全文获取类型
收费全文 | 4396篇 |
免费 | 59篇 |
国内免费 | 10篇 |
专业分类
化学 | 3118篇 |
晶体学 | 20篇 |
力学 | 68篇 |
数学 | 932篇 |
物理学 | 327篇 |
出版年
2019年 | 34篇 |
2016年 | 71篇 |
2015年 | 51篇 |
2014年 | 34篇 |
2013年 | 113篇 |
2012年 | 90篇 |
2011年 | 140篇 |
2010年 | 102篇 |
2009年 | 114篇 |
2008年 | 140篇 |
2007年 | 139篇 |
2006年 | 146篇 |
2005年 | 130篇 |
2004年 | 105篇 |
2003年 | 101篇 |
2002年 | 103篇 |
2001年 | 54篇 |
2000年 | 72篇 |
1999年 | 61篇 |
1998年 | 59篇 |
1997年 | 56篇 |
1996年 | 71篇 |
1995年 | 79篇 |
1994年 | 59篇 |
1993年 | 68篇 |
1992年 | 95篇 |
1991年 | 95篇 |
1990年 | 77篇 |
1989年 | 71篇 |
1988年 | 60篇 |
1987年 | 65篇 |
1986年 | 68篇 |
1985年 | 84篇 |
1984年 | 90篇 |
1983年 | 84篇 |
1982年 | 81篇 |
1981年 | 107篇 |
1980年 | 62篇 |
1979年 | 82篇 |
1978年 | 74篇 |
1977年 | 93篇 |
1976年 | 65篇 |
1975年 | 79篇 |
1974年 | 85篇 |
1973年 | 64篇 |
1972年 | 55篇 |
1971年 | 48篇 |
1970年 | 37篇 |
1967年 | 31篇 |
1962年 | 29篇 |
排序方式: 共有4465条查询结果,搜索用时 0 毫秒
21.
Gáti T Simon A Tóth G Magiera D Moeller S Duddeck H 《Magnetic resonance in chemistry : MRC》2004,42(7):600-604
Adducts of four phosphine chalcogenides with the chiral dirhodium complex ([Rh-Rh]) were investigated by variable-temperature 1H and 31P NMR spectroscopy in order to compare their properties as axial ligands. Whereas the selenide (1) and the sulfide (2) are strong ligands with electrostatic attraction and, in addition, a significant orbital (HOMO-LUMO) interaction, the phosphine oxide compounds (P=O) bind primarily via electrostatic attraction and are relatively weak donors. Moreover, the overall bond strength in these adducts depends on steric congestion around the P=O group. 相似文献
22.
Helmut Sprdizer Andrea Pichler Wolfgang Holzer Irene Toth Bettina Zuchart 《Helvetica chimica acta》1997,80(1):139-145
Khusimone (1), one of the main odor-donating compounds of vetiver oil is subject of the following study on structure/odor relationship. The omittance of the ethano bridge of the tricyclic khusimone leads to a bicyclic system. The stereoselective approach to this degraded structure is described, and the olfactory properties are studied. The key step of the synthesis of the hydrindane nucleus is based on a highly diastereoselective conjugate addition to a chiral oxo-cyclopentene-2-carboxylate. 相似文献
23.
By investigation of isomeric 2H- and 13C-labelled C9H11 Cl-compounds it has been shown that the ion [M ? Cl]+ is transformed mainly to the ion [C7H7]+ via C2H4-elimination from alkyl-substituted tropylium-ions. Phenyl-cyclopropane-cations play only a small part, if at all, in this fragmentation process. 相似文献
24.
25.
26.
"Bare" CeO(2)(+) ions can be prepared in the gas phase by consecutive oxidation of Ce(+) with O(2) and NO(2). The ability to activate saturated and unsaturated hydrocarbons is investigated by use of Fourier-transform ion cyclotron resonance mass spectrometry. In the reactions of CeO(2)(+) with linear and branched alkanes C-H bond activation is observed almost exclusively. In contrast, both oxygen-atom transfer and C-H bond activation processes occur when thermalized CeO(2)(+) cations react with simple alkenes and aromatic compounds. C-C bond activation is not observed at all. Insight into the structural and electronic properties of neutral CeO(2) and cationic CeO(2)(+) is provided by means of quasirelativistic density-functional and ab initio pseudopotential calculations. They reveal a (2)Sigma(u)(+) ground state for CeO(2)(+) which is best described as a linear cerium dioxide with a resonating pi bond. Finally, we discuss the influence of oxo ligands on the chemistry of the cationic CeO(n)()(+) (n = 0-2) species toward hydrocarbons. 相似文献
27.
