全文获取类型
收费全文 | 537篇 |
免费 | 9篇 |
国内免费 | 2篇 |
专业分类
化学 | 447篇 |
晶体学 | 8篇 |
力学 | 2篇 |
数学 | 39篇 |
物理学 | 52篇 |
出版年
2023年 | 3篇 |
2022年 | 8篇 |
2021年 | 7篇 |
2020年 | 10篇 |
2019年 | 11篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 16篇 |
2015年 | 15篇 |
2014年 | 12篇 |
2013年 | 16篇 |
2012年 | 35篇 |
2011年 | 41篇 |
2010年 | 19篇 |
2009年 | 23篇 |
2008年 | 34篇 |
2007年 | 32篇 |
2006年 | 29篇 |
2005年 | 29篇 |
2004年 | 27篇 |
2003年 | 23篇 |
2002年 | 23篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 4篇 |
1998年 | 10篇 |
1997年 | 4篇 |
1996年 | 9篇 |
1994年 | 2篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1969年 | 2篇 |
1954年 | 4篇 |
排序方式: 共有548条查询结果,搜索用时 0 毫秒
31.
Marisa Phelan Fawaz Aldabbagh Per B. Zetterlund Bunichiro Yamada 《Macromolecular theory and simulations》2005,14(2):109-116
Summary: A detailed investigation of chain transfer to polymer during free radical ring‐opening polymerization of the eight‐membered disulfide monomer 2‐methyl‐7‐methylene‐1,5‐dithiacyclooctane (MDTO) is presented. It has been shown that extensive chain transfer to polymer occurs involving both poly(MDTO) radicals and cyanoisopropyl radicals. Significant decreases in molecular weight were observed when cyanoisopropyl radicals were generated in the presence of poly(MDTO) in the absence of monomer. The molecular weight distribution (MWD) obtained from polymerization of MDTO in the presence of pre‐added poly(MDTO) was markedly different from that obtained without pre‐added polymer. A kinetic model was constructed in an attempt to quantitatively describe the chain transfer to polymer process based on the addition fragmentation chain transfer mechanism. It was found however that the simulated MWDs were considerably broader than the experimental MWDs, which were similar to the Schulz‐Flory distribution.
32.
M Kamitani M Itazaki C Tamiya H Nakazawa 《Journal of the American Chemical Society》2012,134(29):11932-11935
The first catalytic double hydrophosphination of alkynes was achieved by reaction with diarylphosphines in the presence of an iron catalyst. The double hydrophosphination proceeded regioselectively and effectively for various secondary arylphosphines and terminal alkynes to give 1,2-bisphosphinoethane derivatives. 相似文献
33.
Masumi?Sato Masahiro?Nobe Toshiaki?DobashiEmail author Takao?Yamamoto Akira?Konno 《Colloid and polymer science》2005,284(3):293-300
We have found that the dialysis of curdlan dissolved in alkaline solution into aqueous solutions of metal salts yielded multifold gel structures. Aqueous sodium chloride and potassium chloride as well as pure water induced isotropic gels. Aqueous calcium salts induced liquid crystalline gel with refractive index gradient/amorphous gel alternative structure. Aqueous salts of trivalent aluminum and ferric cations induced a rigid liquid crystalline gel, which shrank above a threshold concentration of each salt. On the other hand, Liesegang ring-like pattern was observed with aqueous solutions of mixed salts of calcium chloride and magnesium chloride. The patterns have been classified to discuss the mechanism of forming the variety of structures. 相似文献
34.
William C. Neuhaus Marisa C. Kozlowski 《Angewandte Chemie (International ed. in English)》2020,59(20):7842-7847
A facile method to oxidatively trimerize phenols using a catalytic aerobic copper system is described. The mechanism of this transformation was probed, yielding insight that enabled cross‐coupling trimerizations. With this method, the natural product pyrolaside B was synthesized for the first time. The key strategy used for this novel synthesis is the facile one‐step construction of a spiroketal trimer intermediate, which can be selectively reduced to give the natural product framework without recourse to stepwise Ullmann‐ and Suzuki‐type couplings. As a result, pyrolaside B can be obtained expeditiously in five steps and 16 % overall yield. Three other analogues were synthesized, thus highlighting the utility of the method, which provides new accessibility to this area of chemical space. A novel xanthene was also synthesized through controlled Lewis acid promoted rearrangement of a spiroketal trimer. 相似文献
35.
