全文获取类型
收费全文 | 4304篇 |
免费 | 112篇 |
国内免费 | 5篇 |
专业分类
化学 | 2792篇 |
晶体学 | 56篇 |
力学 | 58篇 |
数学 | 433篇 |
物理学 | 1082篇 |
出版年
2021年 | 33篇 |
2020年 | 35篇 |
2019年 | 38篇 |
2016年 | 58篇 |
2015年 | 66篇 |
2014年 | 73篇 |
2013年 | 141篇 |
2012年 | 137篇 |
2011年 | 163篇 |
2010年 | 111篇 |
2009年 | 82篇 |
2008年 | 142篇 |
2007年 | 135篇 |
2006年 | 143篇 |
2005年 | 141篇 |
2004年 | 129篇 |
2003年 | 129篇 |
2002年 | 120篇 |
2001年 | 116篇 |
2000年 | 115篇 |
1999年 | 60篇 |
1998年 | 42篇 |
1997年 | 47篇 |
1996年 | 86篇 |
1995年 | 62篇 |
1994年 | 81篇 |
1993年 | 61篇 |
1992年 | 61篇 |
1991年 | 59篇 |
1990年 | 58篇 |
1989年 | 46篇 |
1988年 | 50篇 |
1987年 | 64篇 |
1986年 | 58篇 |
1985年 | 58篇 |
1984年 | 63篇 |
1983年 | 48篇 |
1982年 | 52篇 |
1981年 | 60篇 |
1980年 | 42篇 |
1979年 | 46篇 |
1978年 | 53篇 |
1977年 | 56篇 |
1976年 | 58篇 |
1975年 | 57篇 |
1974年 | 57篇 |
1973年 | 42篇 |
1968年 | 27篇 |
1967年 | 24篇 |
1957年 | 30篇 |
排序方式: 共有4421条查询结果,搜索用时 672 毫秒
121.
122.
The alkali phosphides of 1.2-bis-phosphinobenzene, and 1-alkyl-derivatives react with (Me2N)2COEt+BF4? to give the title compounds with dicoordinated phosphorus. The structure is proved by their nmr spectra. 相似文献
123.
Friedrich Bossert Horst Meyer Egbert Wehinger 《Angewandte Chemie (International ed. in English)》1981,20(9):762-769
The aryldihydropyridines first prepared by Hantzsch almost 100 years ago have recently been found to be highly effective calcium antagonists with suitable pharmacological profiles. An illustrative example is dimethyl-l,4-dihydro-2,6-dimethyl-4-(o-nitrophenyl)pyridine-3,5-dicarboxylate (Nifedipine) which is already employed therapeutically. This substance lowers the frequency of attack of angina pectoris and reduces blood pressure. The discovery of the therapeutic activity of this class of substances initiated renewed investigation of the Hantzsch condensation and the synthesis of numerous 4-aryldihydropyridines and related compounds. Qualitative and quantitative structure/activity relationships of these substances can be deduced from their biological data. 相似文献
124.
Parac-Vogt TN Pacco A Nockemann P Laurent S Muller RN Wickleder M Meyer G Vander Elst L Binnemans K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,12(1):204-210
Proton nuclear magnetic relaxation dispersion (NMRD) profiles were recorded between 0.24 mT and 1.4 T for lanthanum(III)- and gadolinium(III)-containing [15]metallacrown-5 complexes derived from alpha-aminohydroxamic acids and with copper(II) as the ring metal. The influence of the different R-groups on the proton relaxivity was investigated, and a linear relationship between the relaxivity and the molecular mass of the metallacrown complex was found. The selectivity of the metallacrown complexes was tested by transmetalation experiments with zinc(II) ions. The crystal structure of the copper [15]metallacrown-5 gadolinium complex with glycine hydroximate ligands is reported. 相似文献
125.
Structure of Tetraphenyldisiloxane Tetraphenyldisiloxane 1 crystallizes at 298 K monoclinically (P21/n; a = 1407.6; b = 610.7; c = 1262.7 pm; β = 95.87° Z = 2) and undergoes a second order phase transition at 200 K, in an almost unchanged structure of triclinic symmetry. At 298 K the molecules are already bent (Si? O? Si = 160°) with static or dynamical disorder of the bridging atom. Both Si? O distances are different (156 or 169 pm), because the shift of the bridging O atom is not perpendicular to the Si? to? Si vector. The reason for this remarkable behavior is not yet clear. According to the vibrational spectra, the Si? O? Si bridge is bent in the crystal but, in CCl4 solution a dynamical oscillation through the linear configuration may occur. 相似文献
126.
