全文获取类型
收费全文 | 144篇 |
免费 | 2篇 |
专业分类
化学 | 79篇 |
晶体学 | 6篇 |
力学 | 5篇 |
数学 | 20篇 |
物理学 | 36篇 |
出版年
2022年 | 8篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2017年 | 3篇 |
2016年 | 2篇 |
2015年 | 2篇 |
2013年 | 17篇 |
2012年 | 7篇 |
2011年 | 8篇 |
2010年 | 1篇 |
2009年 | 7篇 |
2008年 | 6篇 |
2007年 | 6篇 |
2006年 | 6篇 |
2005年 | 5篇 |
2004年 | 7篇 |
2003年 | 4篇 |
2002年 | 3篇 |
2001年 | 1篇 |
2000年 | 1篇 |
1999年 | 1篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1990年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1973年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有146条查询结果,搜索用时 0 毫秒
51.
In this study we extend a model, proposed by Dendrinos, which describes dynamics of change of influence in a social system containing a public sector and a private sector. The novelty is that we reconfigure the system and consider a system consisting of a public sector, a private sector, and a non-governmental organizations (NGO) sector. The additional sector changes the model’s system of equations with an additional equation, and additional interactions must be taken into account. We show that for selected values of the parameters of the model’s system of equations, chaos of Shilnikov kind can exist. We illustrate the arising of the corresponding chaotic attractor and discuss the obtained results from the point of view of interaction between the three sectors. 相似文献
52.
Representing a design graphically can make certain properties of a design obvious that otherwise would require tedious reasoning. This holds for symmetries, resolvability, and subdesign inclusion in appropriate cases. We describe how a graphical representation can be achieved in many cases and show several examples, in particular visualizations of the biplanes on up to 16 vertices and subdesigns of the large Witt design. © 2011 Wiley Periodicals, Inc. J Combin Designs 20:1–22, 2012 相似文献
53.
The dissolution of an iron-containing substance in orthophosphoric acid either alone or mixed with nitric acid or nitric acid-sulphuric acid mixture at 270-280 degrees has been examined, to determine the composition of the solutions which are used for analytical determinations. The possibility of condensation of H(3)PO(4) and formation of polyphosphoric acids during the dissolution has been proved. Solutions which may contain pyrophosphoric acid, tripolyphosphoric acid and tetrapolyphosphoric acid have been obtained. The exact composition of these solutions depends on the initial H(3)PO(4) concentration, the heating time, and, in some cases, the presence of nitric acid and sulphuric acid. It is assumed that the nitric and sulphuric acid affect the loss of water and therefore the thermal dehydration of the orthophosphoric acid during the dissolution. No qualitative changes in the phosphoric acid composition have been observed in the presence of Fe(III). Formation of Fe(III) polyphosphates is assumed. 相似文献
54.
Georgieva DN Genov N Nikolov P Aleksiev B Rajashankar KR Voelter W Betzel C 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2003,59(3):617-627
The neurotoxic complex Vipoxin is the lethal component of the venom of Vipera ammodytes meridionalis, the most toxic snake in Europe. It is a complex between a toxic phospholipase A2 (PLA2) and a non-toxic and catalytically inactive protein, stabilizing the enzyme and reducing the activity and toxicity. Structure-function relationships in this complex were studied by spectroscopic methods. A good correlation between the ionization behaviour and accessible surface area (ASA) of the tyrosyl residues was observed. In the toxic PLA2 subunit phenolic groups participate in H-bonding network that stabilizes the catalytically and pharmacologically active conformation. The tryptophan fluorescence decay of Vipoxin is well fitted by two exponentials with lifetimes of 0.1 (54%) and 2.5 (46%) ns. W20P, W31P and W31I are located in the interface between the two subunits and participate in hydrophobic interactions stabilizing the complex. Dissociation of the complex leads to a transition of the tryptophans from hydrophobic to hydrophilic environment, which influences mainly tau2. The longer lifetime is more sensitive to the polarity of the environment. Circular dichroism measurements demonstrate that the two components of the neurotoxin preserve their secondary structure after dissociation of the complex. The results of the spectroscopic studies are in accordance with a mechanism of blockade of transmission across the neuromuscular junctions of the breathing muscles by interaction of a dissociated toxic PLA2 with a membrane. The loss of toxicity is connected with slight changes in the secondary structure of PLA2. CD studies also show a substantial contribution of disulfide bonds to the stability of the neurotoxic complex and its components. 相似文献
55.
