filter effect must be allowed for. Time-resolved and temperature-dependent stationary measurements of fluorescence
led to the separation of dynamic and static quenching components. The static quenching constant is discussed with respect
to the equilibrium constant of complex formation determined by absorption spectroscopy. 相似文献
9.
10.
Horst Szambien 《Results in Mathematics》1992,22(1-2):619-621