全文获取类型
收费全文 | 2797篇 |
免费 | 19篇 |
国内免费 | 13篇 |
学科分类
数理化 | 2829篇 |
出版年
2018年 | 19篇 |
2016年 | 27篇 |
2015年 | 25篇 |
2014年 | 39篇 |
2013年 | 82篇 |
2012年 | 71篇 |
2011年 | 68篇 |
2010年 | 54篇 |
2009年 | 40篇 |
2008年 | 60篇 |
2007年 | 61篇 |
2006年 | 77篇 |
2005年 | 72篇 |
2004年 | 71篇 |
2003年 | 53篇 |
2002年 | 49篇 |
2001年 | 21篇 |
2000年 | 37篇 |
1999年 | 39篇 |
1998年 | 28篇 |
1997年 | 35篇 |
1996年 | 58篇 |
1995年 | 42篇 |
1994年 | 39篇 |
1993年 | 51篇 |
1992年 | 43篇 |
1991年 | 42篇 |
1990年 | 32篇 |
1989年 | 34篇 |
1988年 | 24篇 |
1987年 | 30篇 |
1986年 | 37篇 |
1985年 | 68篇 |
1984年 | 34篇 |
1983年 | 41篇 |
1982年 | 45篇 |
1981年 | 38篇 |
1980年 | 59篇 |
1979年 | 54篇 |
1978年 | 53篇 |
1977年 | 48篇 |
1976年 | 50篇 |
1975年 | 36篇 |
1974年 | 35篇 |
1973年 | 41篇 |
1972年 | 26篇 |
1971年 | 25篇 |
1970年 | 28篇 |
1967年 | 17篇 |
1930年 | 18篇 |
排序方式: 共有2829条查询结果,搜索用时 15 毫秒
51.
Dirk Steinborn Olaf Gravenhorst Clemens Bruhn Dusan Miklo Michal Dunaj-Jur
o Alfred Kolbe 《无机化学与普通化学杂志》1997,623(12):1954-1958
Synthesis and Characterization of Aquapentachloroplatinates(IV) – Structure of [K(18-crown-6)][PtCl5(H2O)] The crown ether complex of the aquapentachloroplatinic acid of the composition [H13O6][PtCl5(H4O2)] · 2(18-cr-6) ( 2 ) reacts with K2CO3 and [NnBu4]OH in aqueous solution to give [K(18-cr-6)][PtCl5(H2O)] ( 5 a ) and [NnBu4][PtCl5(H2O)] · 1/2 (18-cr-6) · H2O ( 5 b ), respectively. Both compounds were characterized by microanalysis, vibrational (IR, Raman) and NMR (1H, 13C, 195Pt) spectroscopy. The X-ray structure analysis of 5 a (orthorhombic, pnma; a = 16,550(4), b = 18,044(3), c = 7,415(1) Å; Z = 4; R1 = 0,0183; wR2 = 0,0414) reveals that the crystal is threaded by chains built up of [PtCl5(H2O)]? and [K(18-cr-6)]+ units. There are tight K …? Cl contacts (d(K? Cl1)) = 3,0881(9) Å and OW? H? Ocr hydrogen bridges (d(O1 …? O2) = 2,806(3) Å) between these units. The coordination polyhedron [PtCl5O] has approximately C4v symmetry. 相似文献
52.
H. Blume R. Färber N. Feldmann K. Heinloth D. Kramarczyk F. Kuckelkorn H. J. Michels J. Päsler J. P. Dowd 《Zeitschrift fur Physik C Particles and Fields》1983,16(4):283-289
The electroproduction of π0-mesons on protons has been measured with electrons of 1.7 and 2.1 GeV energy in the region of the nucleon resonances at \(W = \sqrt {(q + p)^2 } = 1,400 - 2,000MeV\) at rather low values of |q 2|=|(e?e′)2|=0.01–0.1 GeV2 but at high values of the nucleon recoil |t|=|(p?p′)2|≈0.6–2.2 GeV2 corresponding to \(\Theta _\pi ^{cm} \approx 145^ \circ - 180^ \circ\) . Up toW≈1,650 MeV the cross section for π0-production with virtual photons of |q 2|≈0.03 GeV2 is only slightly smaller than that with real photons. ForW?1,700 MeV the cross section at |q 2|≈0.03 GeV2 is on average a factor of 2 smaller than that atq 2=0 but only slightly higher than that at |q 2|≈0.3–1 GeV2. 相似文献
53.
