全文获取类型
收费全文 | 3786篇 |
免费 | 141篇 |
国内免费 | 15篇 |
专业分类
化学 | 3218篇 |
晶体学 | 24篇 |
力学 | 23篇 |
数学 | 193篇 |
物理学 | 484篇 |
出版年
2023年 | 16篇 |
2022年 | 22篇 |
2021年 | 39篇 |
2020年 | 40篇 |
2019年 | 52篇 |
2018年 | 30篇 |
2017年 | 39篇 |
2016年 | 74篇 |
2015年 | 74篇 |
2014年 | 82篇 |
2013年 | 213篇 |
2012年 | 195篇 |
2011年 | 239篇 |
2010年 | 136篇 |
2009年 | 147篇 |
2008年 | 243篇 |
2007年 | 263篇 |
2006年 | 224篇 |
2005年 | 236篇 |
2004年 | 168篇 |
2003年 | 156篇 |
2002年 | 125篇 |
2001年 | 62篇 |
2000年 | 82篇 |
1999年 | 54篇 |
1998年 | 55篇 |
1997年 | 64篇 |
1996年 | 58篇 |
1995年 | 36篇 |
1994年 | 31篇 |
1993年 | 45篇 |
1992年 | 27篇 |
1991年 | 27篇 |
1990年 | 27篇 |
1989年 | 24篇 |
1988年 | 26篇 |
1987年 | 27篇 |
1986年 | 13篇 |
1985年 | 59篇 |
1984年 | 48篇 |
1983年 | 20篇 |
1982年 | 30篇 |
1981年 | 35篇 |
1980年 | 34篇 |
1979年 | 49篇 |
1978年 | 40篇 |
1977年 | 27篇 |
1976年 | 27篇 |
1975年 | 24篇 |
1974年 | 29篇 |
排序方式: 共有3942条查询结果,搜索用时 46 毫秒
101.
Ralf Otte Roland Fröhlich Shuichi Nakamura Takeshi Toru Dieter Hoppe 《Tetrahedron letters》2007,48(49):8636-8642
The enantio- and diastereoselective synthesis of allenyl aryl sulfides by asymmetric lithiation of 2-alkynyl (2-hetero)aryl sulfides is described. A dynamic thermodynamic resolution by selective crystallization of the intermediate lithium complexes derived from deprotonation, applying a bis(oxazoline) ligand, was achieved to give enantioselectivities up to 85% ee. Subsequent stereospecific nickel-catalyzed cross-coupling reactions with arylzinc reagents established a versatile access to threefold carbon-substituted allenes. 相似文献
102.
Tsutomu Yokozawa Jun Takagi Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》1989,27(1):291-299
Spontaneous copolymerization of cyclic ketene acetal, 2-methylene-4-phenyl-1,3-dioxolane ( I ) with common electrophilic vinyl monomers, such as methyl α-cyanoacrylate (MCA), acrylonitrile (AN), and methyl methacrylate (MMA) were investigated to further explore zwitterion polymerization method with cyclic ketene acetals. In the reaction of I with MCA and AN, spontaneous copolymerization took place at ambient temperature. The copolymers of I with MCA gave low molecular weight polymers, but copolymers obtained with I and AN were high molecular weight polymers. In the reaction of I and MMA, high molecular weight copolymer was obtained only at temperatures above 80°C. Thus, obtained polymers were not the alternating copolymers and possessed high I content in all the cases. From the above results, macrozwitterionic mechanism was suggested as discussed. 相似文献
103.
(±)-Tryptoquivaline G was formally synthesized through shortened steps from tryptophan, in which oxidation with thallium(III) trinitrate(TTN) was effected at the crucial stage. The present work also constitutes the first synthesis of oxindole lactones carrying 2-alkyl quinazolinones. 相似文献
104.
Rotational correlation times (τT) of the 5′-AMP molecule deduced from spin-lattice relaxation times (T1) of different protons in the molecule agree fairly well with each other in the temperature range of 3.5–74°C. The same is true with τT values deduced from 13CT1 values. These results indicate that the internal motions are slow as compared to the overall rotation of the 5′-AMP molecule. 相似文献
105.
