全文获取类型
收费全文 | 337篇 |
免费 | 8篇 |
国内免费 | 1篇 |
专业分类
化学 | 287篇 |
晶体学 | 1篇 |
数学 | 10篇 |
物理学 | 48篇 |
出版年
2023年 | 2篇 |
2022年 | 2篇 |
2020年 | 7篇 |
2019年 | 3篇 |
2018年 | 3篇 |
2017年 | 3篇 |
2016年 | 9篇 |
2015年 | 2篇 |
2014年 | 7篇 |
2013年 | 13篇 |
2012年 | 23篇 |
2011年 | 24篇 |
2010年 | 15篇 |
2009年 | 10篇 |
2008年 | 18篇 |
2007年 | 16篇 |
2006年 | 15篇 |
2005年 | 19篇 |
2004年 | 15篇 |
2003年 | 10篇 |
2002年 | 12篇 |
2001年 | 8篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1972年 | 5篇 |
1969年 | 4篇 |
1965年 | 2篇 |
排序方式: 共有346条查询结果,搜索用时 31 毫秒
11.
Shinji Toyota Takashi Nakagawa Masashi Kotani Michinori
ki Hidehiro Uekusa Yuji Ohashi 《Tetrahedron》2002,58(52):10345-10351
All the three possible rotamers of the title compound were separated by chromatography, and unambiguously identified by NMR and X-ray analysis. One of the isomers was optically inactive Ci conformation. The other optical active forms were resolved to give a pair of enantiomers, which were characterized by optical rotation and CD spectra. Thus the optical inactivity of a compound such as meso-tartaric acid that can take Ci conformation in solution, is now ascribed to that the molecule has an optically inactive Ci conformer and equal amounts of optically active conformers, that are enantiomers, in solution. 相似文献
12.
The stability and electronic structure of perovskite hydrides ABH3 were investigated by means of first-principles density functional calculations. Two types of perovskite hydrides are distinguished: (1) When A and B are alkali and alkaline earth metals, the hydrides are ionic compounds with calculated band gaps of around 2 eV and higher. Their stability trend follows basically the concept of Goldschmidt's tolerance factor. (2) When A is one of the heavier alkaline earth metals (Ca, Sr, Ba) and B a transition metal, stable compounds ABH3 result only when B is from the Fe, Co, or Ni groups. This stability trend is basically determined by effects associated with d band filling of both the transition metal and the hydride. In contrast to group (1) perovskites, the transition metal-containing compounds are metals. The synthesis of CaNiH3 and its structure determination from CaNiD3 is reported. This compound is a type (2) perovskite hydride with a fully occupied hydrogen position (CaNiD3: a=3.551(4) Å, dNi-D=1.776(2) Å). Its stability is discussed with respect to transition metal hydrides with complex anions (e.g., Mg2NiH4, Na2PdH2, Sr2PdH4). 相似文献
13.
Cis-fused bicyclo[6.4.0]dodecene 11 was converted into taxane BC ring system 21 in three steps; transformation of the phenylsulfonyl group to an alpha-hydroxy carbonyl group by the treatment with potassium hexamethyldisilazide (KHMDS) and triethyl phosphite under oxygen atmosphere, followed by reductive elimination of the hydroxyl group of alpha-ketol moiety, and inversion of ring juncture. Epimerization of the sulfonyl group of 11 was indispensable for the first oxidation process (17 --> 18) and the second oxidation of 12 leading to hydroxylation at the alpha-position of the carbonyl group proceeded with high regio- and stereoselectivity to give 13. On the other hand, reaction of the cross-conjugated compound 5 with KHMDS at 0 degrees C brought about a complete reorganization of molecular framework to provide the compound 7 in which the five-membered ring and the conjugated seven-membered ring were connected through a single bond. 相似文献
14.
Yasushi OgawaMasahito Toyama Michinori KarikomiKazuo Haga Tadao Uyehara 《Tetrahedron letters》2003,44(10):2167-2170
Both enantiomers of (P)-(+)-2- and (M)-(−)-2-acetoxy-11,14-dimethyl[5]helicenes 8 were synthesized by asymmetric aromatic oxy-Cope rearrangement of the corresponding chiral bridged bicyclic compounds, which were obtained by enzymatic resolution. The absolute configurations of 8 were assigned by their circular dichroism spectra. 相似文献
15.
