全文获取类型
收费全文 | 159篇 |
免费 | 7篇 |
专业分类
化学 | 125篇 |
晶体学 | 7篇 |
力学 | 1篇 |
数学 | 13篇 |
物理学 | 20篇 |
出版年
2022年 | 1篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 7篇 |
2018年 | 5篇 |
2017年 | 6篇 |
2016年 | 2篇 |
2015年 | 4篇 |
2014年 | 9篇 |
2013年 | 18篇 |
2012年 | 6篇 |
2011年 | 8篇 |
2010年 | 14篇 |
2009年 | 6篇 |
2008年 | 17篇 |
2007年 | 16篇 |
2006年 | 8篇 |
2005年 | 4篇 |
2004年 | 3篇 |
2003年 | 1篇 |
2002年 | 1篇 |
2001年 | 5篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 4篇 |
1996年 | 1篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
排序方式: 共有166条查询结果,搜索用时 15 毫秒
101.
The presence of circulating free DNA in the sera of cancer patients has been reported by different authors. Moreover, a hypothetical relationship between the DNA level and different stages of the disease has also been suggested. In this work, the free serum DNA level in various cancer patients was measured and consistent results with the reported data were found. (1,4) To assess the possible mechanism for the release of this fraction and to explain the high levels observed in cancer patients levels of the bivalent intracellularcation, magnesium were determined. Further to relate the process to the antioxidative mechanism of the cell membrane also selenium levels were measured simultaneously. The serum free DNA and the magnesium concentration of the patients before treatment were significally higher and the selenium levels lower than the control group. After appropriate the rapy the magnesium levels were found to decline while the high free DNA and low selenium values still persisted. These findings imply that the membrane stability and permeability is impaired due both to the neoplastic changes and later to the effect of chemotherapeutic agents with the result that intracellular material is released into the circulation. 相似文献
102.
On small scales spacetime can be understood as some kind of spacetime foam of fluctuating bubbles or loops which are expected
to be an outcome of a theory of quantum gravity. One recently discussed model for this kind of spacetime fluctuations is the
holographic principle which allows to deduce the structure of these fluctuations. We review and discuss two scenarios which
rely on the holographic principle leading to holographic noise. One scenario leads to fluctuations of the spacetime metric
affecting the dynamics of quantum systems: (i) an apparent violation of the equivalence principle, (ii) a modification of
the spreading of wave packets, and (iii) a loss of quantum coherence. All these effects can be tested with cold atoms. 相似文献
103.
The effect of agarose content on thermal phase transitions of the agarose gels was investigated by using Steady State Fluorescence
(SSF) method. Scattered light, I
sc
and fluorescence intensity, I
fl
were monitored against temperature during heating and cooling processes to investigate phase transitions. Two regions were
observed during the heating and cooling processes. At the high temperature region, double helix to coil (h-c) transition took
place. However, during the cooling process coil to double helix (c-h) transitions occurred at low temperature region. Transition
energies were determined using the Arrhenius treatment, and found to be strongly correlated with the agarose content in the
gel system. Transition temperatures were determined from the derivative of the sigmoidal transition paths and found to be
increased by increasing agarose content in both cases. 相似文献
104.
Decahydronaphthalene-1,2,3,4,5,6,7-heptol, a new polycyclitol, was synthesized starting from p-benzoquinone. An endo selective Diels-Alder cycloaddition between p-benzoquinone and 1-acetoxybutadiene followed by stereoselective reduction with NaBH4/CeCl3·7H2O led to the formation of an allylic cis-diol. The formed diol was converted into its acetate with Ac2O/pyridine, in a transformation that required inert atmosphere conditions to suppress a competing aromatization. Controlled oxidation by OsO4 of two olefinic bonds followed by acetylation yielded the heptaacetate whose structure was established unequivocally via application of X-ray crystallographic methods. Removal of the acetate groups by NH3 provided the target heptol. In addition, the carbonic anhydrase inhibitory potency of the title compound was investigated and it was shown to be a potent inhibitor compared to the standard CA inhibitors. 相似文献
105.
New polycyclitols were synthesized starting from 1,4-naphthoquinone. An endo-selective Diels–Alder cycloaddition between 1,4-naphthoquinone and 1-acetoxybutadiene afforded a diketone. Reduction of the diketone with a NaBH4-CeCl3 · 7H2O system gave a new cyclitol analog diol acetate. Acetylation of this compound afforded a triacetate. Oxidation of the double bond of the triacetate compound with OsO4 followed by acetylation of hydroxyl groups gave the pentaacetate, whose structure was established unequivocally via application of x-ray crystallographic methods. Hydrolysis of ester groups of pentaacetate under basic condutions furnished the desired and another novel aryl cyclitol. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource: Full experimental and spectral details.] 相似文献
106.
