全文获取类型
收费全文 | 2799篇 |
免费 | 55篇 |
国内免费 | 13篇 |
专业分类
化学 | 2087篇 |
晶体学 | 18篇 |
力学 | 57篇 |
数学 | 343篇 |
物理学 | 362篇 |
出版年
2020年 | 19篇 |
2019年 | 19篇 |
2018年 | 13篇 |
2017年 | 21篇 |
2016年 | 34篇 |
2015年 | 36篇 |
2014年 | 46篇 |
2013年 | 134篇 |
2012年 | 114篇 |
2011年 | 131篇 |
2010年 | 72篇 |
2009年 | 48篇 |
2008年 | 137篇 |
2007年 | 132篇 |
2006年 | 131篇 |
2005年 | 133篇 |
2004年 | 118篇 |
2003年 | 77篇 |
2002年 | 100篇 |
2001年 | 42篇 |
2000年 | 50篇 |
1999年 | 39篇 |
1998年 | 28篇 |
1997年 | 34篇 |
1996年 | 57篇 |
1995年 | 36篇 |
1994年 | 43篇 |
1993年 | 38篇 |
1992年 | 30篇 |
1991年 | 36篇 |
1990年 | 29篇 |
1989年 | 23篇 |
1988年 | 30篇 |
1987年 | 27篇 |
1986年 | 23篇 |
1985年 | 51篇 |
1984年 | 51篇 |
1983年 | 42篇 |
1982年 | 60篇 |
1981年 | 58篇 |
1980年 | 64篇 |
1979年 | 50篇 |
1978年 | 58篇 |
1977年 | 41篇 |
1976年 | 34篇 |
1975年 | 48篇 |
1974年 | 44篇 |
1973年 | 30篇 |
1972年 | 22篇 |
1968年 | 17篇 |
排序方式: 共有2867条查询结果,搜索用时 15 毫秒
81.
Dieter Leibfritz Erhard Haupt Norbert Dubischar Heinrich Lachmann Raymond Oekonomopulos Günther Jung 《Tetrahedron》1982,38(14):2165-2181
The uncommon amino-acid 2-methylalanine (α-aminoisobutyric acid, Aib) was investigated by 13C-NMR. The chemical shifts of amino- or carboxy-protected derivatives of Aib and of protected oligopeptides are discussed with respect to neighbouring groups and amino acids. The pH-dependence of the 13C-NMR spectra of Aib, Aib-Ala, Ala-Aib, Aib-Ala-Aib and Aib-Ala-Aib-Ala-Aib was studied. Using these examples, a new and advantageous method is demonstrated for the first time for the evalutions of NMR titration curves, which uses so-called chemical shift diagrams (CS diagrams). 相似文献
82.
Raymond Gerdil 《Helvetica chimica acta》1976,59(2):650-655
An empirical linear correlation exists between nuclear quadrupole resonance frequencies of halogen nuclei (35Cl, 79Br, 127I) and the polarographic reduction potentials in series of organic halides. In terms of electronic structure this is interpreted as a significant relationship between the average p-electron population at halogen atoms and the first vacant molecular orbital of halogenated molecules. On the basis of the present correlations conclusions are drawn about some controversial mechanistic aspects of the electroreduction of the carbon-halogen bond. 相似文献
83.
Lo?&#x;c J. CharbonnièreRaymond Ziessel 《Tetrahedron letters》2003,44(33):6305-6307
The synthesis of tris[3-(6-carboxypyridin-2-yl)pyrazol-1-yl]methane is described in a linear multi-step protocol. The pyridyl-pyrazolyl arms are first constructed before being condensed with chloroform. Careful study of the condensation reaction shows the presence of an isomeric form of the tris(pyrazolyl)methane derivative in which one of the pyrazolyl substituents is linked through the nitrogen atom at the 2 position of the pyrazol. After acid-catalysed isomerisation to the desired isomer, the intermediate compound was subjected to a carboalkoxylation reaction and a subsequent hydrolysis. These are some rare examples of reactions directly occurring on the tris(pyrazolyl)methane platforms. 相似文献
84.
