全文获取类型
收费全文 | 2623篇 |
免费 | 102篇 |
国内免费 | 12篇 |
专业分类
化学 | 1884篇 |
晶体学 | 17篇 |
力学 | 48篇 |
数学 | 491篇 |
物理学 | 297篇 |
出版年
2023年 | 14篇 |
2022年 | 28篇 |
2021年 | 38篇 |
2020年 | 44篇 |
2019年 | 53篇 |
2018年 | 25篇 |
2017年 | 23篇 |
2016年 | 76篇 |
2015年 | 43篇 |
2014年 | 55篇 |
2013年 | 87篇 |
2012年 | 123篇 |
2011年 | 124篇 |
2010年 | 62篇 |
2009年 | 87篇 |
2008年 | 149篇 |
2007年 | 128篇 |
2006年 | 122篇 |
2005年 | 110篇 |
2004年 | 99篇 |
2003年 | 102篇 |
2002年 | 102篇 |
2001年 | 54篇 |
2000年 | 67篇 |
1999年 | 40篇 |
1998年 | 48篇 |
1997年 | 53篇 |
1996年 | 50篇 |
1995年 | 38篇 |
1994年 | 49篇 |
1993年 | 32篇 |
1992年 | 29篇 |
1991年 | 37篇 |
1990年 | 29篇 |
1989年 | 19篇 |
1988年 | 32篇 |
1987年 | 24篇 |
1986年 | 19篇 |
1985年 | 37篇 |
1984年 | 45篇 |
1983年 | 28篇 |
1982年 | 47篇 |
1981年 | 36篇 |
1980年 | 29篇 |
1979年 | 23篇 |
1978年 | 20篇 |
1977年 | 26篇 |
1976年 | 22篇 |
1975年 | 13篇 |
1974年 | 9篇 |
排序方式: 共有2737条查询结果,搜索用时 15 毫秒
61.
Maria E. Sulbaran Gerzon E. Delgado Asilo J. Mora Ali Bahsas Hector Novoa de Armas Norbert Blaton 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(9):o543-o545
In the title compound, C6H8N2O2S, also known as N‐acetyl‐2‐thiohydantoin–alanine, the molecules are joined by N—H...O hydrogen bonds, forming centrosymmetric R22(8) dimers; these dimers are linked by C—H...O interactions to form R22(10) rings, thus forming C22(10) chains that run along the [101] direction. 相似文献
62.
Osipov SN Golubev AS Sewald N Michel T Kolomiets AF Fokin AV Burger K 《The Journal of organic chemistry》1996,61(21):7521-7528
A new method for the preparation of alpha-chlorodifluoromethyl-, alpha-bromodifluoromethyl-, and alpha-difluoromethyl-substituted alpha-hydroxy and alpha-amino acid esters 11, 19-21 is described. The key step of the synthesis is the regioselective alkylation of ketones 5, 7-9 and imines 16-18 with C-nucleophiles. The ketones 7-9 are readily available from 3,3,3-trifluorolactate 1 by a five-step procedure. Subsequent removal of the protecting groups from 19-21 provides the corresponding free amino acids 25, 26, 28. 相似文献
63.
On singlet excitation (λ = 254 nm, THF, pentane or hexane), the diastereoisomeric methano-epoxydienes (E)- 6 and (E)- 7 undergo interconversion and yield the products 8 – 11 . The main process is the cleavage of the oxirane ring to the vinyl carbene intermediate e which undergoes addition to the adjacent double bond furnishing the cyclopropene 8 . The alternative carbene intermediate f is evidenced by the formation of the cyclobutene 10 . For the fragmentation leading to 11 , the carbene f as well as the dipolar species h are considered as possible intermediates. On triplet sensitization (acetone, λ > 280 nm), (E)- 7 shows behavior typical of epoxydienes, undergoing fission of the C? O bond of the oxirane ring and isomerization to the compounds 13 , 14 and (E/Z)- 15 . 相似文献
64.
Hans Günther Wahl Silke-Yvonne Habel Norbert Schmieder Hartmut M. Liebich 《Journal of separation science》1994,17(7):543-548
Identification of cis/trans isomers of unsaturated fatty acids cannot usually be achieved by GC-MS (gas chromatography-mass spectrometry) without reference substances. In this study a GC-FTIR-MS system (gas chromatography-Fourier transform-mass spectrometry) was used to identify fatty acid methyl esters (FAMEs) and differentiate between the cis/trans isomers. Besides methyl esters, 2-alkenyl-4,4-dimethyloxazoline derivatives (DMOX), which have been used to locate double bond positions of unsaturated fatty acids, were examined with respect to their suitability for cis/trans differentiation. A combined GC-FTIR-MS system with a wide band (4000–550 cm?1) mercury cadmium telluride (MCT) detector was used in series and parallel to identify 31 reference unsaturated fatty acids, including 7 pairs of cis/trans isomers. Serum samples of healthy persons and commercially available fish oil were analyzed as examples of complex mixtures. Using splitless injection the detection limit for the less sensitive IR detector was 25 ng/μl in case of the weak cis and trans bands. In the FTIR spectra cis/trans isomers were identified by analysis of bands arising from C? H out-of-plane (oop) bending: for both the FAME and DMOX derivatives cis-1,2-disubstituted double bonds give a strong band near 720 cm?1 and the corresponding trans isomers near 967 cm?1. cis Isomers could be identified further by a band at 3012 cm?1. With the combined data of the GC-FTIR-MS system it is now possible to identify polyunsaturated fatty acids with regard to the discrimination of cis/trans isomers. 相似文献
65.