Nitrogen adsorption at 77 K on mesoporous silica materials (MPS) with varying synthesis time and treatment conditions was
investigated. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were also used to characterize the mesoporous
materials. This study was performed at 6, 24 and 72-h synthesis times. It is shown that 6-h is not enough for complete formation
of the MPS material and at least 24-h is necessary. The pore structure starts decaying for the 72-h synthesis time. The three-after-synthesis
treatment conditions used were 1) washed, 2) washed and calcined and 3) directly calcined after synthesis. Ethanol/HCl mixtures
were used for washing and calcinations were performed at 550°C. Among these samples, directly washed sample yields the lowest
adsorption capacity while washed and calcined sample yields the highest adsorption capacity. Hence, it is concluded that washing
stabilizes the structure before high temperature treatment. 相似文献
107.
Mustafa Çelebier Ertan Şahin Nilgün Ancın Nurşen Altuntaş Öztaş Selma Gül Öztaş 《应用有机金属化学》2007,21(10):913-918
A series of neutral complexes, namely, [N‐(2‐hydroxy‐4‐nitrophenyl)‐3‐hydroxysalicylideneiminato]‐ diphenyltin(IV) ( Ia ), [N‐(2‐hydroxy‐4‐nitrophenyl)‐3‐methoxysalicylideneiminato]diphenyltin(IV) ( IIa ) and [N‐(2‐hydroxy‐4‐nitrophenyl)‐3‐ethoxysalicylideneiminato]diphenyltin(IV) ( IIIa ) were prepared by the reaction of diphenyltin dichloride on the corresponding Schiff bases. The Schiff bases were the reaction products of 2‐hydroxy‐4‐nitroaniline and appropriate salicylaldehydes. All the compounds were characterized by elemental analysis, 1H‐NMR, 13C‐NMR, IR and mass spectroscopy. Compound IIIa was also characterized by single crystal X‐ray diffraction and shows a C2NO2 coordination geometry nearly half‐way between a trigonal bipyramidal and square pyramidal arrangement. In the solid state, π? π interactions exist between the aniline fragments of neighbouring molecules. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
108.
A new, fast and easy method for one‐pot reaction of aromatic aldehydes and dialdehydes with malononitrile and cyanogen bromide has been developed to afford full substituted 3‐arylcyclopropane‐1,1,2,2‐tetracarbonitriles in excellent yields in very short time (about 5 seconds). The structures elucidations werev characterized by IR, 1H NMR, 13C NMR, mass spectrometry and X‐ray crystallography techniques. For vthese compounds the crystallographic data showed two structures in mirror image in solid case and one distinct structure in solution. The reaction mechanism was discussed. 相似文献
109.
M. Duran Toksarı Selçuk K. İşleyen Ertan Güner Ömer Faruk Baykoç 《Applied Mathematical Modelling》2008
In this paper, we introduced learning effect into assembly line balancing problems. In many realistic settings, the produced worker(s) (or machine(s)) develops continuously by repeated the same or similar activities. Therefore, the production time of product shortens if it is processed later. We show that polynomial solutions can be obtained for both simple assembly line balancing problem (SALBP) and U-type line balancing problem (ULBP) with learning effect. 相似文献
110.
A new aminoquercitol derivative was synthesized starting from 1,4-cyclohexadiene. Photooxygenation of cyclohexa-1,4-diene afforded anti-2,3-dioxabicyclo[2.2.2]oct-7-en-5-yl hydroperoxide as the main product. The formed hydroperoxy endoperoxide was reduced with LiAlH4 to produce anti-2,3-dioxabicyclo[2.2.2]oct-7-en-5-ol. Protection of alcohol with acetyl chloride followed by reduction of the endoperoxide with thiourea, and then palladium-catalyzed ionization/cyclization reaction gave an oxazolidinone derivative. Hydrolysis of the oxazolidinone ring and acetylation gave an amino compound. Oxidation of the double bond in the amino compound with OsO4 followed by acetylation gave the amino tetraacetate and removal of the acetate groups furnished the desired aminoquercitol whose exact configuration was determined by X-ray diffraction analysis. 相似文献