Doz. Dr. Heinz Falk Otmar Hofer Alfred Leodolter 《Monatshefte für Chemie / Chemical Monthly》1975,106(4):983-990
The structure of squaric acid derivatives was investigated on the basis of the PPP—SCF—LCAO—MO—CI method utilizing the π-electron densities and bond orders. The parameters used in the calculations were modified to fit the experimental data for representative compounds. The data were obtained by X-ray photoelectron spectrometry and electron absorption spectroscopy (including the polarization of the bands as determined by liquid crystal induced circular dichroism measurements). 相似文献
54.
In the AlBr3-catalyzed adamantane rearrangement in CS2 of 1,2-exo-trimethylenenorbornane ( 1 ) to 2-endo,6-endo-trimethylenenorbornane ( 3 ), hydride-ion abstraction occurs at C(6) from the exo-side. The kH/kD value for competition between 1 and 5 (Dexo-C(6)) was 1.58 ± 0.05, whereas no kinetic isotope effect was operative for competition between unlabeled 1 and 4 (Dendo-C(5)) and between 1 and 6 (Dendo-C(6)). 相似文献
55.
The interaction of 2-aminobenzimidazole with the appropriate β-diketones carrying fluoroalkyl groupings has led to the 2-perfluoroalkylpyrimido[1,2-a]benzimidazoles as follows: 4-phenyl-, 4-(2'-naphthyl)-, 4-(3'-pyridyl)-, 4-(2'-furyl)-, and 4-(2,-thienyl)-2-trifluoromethylpyrimido[1,2-a]benzimidazole, and 4-(2'-thienyl)-2-(heptafluoropropylpyrimido [1,2-a]benzimidazole. 相似文献
56.
The "epimerisation" of UDP-GlcNAc to ManNAc, the first step in the biosynthesis of sialic acids, is catalyzed by UDP-GlcNAc 2-epimerase. In this paper we report the synthesis of transition state based inhibitors of this enzyme. To mimic the assumed first transition state of this reaction (TS 1), we designed and synthesized the novel UDP-exo-glycal derivatives 1-4. We also report herein the synthesis of 5 and 6, the first C-glycosidic derivatives of 2-acetamidoglucal, and the synthesis of the ketosides 7 and 8, which were designed as bis-substrate analogue and bis- product analogue, respectively, to mimic the second step of the reaction via the assumed second transition state TS 2. 相似文献
57.
Zusammenfassung Zur Bestimmung von phosphoriger und von unterphosphoriger Säure in Bädern zur stromlosen Herstellung von Metallüberzügen werden zwei neue Methoden beschrieben.Unterphosphorige Säure wird mit Silberperchlorat oxydiert und der Überschuß an Silberionen mit Natriumchlorid zurücktitriert. Die Summe von phosphoriger Säure und unterphosphoriger Säure wird bromatometrisch in 0,11 N salzsaurer Lösung bestimmt. Zur Erfassung der phosphorigen Säure eignet sich das von Norkus, Lunjackas u. Carankute beschriebene jodometrische Verfahren.
1 Diplomarbeit, Bergakademie Freiberg 1967. 相似文献
Summary Two new methods are described for the determination of phosphorous and hypophosphorous acid in bath solutions for the production of metal coatings without current. Hypophosphorous acid is oxidized with silver perchlorate and the excess of silver ions is back-titrated with sodium chloride. The sum of phosphorous and hypophosphorous acid is determined bromatometrically in 0.11 N hydrochloric acid solution. Phosphorous acid can be determined by the iodometric procedure according to Norkus, Lunjackas and Carankute.
1 Diplomarbeit, Bergakademie Freiberg 1967. 相似文献
58.
The equilibrium structures, binding energies, and vibrational spectra of the clusters CH3F(HF)1 n 3 and CH2F2(HF)1 n 3 have been investigated with the aid of large-scale ab initio calculations performed at the Møller–Plesset second-order level. In all complexes, a strong C–FH–F halogen–hydrogen bond is formed. For the cases n = 2 and n = 3, blue-shifting C–HF–H hydrogen bonds are formed additionally. Blue shifts are, however, encountered for all C–H stretching vibrations of the fluoromethanes in all complexes, whether they take part in a hydrogen bond or not, in particular also for n = 1. For the case n = 3, blue shifts of the ν(C–H) stretching vibrational modes larger than 50 cm−1 are predicted. As with the previously treated case of CHF3(HF)1 n 3 complexes (A. Karpfen, E. S. Kryachko, J. Phys. Chem. A 107 (2003) 9724), the typical blue-shifting properties are to a large degree determined by the presence of a strong C–FH–F halogen–hydrogen bond. Therefore, the term blue-shifted appears more appropriate for this class of complexes. Stretching the C–F bond of a fluoromethane by forming a halogen–hydrogen bond causes a shortening of all C–H bonds. The shortening of the C–H bonds is proportional to the stretching of the C–F bond. 相似文献
59.
