全文获取类型
收费全文 | 336431篇 |
免费 | 3174篇 |
国内免费 | 1036篇 |
专业分类
化学 | 177623篇 |
晶体学 | 5416篇 |
力学 | 14680篇 |
综合类 | 8篇 |
数学 | 36459篇 |
物理学 | 106455篇 |
出版年
2021年 | 2626篇 |
2020年 | 2959篇 |
2019年 | 3203篇 |
2018年 | 4113篇 |
2017年 | 4077篇 |
2016年 | 6129篇 |
2015年 | 3834篇 |
2014年 | 5822篇 |
2013年 | 14674篇 |
2012年 | 11664篇 |
2011年 | 14055篇 |
2010年 | 9761篇 |
2009年 | 9700篇 |
2008年 | 13100篇 |
2007年 | 13011篇 |
2006年 | 12587篇 |
2005年 | 11337篇 |
2004年 | 10420篇 |
2003年 | 9309篇 |
2002年 | 9096篇 |
2001年 | 11065篇 |
2000年 | 8406篇 |
1999年 | 6517篇 |
1998年 | 4953篇 |
1997年 | 5027篇 |
1996年 | 4685篇 |
1995年 | 4325篇 |
1994年 | 4198篇 |
1993年 | 3899篇 |
1992年 | 4675篇 |
1991年 | 4742篇 |
1990年 | 4542篇 |
1989年 | 4299篇 |
1988年 | 4290篇 |
1987年 | 4326篇 |
1986年 | 3963篇 |
1985年 | 5134篇 |
1984年 | 5349篇 |
1983年 | 4340篇 |
1982年 | 4524篇 |
1981年 | 4341篇 |
1980年 | 4194篇 |
1979年 | 4538篇 |
1978年 | 4701篇 |
1977年 | 4667篇 |
1976年 | 4675篇 |
1975年 | 4343篇 |
1974年 | 4296篇 |
1973年 | 4451篇 |
1972年 | 2890篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Zusammenfassung Anhand der Methode der Konfigurationenwechselwirkung wurde die Ladungsdichteverteilung in einer Reihe von aromatischen Aminen und N-haltigen Heteroaromaten im angeregten Singulett- und Triplettzustand untersucht. Durch die Untersuchungen wurde die Formulierung der folgenden Regel ermöglicht: im angeregten Singulett- und Triplettzustand ist die Basizität bei den Aminen wesentlich vermindert; die N-haltigen Heteroaromaten weisen im angeregten Singulettzustand eine erhöhte Basizität auf, und im Triplettzustand tritt eine Verminderung der Basizität ein.
The electron density distribution in excited singlet and triplet states of a number of aromatic amines and heterocyclic compounds containing nitrogen atoms is studied using the CI-method. The investigation allows the formulation of the following rule: in excited singlet and triplet states the amines possess significantly reduced basicity; the nitrogen heterocyclic compounds in excited singlet states increase their basicity, whereas in excited triplet states their basicity is decreased.
Résumé Au moyen de la méthode d'interaction de configuration, on a étudié la répartition de la densité électronique dans un certain nombre d'aminés aromatiques et de composés hétérocycliques azotés dans l'état excité singulet et triplet.Les études effectuées ont permis d'établir la règle suivante: à l'état excité singulet ou triplet la basicité des aminés est sensiblement réduite; la basicité des composés N-hétérocycliques augmente dans l'état excité singulet et diminue dans le triplet.相似文献
962.