Norbert Kuhn Stefan Stubenrauch
Roland Boese
Dieter Bl ser 《Journal of organometallic chemistry》1992,440(3):289-296The azacyclopentadienyl compounds (2,5-C4tBu2RHN)MCl3 (M = Ti, Zr, Hf; R = H, SiMe3) have been prepared as stable solids from the lithiated pyrroles and MCl4. The π-coordination of the azacyclopentadienyl ligands, as suggested from 13 C NMR data, has been confirmed for (2,5-C4tBu2H2N)TiCl3 by an X-ray diffraction study. 相似文献
66.
Norbert Quien 《manuscripta mathematica》1982,39(2-3):313-338
We prove finiteness results of Plateau's problem for boundary curves in a normal chart of radius R < /2 in an analytic Riemannian manifold. It is shown that for analytic there exist only finitely many minimal surfaces of the type of the disc bounded by , which represent an absolute minimum of area, and if boundary branch points are excluded, there exist only finitely many relative minima in the normal chart. If lies on the boundary of a strict convex manifold, there cannot exist any boundary branch points.
Die vorliegende Arbeit entstand als Dissertation am Mathematischen] Institut der Universität Saarbrücken.
Herrn Prof. Dr. F. Tomi möchte ich für die Bereitstellung des Themas und für seine Betreuung sehr herzlich danken. 相似文献
Die vorliegende Arbeit entstand als Dissertation am Mathematischen] Institut der Universität Saarbrücken.
Herrn Prof. Dr. F. Tomi möchte ich für die Bereitstellung des Themas und für seine Betreuung sehr herzlich danken. 相似文献
67.
The mass spectra of two series of 2-substituted-2,3-dihydro-4H-pyrans (six ethers and six carbonyl derivatives) showed a marked dependence upon the nature of the substituent. α-Cleavage (substituent loss) was the only primary decomposition observed in the carbonyl series, while ‘retro Diels-Alder’ reactions competed with α-cleavage in the ether series. An unambiguous interpretation of this difference in behaviour, based on the consequence of charge localization and thermochemical data, is proposed. 相似文献
68.
The organoaluminium and -gallium hydroxylamides (Me2GaONMe2)2, (tBu2AlONMe2)2, (tBu2GaONMe2)2 and (Me2AlONiPr2)2 have been prepared by the reaction of the hydroxylamines Me2NOH and iPr2NOH with the trialkylmetal compounds trimethylgallium, tri-tbutylaluminium and tri-tbutylgallium, respectively. All compounds have been characterised by NMR spectroscopy (1H, 13C, 15N, 17O and 27Al), by mass spectrometry and elemental analyses. The crystal structures of the four compounds have been determined, showing that they all form dimers but with different modes of aggregation: (Me2GaONMe2)2 has a Ga2O2N2 six-membered ring, (tBu2AlONMe2)2 and (Me2AlONiPr2)2 have Al2O2 four-membered rings, (tBu2GaONMe2)2 forms a Ga2O2N five-membered ring. 相似文献
69.
Norbert Kuhn Martin Ghner Martin Grathwohl Jrg Wiethoff Gernot Frenking Yu Chen 《无机化学与普通化学杂志》2003,629(5):793-802
2‐Iminoimidazolines — Strong Nitrogen Bases als Ligands in Inorganic Chemistry Due to the tendency of the 5‐membered cyclic fragment to accept a positive charge which yields an ylide type bonding situation, 2‐iminoimidazolines are strong nitrogen bases. They can serve as neutral ligands being 2+2 electron donors. Deprotonation leads to the anions which are potential 2+4 electron donors. We describe first the synthesis and characterization of the title compound 2‐imino‐1, 3‐dimethylimidazoline (ImNH, 8 ) and its anion 9 . Next we demonstrate their properties as ligands in complexes of main group elements and transition metals. In a third chapter we describe attempts to functionalize iminoimidazolines with the goal to create neutral ligands that coordinate in a semistable fashion. On this way we want to make a contribution to the chemistry of complex compounds directed towards catalysis. 相似文献
70.
Dr. Norbert Köhler 《Monatshefte für Mathematik》1981,92(2):105-116
Those non-hamiltonian graphsG withn vertices are characterized, which satisfy the Ore-type degree-conditiond(x)+d(y)n–2 for each pairx,yM of different nonadjacent vertices whereM consists of two vertices ofG. As an application a theorem on hamiltonian connectivity of graphs is given. Furthermore, a condition is presented which is sufficient for the existence of a covering of a graph by two disjoint paths with prescribed set of startpoints and prescribed set of endpoints. A class of graphs is described which have no covering of this kind. 相似文献