OZONE, MIDDLE ULTRAVIOLET RADIATION AND THE AQUATIC ENVIRONMENT 总被引:3,自引:0,他引:3
Marine organisms in the upper layers of the sea may be endangered by increased ultraviolet radiation resulting from declines in the thickness of stratospheric ozone. Evidence supporting this hypothesis includes the fact that wavelengths of potentially damaging ultraviolet radiation can penetrate to ecologically significant depths and laboratory findings that many marine organisms are extremely sensitive to this radiation. Estimated effects of increased ultraviolet radiation on populations of marine organisms range from insignificant to catastrophic. Direct estimation of population effects have not been made, although this is the only adequate measure of the potential impact. The extreme diminution of ozone during the Antarctic spring, coupled with the dynamics of phytoplankton production in this region, may provide conditions suitable for an environmental test of the hypotheses that marine organisms are endangered by reduced stratospheric ozone. There is an urgency to the testing of this hypothesis since these populations may be directly impacted at the present time. 相似文献
85.
Raymond Constanciel 《Theoretical chemistry accounts》1986,69(5-6):505-523
We derive from exact integral equations of classical electrostatics some approximate expressions of the interaction energy of a point charge distribution with a dielectric medium. We show how they can be used for computing the wave functions of solvated species imbedded in cavities formed by interlocking spheres in a polarizable continuum. We discuss the relation between these formulae and the ones proposed earlier on empirical basis and we especially emphasize the improvement that they bring out. 相似文献
86.
The concentration sensitivity of a thermal conductivity detector (TCD) depends, among other factors, on the amount of sample mixture in the detector's sensing cell. Since the cell volume has to be appropriately matched with column diameter, it makes the concentration sensitivity of a TCD dependent on column diameter and, therefore, on the speed of gas chromatography. Through reduction of column diameter, higher speed tends to lead to a reduction in the concentration sensitivity of the cell. The factor which the most directly affects the concentration sensitivity of a TCD cell is the heat power conducted through the cell. The higher the power, the greater the sensitivity. The limit of detection of a TCD depends on the concentration-sensitivity of its cell and on the level of statistical errors in the measurement. The errors increase with increasing analysis speed. As the column diameter is reduced, the errors cause additional worsening (on top of the decrease in concentration sensitivity) of the detection limit, dynamic range, and other performance characteristics of the TCD. 相似文献
87.
88.
Benniston AC Harriman A Romero FM Ziessel R 《Dalton transactions (Cambridge, England : 2003)》2004,(8):1233-1238
The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) tris(2,2'-bipyridine) complexes via an alkynylene group differ significantly from those of the relevant mononuclear complex. In particular, the energy of the first triplet excited state is lowered relative to the parent complex, because of the presence of the alkynylene substituent, while the triplet lifetime is prolonged, in part, because of extended electron delocalization. We now report that the triplet lifetime is also affected by the nature of the spectator 2,2'-bipyridyl ligands. Thus, replacing the parent 2,2'-bipyridine ligands with the corresponding 4,4'-dinitro-substituted ligands serves to decrease the luminescence yield and lifetime. With the corresponding carboxylate ester, the luminescence yield and lifetime are increased. Perdeuteration of the parent 2,2'-bipyridine ligands also leads to a modest increase in the luminescence yield. Such observations are indicative of electronic coupling between the various metal-to-ligand, charge-transfer excited triplet states. Temperature dependence studies confirm that these excited states are closely spaced and thermally accessible at ambient temperature. For some of the binuclear complexes, the quantum yield for formation of the lowest-energy triplet state is significantly less than unity. 相似文献
89.
A quantitative compressed pellet infrared method used together with a micro-distillation to effect separation of the isomer has been developed and reported here for the analysis of ortho-, meta-, and para-terphonyl. The terphenyl content is determined on fractions isolated by distillation from radiation damaged samples. The compressed pellet infrared method is preferred instead of the more conventional solution technique since smaller quantities of isolated terphcnyl are required. The pellet method is applicable also to polyphenyls higher than the terphenyls. The solution technique is not due to the insolubility of the polyphenyls. It is expected that the compressed pellet spectral method developed here might well find application in the analysis of compounds other than polyphenyls. Results presented here appear to be the, first quantitative infrared data reported in the literaturu on polyphenyl compounds. 相似文献
90.
The solution conformation of bicyclo [3.1.0] hexan-3-one has been obtained by the use of the shifts induced in its 1H and 13C NMR by Yb(fod)3. Refinement of the angle of pucker, ∝, of the 5-membered ring indicates that the molecule adopts a flattened boat conformation with ∝ = 195°. This prediction is supported by ab initio, STO-3G, calculations on the isolated molecule. Use of a two- or four-site model for lanthanide-substrate complexing adequately reproduces the experimental data whereas a one-site binding model is unsatisfactory. The importance of multi-site binding is further emphasised by results for the C2v-symmetric ketone, adamantanone, where only a four-site model gives satisfactory agreement between observed and calculated lanthanide-induced shifts. 相似文献