Takeshita N Paradis LA Oner D McCarthy TJ Chen W 《Langmuir : the ACS journal of surfaces and colloids》2004,20(19):8131-8136
Wettability was controlled in a rational manner by individually and simultaneously manipulating surface topography and surface chemical structure. The first stage of this research involved the adsorption of charged submicrometer polystyrene latex particles to oppositely charged poly(ethylene terephthalate) (PET) film samples to form surfaces with different topographies/roughness; adsorption time, solution pH, solution ionic strength, latex particle size, and substrate charge density are external variables that were controlled. The introduction of discrete functional groups to smooth and rough surfaces through organic transformations was carried out in the second stage. Amine groups (-NH(2)) and alcohol groups (-OH) were introduced onto smooth PET surfaces by amidation with poly(allylamine) and adsorption with poly(vinyl alcohol) (PVOH), respectively. On latex particle adsorbed surfaces, a thin layer of gold was evaporated first to prevent particle redistribution before chemical transformation. Reactions with functionalized thiols and adsorption with PVOH on patterned gold surfaces successfully enhanced surface hydrophobicity and hydrophilicity. Particle size and biomodal particle size distribution affect both hydrophobicity and hydrophilicity. A very hydrophobic surface exhibiting water contact angles of 150 degrees /126 degrees (theta(A)/theta(R)) prepared by adsorption of 1-octadecanethiol and a hydrophilic surface with water contact angles of 18 degrees /8 degrees (theta(A)/theta(R)) prepared by adsorption of PVOH were prepared on gold-coated surfaces containing both 0.35 and 0.1 microm latex particles. The combination of surface topography and surface-chemical functionality permits wettability control over a wide range. 相似文献
16.
A method was investigated in which all of the phthalate esters in biological samples were determined as phthalic acid by gas-liquid chromatography. The method is based on the separation of phthalate esters from the sample with n-hexane, saponification of the esters with an alkaline ethanolic solution to give phthalic acid, purification of the acid by extraction with diethyl ether and column chromatography using silica gel, and conversion of the acid into bis(2,2,2-trifluoroethyl) phthalate with a 2,2,2-trifluoroethanol solution containing boron trifluoride. The derivative obtained is highly sensitive to an electron-capture detector, giving a sensitivity of 0.1 pg. Biological samples fortified with di(2-ethylhexyl) phthalate at levels of 5-100 ppb were analyzed, with recoveries of 70-100%. 相似文献
17.
Suginome M Ohmura T Miyake Y Mitani S Ito Y Murakami M 《Journal of the American Chemical Society》2003,125(37):11174-11175
Enantioenriched beta-borylallylsilanes were synthesized by palladium-catalyzed enantioface-selective addition of the silicon-boron bond to terminal allenes using a palladium catalyst possessing a chiral monodentate phosphine ligand. Use of a silylborane bearing a chiral auxiliary on the boron atom was beneficial to gain enantioface selectivities as high as 96% de. 相似文献
18.
Fujishima H Takeshita H Toyota M Kim HS Wataya Y Tanaka M Sasaki T Ihara M 《Chemical & pharmaceutical bulletin》2001,49(5):572-575
Biological evaluations of bicyclo[6.4.0]dodecenone derivatives on antimalarial activity in vitro against Plasmodium falciparum and cytotoxicity against human KB cells were made. (+/-)-(1R*,4S*,7R*,8S*)-4-tert-Butyl-dimethylsiloxy-5,5-dimethyl-1-methyl-9-methylene-7-phenylsulfonylbicyclo[6.4.0]dodec-2,11-dien-10-one (15) exhibited potent antimalarial activity, whereas (+/-)-(1R*,7R*,8S*)-1-methyl-9-methylene-7-phenylsulfonylbicyclo[6.4.0]dodec-2,11-dien-10-one (14) showed significant cytotoxic activity in human KB cells. Both 14 and 15 possess, as a structural character, the exo-methylene moiety in their 6-membered ring of the 8-6 fused ring system. 相似文献
19.
Oki Michinori Toyofuku Yoshitaka Sakaue Tatsuya Hirose Takanori Asakura Mitsuhiro Morita Nobuhiro Toyota Shinji 《Russian Journal of Organic Chemistry》2003,39(4):542-553
X-ray crystallography of the ap-form of the 1-(9-fluorenyl)-2-(2-methyl-2-oxiranyl)naphthalene has revealed that the carbon atom of the oxirane which is connected to the naphthyl group in this compound is almost planar. The specific structural features of the epoxy ring in this compound are caused by sterical effects and by the presence of a -system in the immediate vicinity of the oxirane ring. Certain differences have been found also in reactivity of rotational isomers of 2-X-substituted 1-[1-(9-fluorenyl)-2-naphthyl]ethyl cations (X = O, S, Se). At X = O arose more deprotonation product from the -position of the oxygen than in reactions of sulfur and selenium-containing analogs. Reactions of epoxides with zinc chloride almost exclusively gave the corresponding aldehydes. 相似文献
20.
Abstract 2-(4-Alkoxybenzoyloxy)-5-alkylaminotropone and 5-alkoxy-2-(4-alkylaminobenzoyloxy)tropone liquid crystals showed the smectic C phase exclusively. Variable temperature FT-IR spectra indicated that intermolecular hydrogen bonding between the NH and tropone C=O groups assisted the appearance of the mesophase. 4-Alkoxyphenyl 4-alkylaminobenzoates were non-mesogenic. 相似文献