Colourless single crystals of the caffeine adduct of mercurous perchlorate dihydrate, [Hg2(Caf)2](ClO4)2(H2O)2, were grown from aqueous solutions of mercurous perchlorate and caffeine by isothermal evaporation at ambient temperature. The crystal structure (monoclinic, P21/n, Z = 4, a = 1628.0(2), b = 780.4(1), c = 2229.6(3) pm, β = 99.84(1)°, R1(all data) = 0.0894) contains [trans‐Caf‐Hg‐Hg‐Caf]2+ cations with a Hg‐Hg distance of 250.88(6) pm, Hg‐N (bond) distances of 214.4(6) and 215.1(6) pm and Hg‐Hg‐N angles of 176.9(2) and 165.1(2)°, respectively. These cations are attached via weak Hg‐O contacts to dimers which are further arranged to leave large channels into which one crystal water molecule is included. The second water molecule and the two perchlorate anions are weakly attracted to one Hg atom. 相似文献
127.
Abstract— Clover yellow mosaic virus, in common with other plant viruses with flexuous, rod-like particles, can be photoreactivated. At lower U.V. doses, photoreactivation is masked by an inhibitory effect on virus infection of visible light alone. 相似文献
128.
Ernst Meyer 《Angewandte Chemie (International ed. in English)》1970,9(8):583-589
The principles of the topological encoding of formulas and machine searching are briefly described. These methods have been developed to the point where it is now possible to carry out a computer search for a given structural fragment without the risk of false drops. It is also possible to carry out searches whose requirements, while well-defined and readily used by chemists, cannot be represented exclusively by element symbols and chemical bonds. 相似文献
129.
H. Jehring U. Chevallerie-Haaf A. Meyer G. Henze 《Fresenius' Journal of Analytical Chemistry》1989,332(8):890-897
Summary The dp-polarographic behaviour of different pesticides with carbon-halogen bonds was investigated in dimethylsulfoxide and methanol/water as solvents; the peak-potentials are demonstrated graphically. From peak-height the pesticides are detectable up to 100 ng · ml–1. The investigations are of interest for the development of multistage-combined procedures by h.p.l.c. with amperometric detection.
Herrn Prof. Dr. R. Neeb zum 60. Geburtstag gewidmet 相似文献
Herrn Prof. Dr. R. Neeb zum 60. Geburtstag gewidmet 相似文献
130.
Step-scan FTIR time-resolved spectroscopy (S2FTIR TRS) in acetonitrile-d3 has been used to probe the acceptor ligand in metal-to-ligand charge transfer (MLCT) excited states of amide-substituted polypyridyl complexes of RuII and in analogues appended to polystyrene. On the basis of ground-to-excited state shifts in v(C = O) of -31 cm-1 for the amide group in [RuII(bpy)2(bpyCONHEt')]2+ (bpyCONHEt' = 4'-methyl-2,2'-bipyridine-4-carboxamide-Et'; Et' = -CH2CH2BzCH2CH3) (1) and in the derivatized polystyrene abbreviated [PS-[CH2-CH2NHCObpy-RuII(bpy)2]20]40+ (3), the excited-state dipole is directed toward the amide-containing pyridyl group in the polymer side chain. Smaller shifts in v(C = O) of -17 cm-1 in [RuII(4,4'-(CONEt2)2bpy)2-(bpyCONHEt')]2+ (2) and in the derivatized polystyrene abbreviated [PS-[CH2CH2NHCObpy-RuII(4,4'-(CONEt2)2bpy)2]20]40+ (4) indicate that the excited-state dipole is directed toward one of the diamide bpy ligands. The nearly identical results for 1 and 3 and for 2 and 4 show that the molecular and electronic structures of the monomer excited states are largely retained in the polymer samples. These conclusions about dipole orientation in the polymers are potentially of importance in understanding intrastrand energy transfer dynamics. The excited-state dipole in 3 is oriented in the direction of the covalent link to the polymer backbone, and toward nearest neighbors. In 4, it is oriented away from the backbone. 相似文献