全文获取类型
收费全文 | 4870篇 |
免费 | 69篇 |
国内免费 | 11篇 |
专业分类
化学 | 3425篇 |
晶体学 | 24篇 |
力学 | 80篇 |
数学 | 994篇 |
物理学 | 427篇 |
出版年
2019年 | 41篇 |
2016年 | 81篇 |
2015年 | 65篇 |
2014年 | 46篇 |
2013年 | 126篇 |
2012年 | 118篇 |
2011年 | 167篇 |
2010年 | 119篇 |
2009年 | 135篇 |
2008年 | 163篇 |
2007年 | 153篇 |
2006年 | 164篇 |
2005年 | 147篇 |
2004年 | 114篇 |
2003年 | 108篇 |
2002年 | 111篇 |
2001年 | 58篇 |
2000年 | 86篇 |
1999年 | 65篇 |
1998年 | 61篇 |
1997年 | 56篇 |
1996年 | 75篇 |
1995年 | 84篇 |
1994年 | 63篇 |
1993年 | 70篇 |
1992年 | 104篇 |
1991年 | 97篇 |
1990年 | 87篇 |
1989年 | 77篇 |
1988年 | 69篇 |
1987年 | 75篇 |
1986年 | 75篇 |
1985年 | 90篇 |
1984年 | 97篇 |
1983年 | 93篇 |
1982年 | 88篇 |
1981年 | 112篇 |
1980年 | 69篇 |
1979年 | 89篇 |
1978年 | 83篇 |
1977年 | 102篇 |
1976年 | 74篇 |
1975年 | 87篇 |
1974年 | 85篇 |
1973年 | 70篇 |
1972年 | 58篇 |
1971年 | 50篇 |
1970年 | 37篇 |
1967年 | 33篇 |
1962年 | 30篇 |
排序方式: 共有4950条查询结果,搜索用时 15 毫秒
31.
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
32.
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
33.
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
34.
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
35.
Helmut Schmidhammer Felizia Kaspar Arpad Marki Anna Borsodi 《Helvetica chimica acta》1994,77(4):999-1002
The mixed azines 4 and 5 were prepared by reaction of naloxazone ( 2 ) with either oxymorphone ( 6 ) or 14-O-methyloxymorphone ( 7 ) and tested in vitro using opioid receptor binding assays and in vivo using the AcOH-writhing test in mice. Compound 4 was found to be a partial agonist, while compound 5 was a potent opioid agonist with higher potency than morphine. 相似文献
36.
The photoprocesses of a series of N-substituted dibenz[b,f]azepines (iminostilbenes) were studied by absorption and emission spectroscopy, by laser flash photolysis, and by preparative irradiation with NMR analysis. In solutions, 2pi+2pi photodimers of N-cyano and N-acyl dibenzazepines are formed via the triplet state upon acetone- or benzophenone-sensitized energy transfer. T-T absorption spectra were measured and absorption coefficients were determined. The triplet energy transfer is equally efficient for N-alkyl dibenzazepines, which do not dimerize. Excited states of npi* character in the latter cases are discussed to rationalize the different reactivities. In spite of negligible intersystem crossing of 21 dibenzazepine derivatives, photodimers of N-acyl and N-cyano dibenzazepines are formed upon direct excitation in concentrated solutions (0.01-0.1 mol dm(-3)) as well as in the solid state. A selective anti-configuration of the photodimers was found throughout. 相似文献
37.
ValeryN. Kalinin IgorV. Shishkov SergeyK. Moiseev ElviraE. Shults GenrikhA. Tolstikov NataliaI. Sosnina PavelV. Petrovskii KonstantinA. Lyssenko Helmut Schmidhammer 《Helvetica chimica acta》2006,89(5):861-869
In a search for starting materials for the preparation of 7,8‐fused morphine alkaloid derivatives, 8‐[(1E‐2‐phenylethenyl]codeinone dimethyl ketal ( 4 ) and 8‐[(1E‐2‐phenylethenyl]codeine ( 5 ) were prepared. These dienes were used as substrates in the Diels–Alder reactions. Compound 5 formed the ‘normal’ adduct 12 with N‐phenylmaleimide, while compound 4 behaved in reactions with dienophiles as the ‘masked’ diene 11 , a 8‐[(1E)‐2‐phenylethenyl]‐substituted thebaine, yielding the corresponding 19‐substituted 6,14‐endo‐etheno‐6,7,8,14‐tetrahydrothebaines. Specifically, reaction of 4 with methyl vinyl ketone gave rise to 19‐[(1E)‐phenylethenyl]thevinone ( 14 ) whose structure was elucidated by an X‐ray diffraction analysis. The thebaine derivative 11 was also prepared from 4 . 相似文献
38.
Joachim Heidberg Elisabeth Kampshoff Helmut Stein Helmut Weiss Michael Warskulat 《Mikrochimica acta》1988,94(1-6):105-108
Sample/spectrum relationships are investigated using both a low resolution rapid-scanning NIR monochromator and a Fourier transform instrument capable of high resolution and are evaluated in terms of whether or not the resolution of the instrument is sufficient for measuring the natural bandwidths corresponding to the sample. Based on the sample/spectrum relationship a criterion is developed which must be followed in order to apply either derivative spectroscopy or deconvolution to enhance the resolution of overlapped bands without generating spectral artifacts. 相似文献
39.
Gas-phase derivatization of [C3H5]+ ions using tandem mass spectrometry methods A13C labelling study
Nguyen Kieu My Marita Schilling Helmut Schwarz 《Journal of mass spectrometry : JMS》1987,22(5):254-258
The study of specifically 13C-labelled precursors sheds further light on the gas-phase chemistry of allyl and 2-propenyl cations. It is demonstrated that both species are formed from allyl and 2-propenyl bromide upon 70 eV electron impact ionization without skeletal reorganization. Gas-phase derivatization of the [C3 H5]+ ions with benzene facilitates, as suggested and observed earlier, the distinction of the two isomers using collision-induced dissociation of the Wheland complexes (or isomers thereof). The 13C labelling data clearly demonstrate that 64% of allyl cations survive the derivatization while 36% isomerize to 2-phenylpropyl cations; the latter are also formed via the reaction of 2-propenyl cation with benzene, protonation of α-methylstyrene and water loss from protonated 2-phenyl-2-propanol, respectively. Unimolecular loss of C2H4 from protonated allylbenzene proceeds via two competing reaction channels: one involves heterolysis of 1-phenylpropyl cations (~30%); the major pathway (~70%), however, involves decomposition via propylene benzenium ions. 相似文献
40.
The successive reaction of (CO)6M with Na[NCR21] and [Et3O]BF4 yields (CO)5M[C(NCR21)OEt] (II: M = Cr; III: M = W; CR21 = C(C6H4Br-p)2 (a), CPh2 (b), C(C6H4OMe-p)2 (c), C(C6H4)2O (d), CBu2tt (e)). Hexacarbonyltungsten, (CO)6W, reacts with Na[NCPh2] and MeOSO2F to give (CO)5W[C(NCPh2) OMe] (IV). X-Ray analysis of IIe shows that: (1) the CNC fragment is almost linear (171.7°); (2) the two NC bond lengths are equal within experimental error; and (3) the O,C,Cr,N plane is perpendicular to the C(Me3),C,N,C(Me3) plane (90.0°). Therefore compounds II–IV are best described as 1-alkoxy-2-azaallenyl complexes. 相似文献