Gerhard Laus Johannes Schütz Herwig Schottenberger Max Andre Klaus Wurst Mariana Spetea Karl‐Hans Ongania AdrianG. Müller Helmut Schmidhammer 《Helvetica chimica acta》2003,86(10):3274-3280
The two diastereoisomeric ferrocenyl‐substituted orvinols 2 and 3 were prepared. The modified alkaloids are still able to interact with opioid receptors (see Table). The ferrocene moiety allows highly selective and sensitive electrochemical detection. The X‐ray crystal structure of the major isomer 2 was determined. The combination of a metallocene and a morphinan alkaloid holds promise for useful antitumor activity. 相似文献
28.
Helmut Goldmann 《manuscripta mathematica》1987,59(2):229-244
Let M be a C-smooth submanifold of a domain GCn. Denote by 0(M) the algebra of germs of holomorphic functions on M, that is, each f0(M) is holomorphic in some neighbourhood of M, the neighbourhood dependent of f. Now define A(M) as the closure of 0(M) with respect to the topology of compact convergence on M.If M coincides with A(M), the spectrum of A(M), we give a necessary and sufficient condition for A(M) such that M is a complex manifold. 相似文献
29.
Helmut Pottmann 《Results in Mathematics》1987,11(1-2):122-143
Im ersten Teil dieser Arbeit1 wurden die orientierten Flächeninhalte der Punktbahnen ebener äquiformer Zwangläufe studiert und einige Anwendungen der erzielten Resultate angeführt. Der vorliegende zweite Teil beschäftigt sich - unter Beibehaltung der Bezeichnungen, der Numerierung der Abschnitte, Sätze usw. - einerseits mit Geradenhüllbahnen, andererseits mit globalen Eigenschaften spezieller äquiformer Zwangläufe, und zwar mit der äquiformen Bewegung der Krümmungsstrecken einer ebenen Kurve, mit äquiformen Konchoiden-bewegungen und mit der äquiformen Bewegung der Durchmesser eines beschränkten konvexen Bereichs. Beim Studium der durch formale Integration gewonnenen, vorzeichenbehafteten “Längen” der Geradenhüllbahnen wurden zur Interpretation des Vorzeichens orientierte Geraden des Gangsystems betrachtet. Dieser anscheinend notwendige Übergang zu Speeren wurde bislang bei der Formulierung von Holditch-Sätzen über Längen von Geradenhüllbahnen (vgl.[11],[39]) nicht beachtet. Wie schon im ersten Teil gestatten die Ergebnisse eine Anwendung in der globalen euklidischen n-Lagentheorie sowie auf nichtgeschlossene euklidische Zwangläufe. 相似文献
30.
Abstract Photoreactions, such as base release and decomposition of the base moiety, induced by either 20 ns laser pulses at 193 nm or continuous 254 nm irradiation, were studied for a series of uracil and adenine derivatives in neutral aqueous solution. The quantum yield of chromophore loss (φ) depends significantly on the nature of the nucleic acid constituent and the saturating gas (Ar, N2O or O2). In the case of polynucleotides the destruction of nucleotides was measured by high-performance liquid chromatography after hydrolysis; the quantum yields (φ) are comparable to those of chromophore loss or larger. The φcl and aφdn of 0.04–0.1 for poly(U) and poly(dU), obtained for both wavelengths of irradiation, are due to processes originating from the lowest excited singlet state, i.e. formation of photohydrates and photodimers, and a second part from photoionization using λirr= 193 nm. Irradiation at 193 nm effectively splits pyrimidine dimers and thus reverts them into monomers. The quantum yield for release of undamaged bases (φbr) from nucleosides, nucleotides and polynucleotides upon irradiation at 254 nm is typically φbr= (0.1–1) × 10?4 Breakage of the N-glycosidic bond is significantly more efficient for λirr=193 nm, e.g. φbr= 1.1 × 10?3, 0.8 × 10?3, 4.3 × 10?3 and 0.5 × 10?3 for poly(A), poly(dA), poly(U) and poly(dU) in Ar-saturated solution, respectively. Enhanced φ values for λirr= 193 nm, essentially for adenine and its derivatives, are caused by photo-processes that are initiated by photoionization. 相似文献