Zusammenfassung Leitfähigkeitsmessungen an Et4NCl-Lösungen zeigen die Gültigkeit derBjerrumschen Theorie in PhPOCl2; die Dissoziationskonstante beträgt 1,5·10–2. FeCl3 verhält sich nicht wie eine Ionenverbindung, doch ähnlich wie in POCl3. Zur Verfolgung von Ionenreaktionen wurden konduktometrische Titrationen ausgeführt. ZnCl2 und AlCl3 geben 2 Chloridionen, BCl3, TiCl4, SnCl4 und PCl5 je 1 Chloridion an die Akzeptoren FeCl3 oder SbCl5, ab. In Gegenwart von polaren Chloriden nehmen BCl3, AlCl3, FeCl3, PCl5 und SbCl5 je 1 Chloridion, ZnCl2, TiCl4 und SnCl4 je nach Angebot 1 oder 2 Chloridionen auf. Sowohl die Chlorometallate, als auch die Chloroniumverbindungen sind im gleichen Ausmaß wie Et4NCl ionisiert.Mit 4 Abbildungen2. Mitt.:M. Baaz, V. Gutmann, M. Y. A. Talaat undT. S. West, Mh. Chem.92, 150 (1961).Herrn Prof. Dr.L. F. Audrieth zum 60. Geburtstag gewidmet. 相似文献
963.
Density functional theory has been used to explore cycloaddition reactions of organic molecules containing carbonyl functional groups on the Si(100) surface. As with other pi bonds, carbonyl groups can add to the surface by a [2+2] cycloaddition with negligible activation barrier, as previously shown through experiment. However, the present calculations indicate that 1,2-dicarbonyls, such as glyoxal, may also react by means of a [4+2] addition to form a hetero-Diels-Alder product in which the organic ring stands normal to the surface. Calculations of [2+2] and [4+2] pathways indicate that both reactions proceed without significant barriers. This reactivity is analogous to that of conjugated dienes, in which evidence for both reactions has been observed. In contrast to unsaturated alkyl systems, which must react through the pi electron system, the reactions of carbonyls may proceed through a very different mechanism, in which the initial surface interaction is through the oxygen lone pair. The presence of lone pairs affects the geometry of the [4+2] adduct, and may alter the competition between [2+2] and [4+2] addition. Some potential rearrangement reactions of the initial binding products are described. Recent experimental studies of a 1,2-dicarbonyl on Si(100) are reinterpreted in light of these calculations, and found to be consistent with the presence of the [4+2] adduct. Finally, some molecules are suggested as cycloaddition reagents for experimental tests of the conclusions presented here. 相似文献
964.
Assessment of the retention properties of poly(vinyl alcohol) stationary phase for lipid class profiling in liquid chromatography 总被引:3,自引:0,他引:3
Potentialities of polymerized vinyl alcohol on silica gel were assessed for class separation of simple lipids, sphingolipids, glyceroglycolipids and phospholipids by high-performance liquid chromatography. A screening of pure solvents in binary gradient elution and a chemometric approach was used to define a rugged two segment linear gradient formed from four solvents for total lipid class separation. Triethylamine and formic acid were added in all mobile phase components for acidic phospholipid separation and evaporative light scattering response enhancement. Simple analytical procedures are described for the analysis of complex lipid materials. 相似文献
965.
V. A. Tartakovskii A. S. Ermakov Yu. A. Strelenko D. B. Vinogradov E. Yu. Petrov 《Russian Journal of Organic Chemistry》2005,41(1):120-123
A practical procedure has been proposed for the synthesis of functionally substituted 4,5-dihydro-1,2,3-oxadiazole 2-oxides on the basis ofsulfamic acid derivatives.__________Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 1, 2005, pp. 121–123.Original Russian Text Copyright © 2005 by Tartakovskii, Ermakov, Strelenko, Vinogradov, Petrov. 相似文献
966.
V. Kalcheva Kh. Lozanova D. Simov P. B. Terent'ev 《Chemistry of Heterocyclic Compounds》1985,21(9):985-987
3-Acetylaminomethyl-2-benzoxazolones are obtained by Beckmann rearrangement of the anti-oximes of 3-acetonyl-2-benzoxazolones. The isomeric 3-(N-methylcarbamoylmethyl)-2-benzoxazolones are synthesized by alkylation of 2-benzoxazolone with chloroacetic acid, followed by reaction of the 3-carboxymethyl-2-benzoxazolones obtained with thionyl chloride and methylamine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1185–1188, September, 1985. 相似文献
967.