Gaetano Riggio Wolfgang Herbert Hopff Alfred Andr Hofmann Peter Gaudenz Waser 《Helvetica chimica acta》1983,66(4):1039-1045
Hydroxyphenyl-1-methylpyridinium-iodide as Potential Reactivators of Acetylcholinesterase Poisoned with Organophosphorus Compounds . It was our aim to reactivate acetylcholinesterase poisoned with sarin. We synthesized 2-(o-hydroxyphenyl)-1-methylpyridinium-iodide ( 9 ), 2-(p-hydroxyphenyl)-1-methylpyridinium-iodide ( 19 ) and 4-(o-hydroxyphenyl)-1-methylpyridinium-iodide ( 14 ) as potential reactivators. All substances showed moderate toxicity against mice; their reactivity potency in vitro and in vivo was negligible. 相似文献
60.
On the Synthesis of Sulfonated Derivatives of 2,3-Dimethylaniline and 3,4-Dimethylaniline Baking the hydrogensulfate salt of 2,3-dimethylaniline ( 1 ) or of 3,4-dimethylaniline ( 2 ) led to 4-amino-2,3-dimethylbenzenesulfonic acid ( 4 ) and 2-amino-4,5-dimethylbenzenesulfonic acid ( 5 ), respectively (Scheme 1). The sulfonic acid 5 was also obtained by treatment of 2 with sulfuric acid or by reaction of 2 with amidosulfuric acid. 3-Amino-4,5-dimethylbenzenesulfonic acid ( 3 ) and 5-Amino-2,3-dimethylbenzenesulfonic acid ( 6 ) were prepared by sulfonation of 1,2-dimethyl-3-nitrobenzene ( 9 ) to 3,4-dimethyl-5-nitrobenzenesulfonic acid ( 11 ) and of 1,2-dimethyl-4-nitrobenzene ( 10 ) to 2,3-dimethyl-5-nitrobenzenesulfonic acid ( 12 ), respectively, with subsequent Béchamp reduction (Scheme 1). Preparations of 2-amino-3,4-dimethylbenzenesulfonic acid ( 7 ) and of 6-amino-2,3-dimethylbenzenesulfonic acid ( 8 ) were achieved by the sulfur dioxide treatment of the diazonium chlorides derived from 3,4-dimethyl-2-nitroaniline ( 24 ) and from 2,3-dimethyl-6-nitroaniline ( 31 ) to 3,4-dimethyl-2-nitrobenzenesulfonyl chloride ( 29 ) and 2,3-dimethyl-6-nitrobenzenesulfonyl chloride ( 32 ), respectively, followed by hydrolysis to 3,4-dimethyl-2-nitrobenzenesulfonic acid ( 30 ) and 2,3-dimethyl-6-nitrobenzenesulfonic acid ( 33 ), and final reduction (Scheme 3). Compound 7 was also synthesized by reaction of 4-chloro-2,3-dimethylaniline ( 23 ) with amidosulfuric acid to 2-amino-5-chloro-3,4-dimethylbenzenesulfonic acid ( 20 ) and subsequent hydrogenolysis (Scheme 2). 4′-Bromo-2′, 3′-dimethyl-acetanilide ( 13 ) and 4′-chloro-2′, 3′-dimethyl-acetanilide ( 14 ) on treatment with oleum yielded 5-acetylamino-2-bromo-3,4-dimethylbenzenesulfonic acid ( 17 ) and 5-acetylamino-2-chloro-3,4-dimethylbenzenesulfonic acid ( 18 ), respectively. Their structures were proven by hydrolysis to 5-amino-2-bromo-3,4-dimethylbenzenesulfonic acid ( 21 ) and 5-amino-2-chloro-3,4-dimethylbenzenesulfonic acid ( 22 ), followed by reductive dehalogenation to 3 . 相似文献