The percentage of thermal dehydration and decomposition of ordinary and deuterated hydrates of barium acetate (tri- and monohydrates) were studied with a derivatograph and by DSC method. The observed phase transitions were identified and their corresponding enthalpy changes determined. The latter were compared with those estimated on thermodynamic data. The conclusions made on the dehydration process in correlation with IR spectra were used as information about the structure of the dehydrated water in the compounds investigated. The end product was also identified as barium carbonate. 相似文献
56.
Galabov B Nikolova V Wilke JJ Schaefer HF Allen WD 《Journal of the American Chemical Society》2008,130(30):9887-9896
The S N2 identity exchange reactions of the fluoride ion with benzyl fluoride and 10 para-substituted derivatives (RC6H 4CH 2F, R = CH3, OH, OCH 3, NH2, F, Cl, CCH, CN, COF, and NO2) have been investigated by both rigorous ab initio methods and carefully calibrated density functional theory. Groundbreaking focal-point computations were executed for the C6H5CH 2F + F (-) and C 6H 5CH2Cl + Cl (-) SN2 reactions at the highest possible levels of electronic structure theory, employing complete basis set (CBS) extrapolations of aug-cc-pV XZ (X = 2-5) Hartree-Fock and MP2 energies, and including higher-order electron correlation via CCSD/aug-cc-pVQZ and CCSD(T)/aug-cc-pVTZ coupled cluster wave functions. Strong linear dependences are found between the computed electrostatic potential at the reaction-center carbon atom and the effective SN2 activation energies within the series of para-substituted benzyl fluorides. An activation strain energy decomposition indicates that the SN2 reactivity of these benzylic compounds is governed by the intrinsic electrostatic interaction between the reacting fragments. The delocalization of nucleophilic charge into the aromatic ring in the SN2 transition states is quite limited and should not be considered the origin of benzylic acceleration of SN2 reactions. Our rigorous focal-point computations validate the benzylic effect by establishing SN2 barriers for (F (-), Cl (-)) identity exchange in (C6H5CH2F, C6H 5CH2Cl) that are lower than those of (CH3F, CH3Cl) by (3.8, 1.6) kcal mol (-1), in order. 相似文献
57.
A new and efficient conjugate addition of trialkylphosphites to 3-ω-bromoacetylcoumarin 1 catalysed by p-toluenesulfonic acid (TsOH) has been studied. Under the same conditions, an enolphosphate gave the corresponding esters of 3-acetyl-4-phosphono-2-oxochromans in high yields. The use of TsOH in the reaction of 3-acetyl-, 3-benzoyl-, and 3-ethoxycarbonyl coumarins led mainly to 1,4-addition products—the corresponding 3-acyl-4-dialkylphosphono-2-oxochromans—in very good yields. 相似文献
58.
59.
L. Minkova E. Lefterova Ts. Koleva E. Nedkov M. Nikolova 《Colloid and polymer science》1988,266(10):898-905
Isotactic polypropylene films, Buplen Type, 40m thick, irradiated by a60Co source to doses 0.37–37 Mrad, are investigated by means of optical microscopy, WAXS, thermogravimetry, DSC and DTA. The original film exhibits a paracrystal structure. Irradiation does not change the films' structure. The kinetic parameters of the non-isothermal destruction and the thermodynamic parameters of melting are obtained. The samples irradiated to small doses (up to 3 Mrad) are thermally more stable; the activating energy of the destruction is higher than that of the original film. The temperature of melting slightly increases, while the enthalpy of melting decreases. For the range of doses of 3.7–37 Mrad, the films show low thermal stability and the destruction proceeds with low activating energy. From the results of the data obtained, the following assumptions are made: the-irradiation causes simultaneous crosslinking and chain scission at random sites along the chains. Fragments of partially crosslinked molecules and fractions of low molecular linear segments are formed. The destruction caused by radiation prevails above 3 Mrad. 相似文献
60.
Malinka P. Stoyanova Boris L. Shivachev Rosica P. Nikolova Vladimir Dimitrov 《Tetrahedron: Asymmetry》2013,24(21-22):1426-1434
Methylidene substituted camphor was easily epoxidized to give bridgehead oxirane-substituted camphor in the form of two diastereoisomers, isolated in pure form. These were used in aminolysis reactions with different secondary amines leading to series of chiral aminoalcohols bearing a ketone functionality. The latter could be reduced by using LAH or DIBAL to a series of aminodiols. For the determination of the configuration of the newly formed stereogenic centre, advanced NMR experiments and X-ray crystallography were used. The new aminoalcohols and aminodiols were tested as pre-catalysts for the enantioselective addition of Et2Zn to benzaldehyde showing moderate enantioselectivity. 相似文献