V. P. Lesnyak D. I. Shiman L. V. Gaponik F. N. Kaputskii A. I. Lamotkin Zh. V. Bondarenko 《Russian Journal of Applied Chemistry》2007,80(2):295-299
The influence exerted by copolymers of the C9 fraction of liquid pyrolysis products of petroleum raw materials with maleic anhydride and by their esterification products, added to paper pulp, on the main paper properties was examined. 相似文献
968.
F. dall'Acqua S. Caffieri D. Vedaldi A. Guiotto P. Rodighiero 《Photochemistry and photobiology》1983,37(4):373-379
By irradiating (365 nm) an aqueous liquid solution of 4,5'-dimethylangelicin. a monofunctional photosensitizing furocoumarin, in the presence of an excess of thymine, two new compounds, I and II, have been obtained; they do not show fluorescence when observed with Wood's light. The nuclear magnetic resonance data, the marked similarity of UV absorption and fluorescence spectra of these compounds with those of synthetic 3.4-dihydro-4,5'-dimethylangelicin and their capacity to undergo photodissociation (254 nm) yielding the starting thymine and 4,5'-dimethylangelicin in equimolecular amounts, are consistent with C4 -cycloadducts between the 3,4-double bond of the furocoumarin and 5,6-double bond of thymine. Nuclear magnetic resonance data indicate for I and II a head-to-head and a head-to-tail structure, respectively. When irradiation is carried out in the frozen state, two adducts. III and IV, fluorescent at Wood's light, have been obtained other than the two above-mentioned compounds I and II. Compounds III and IV have been identified as 4'.5'-fluorescent adducts between the 4',5'-double bond of the furocoumarin and the 5.6-double bond of thymine; one of them (III) is identical to that formed in the photoreaction between DNA and 4,5'-dimethylangelicin; for this last compound a cis head-to-head structure has been suggested. 相似文献
969.
T. Paryjczak J. M. Farbotko K. W. Jówiak 《Reaction Kinetics and Catalysis Letters》1982,20(1-2):227-231
Temperature-programmed reduction and oxidation of bimetallic catalysts Pd–Ni/Al2O3 seem to indicate phase segregation of palladium and nickel oxides and alloying of palladium and nickel in the process of reduction of oxidized catalysts.
- Pd–Ni/Al2O3 , .相似文献
970.
Tandem purification of mouse IgM monoclonal antibodies produced in vitro using anion-exchange and gel fast protein liquid chromatography 总被引:1,自引:0,他引:1
P Clezardin N R Hunter I R MacGregor J L MacGregor D S Pepper J Dawes 《Journal of chromatography. A》1986,358(1):209-218
A tandem chromatographic procedure was used to isolate rapidly mouse IgM monoclonal antibodies produced by cultivation of hybridomas in vitro. Hybridoma culture supernatants containing mouse IgM monoclonal antibodies were first chromatographed on an anion-exchange Mono Q column connected to a fast protein liquid chromatography system. This anion-exchange step offers the advantage of obtaining IgM antibodies in a concentrated form. The IgM-rich fractions from the Mono Q column were then injected on a gel filtration Superose 6 column equilibrated with a low-ionic strength buffer and eluted with a high-ionic strength buffer. Assessment of the purity of isolated IgM monoclonal antibodies was performed by sodium dodecyl sulphate polyacrylamide gel electrophoresis together with a Coomassie Brillant Blue R 250 staining technique. Assessment of the immunoreactivity of isolated IgM monoclonal antibodies was performed by an enzyme linked immunosorbent assay using a solid phase adsorbed antigen against which IgM monoclonal antibodies were directed. The chromatographic procedures described allows the rapid isolation of mouse IgM monoclonal antibodies produced in vitro at a high degree of purity and in an immunoreactive state. 相似文献