Marcelo Kobelnik Douglas Lopes Cassimiro Diógenesdos Dos Santos Clóvis Augusto Ribeiro Marisa Spirandeli Crespi 《中国化学》2011,29(11):2271-2277
Solid state compounds of general formula Sr(Diclof)2·5.7H2O and Ba(Diclof)2·4.8H2O were obtained. Thermogravimetry (TG), differential scanning calorimetry (DSC), X‐ray diffraction powder patterns and microscopy analysis were used to characterize these compounds. Details concerning the dehydration and thermal decomposition as well as data of kinetic parameters have been described here. The kinetic behaviors of these stages were evaluated from several heating rates with samples masses of 2 and 5 mg in open crucibles under a nitrogen atmosphere. The results of the present study improve the knowledge on these compounds including their dehydration and thermal stability. The obtained data depend on the mass of the sample which results in two kinetic behavior patterns. 相似文献
36.
Dr. Yukio Hitotsuyanagi Masumi Odagiri Saori Kato Jun‐ichi Kusano Tomoyo Hasuda Haruhiko Fukaya Prof. Dr. Koichi Takeya 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(10):2839-2846
Two bicyclic hexapeptides, allo‐RA‐V ( 4 ) and neo‐RA‐V ( 5 ), and one cyclic hexapeptide, O‐seco‐RA‐V ( 6 ), were isolated from the roots of Rubia cordifolia L. Their gross structures were elucidated on the basis of spectroscopic analysis and X‐ray crystallography of compound 5 . The absolute stereochemistry of compounds 4 and 5 were established by their total syntheses, and the absolute stereochemistry of compound 6 by chemical correlation with deoxybouvardin ( 3 ). Comparison of the 3D structures of highly active RA‐VII ( 1 ) with less‐active compounds 4 and 5 suggests that the orientation of the Tyr‐5 and/or Tyr‐6 phenyl rings plays a significant role in their biological activity. The isolation of peptides 4 – 6 , along with compound 3 , and the comparison of their structures seem to indicate that peptide 6 may be the common precursor to bicyclic peptides 3 – 5 in the plant. 相似文献
37.
38.
White JM Lambert JB Spiniello M Jones SA Gable RW 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(12):2799-2811
The mechanism of stabilization of positive charge on carbon by sulfur, selenium, or tellurium at the beta-position has been investigated kinetically, by measurement of rate enhancements, and structurally, by variation of the bond strength to the leaving group. Stabilization can occur either nonvertically with formation of a bridged intermediate or vertically through hyperconjugation within an open carbocation. We observed large rate enhancements (10(5) for S, 10(6) for Se) in 97% aqueous trifluoroethanol with trifluoroacetate as the leaving group. These enhancements are consistent with either mechanism. Product structures also are consistent with either mechanism. Nine crystal structures revealed that the bond to the leaving group (C-O) is lengthened by the presence of S or Se at the beta-position, in proportion to the basicity of the leaving group. This lengthening is not accompanied by angle distortions expected for the bridging mechanism. The crystallographic data support vertical (hyperconjugative) character along the reaction coordinate, more so for selenium than sulfur. 相似文献
39.
[reaction: see text] Computer-aided design protocols to identify new chiral ligands for reactions proceeding through well-defined transition states are outlined. Ligand families are discovered via computational screening of large structural databases such as the Cambridge Structural Database. Using this method, a novel cis-decalin ligand has been identified as a chiral auxiliary for the allylboration of aldehydes. Synthesis, resolution, and evaluation revealed that this new auxiliary provided the aldehyde facial approach upon which the design was predicated. 相似文献
40.
Mathematische Zeitschrift - 相似文献