Tomooka K Komine N Fujiki D Nakai T Yanagitsuru S 《Journal of the American Chemical Society》2005,127(35):12182-12183
Remarkably stable planar chirality was found in a nine-membered diallylic cyclic ether. Enantiomerically enriched ether was prepared by a kinetic resolution of a racemic one, which is valuable as a novel type of chiral building block. 相似文献
106.
The polymerization of methyl methacrylate (MMA) was carried out with the system polyethyleneglycol (PEG), copper(II) chloride and water at 85°. The effects of the amount of each component on the conversion of MMA were studied. Some polymerization occurred even if copper(II) ion was not present. It is suggested that there were three reactions: (1) polymerization initiated by the complex of PEG, copper(II) ion and water, (2) polymerization by PEG in water, (3) in the water phase, inhibition by copper(II) ion. The polymerization proceeded through a radical mechanism. 相似文献
107.
Norio Tsubokawa Shingo Ohshima Yasuo Sone Takeshi Endo 《Journal of polymer science. Part A, Polymer chemistry》1987,25(3):935-943
The ring-opening transfer polymerization of spiro ortho esters (SOE) initiated by carbon black was investigated. In the absence of carbon black, no polymerization occurred at all. In the presence of channel black containing carboxyl group, the ring-opening transfer polymerization of SOE was initiated at 50-70°C. to give polyether ester, namely alternating copolymer of epoxide and lactone. The rate of polymerization of 1,4,6-trioxaspiro[4.4]nonane and 1,4,6-trioxaspiro[4.5]decane was considerably small compared with that of 1,4,6-trioxaspiro[4.6]undecane. The activation energy of the polymerization of 2-chloromethyl-1,4,6-trioxaspiro[4.6]undecane was estimated to be 6.0 kcal/mol. The initiating activity of carbon black increased with an increase in carboxyl group content of carbon black. Furnace black that contained no carboxyl group was unable to initiate the polymerization. Furthermore, the carbon black lost the initiating ability of the polymerization upon the blocking of carboxyl group on the surface by the treatment with potassium hydroxide or diazomethane. Based on these results, it was concluded that carboxyl group on carbon black plays an important role in the initiation. During the polymerization, a part of the polymer formed was grafted onto carbon black: the grafting ratio was 10–30%. The mechanisms of initiation and grafting were discussed. 相似文献
108.
Satoshi ObikaJyun-ichi Andoh Mayumi OnodaOsamu Nakagawa Akiko HirotoTomomi Sugimoto Takeshi Imanishi 《Tetrahedron letters》2003,44(28):5267-5270
A novel bridged nucleic acid monomer, 3′-amino-3′-deoxy-5-methyl-3′-N,4′-C-methyleneuridine, was successfully synthesized via a useful and convenient azetidine ring formation under Staudinger's conditions. A 1H NMR experiment and a PM3 calculation revealed that the sugar moiety of the novel bridged nucleic acid monomer, 3′-amino-3′,4′-BNA, was restricted to S-type conformation. 相似文献
109.
Takeshi Takeda Tomohiro ShonoKenji Ito Hironori SasakiAkira Tsubouchi 《Tetrahedron letters》2003,44(43):7897-7900
Aldehydes, ketones, esters and lactones are transformed into enol ethers or 1,2-dialkoxy-1-alkenes by treatment with organotitanium species prepared from alkoxymethyl chlorides and a titanocene(II) complex. 相似文献
110.
Terutaka Watanabe Takeshi Wada Masaaki Takehisa Sueo Machi 《Journal of polymer science. Part A, Polymer chemistry》1974,12(8):1619-1628
The heterogeneous polymerization of ethylene initiated by radiation in tert-butyl alcohol was studied. The polymerization was carried out in a 100-ml reactor at 25–100°C and pressures of 200–300 kg/cm2 in the presence of 50 ml of tert-butyl alcohol containing 7 wt-% water. The amounts of polymerized monomer, the average molecular weight of polymer formed, and the molecular weight distribution of polymer were measured at various stages of reaction and at various temperatures. The molecular weight distribution was found to be very much dependent on the reaction time and temperature. For the polymer formed at 50–60°C in the very early stages of reaction, the molecular weight distribution is unimodal, and in the intermediate stage a shoulder appears at a molecular weight higher than the first peak which increases as the polymerization proceeds; eventually a bimodal curve is formed. The bimodal distribution curves were analyzed to determine the fractions and average molecular weights of the each peak. On the basis of these data for the molecular weight distribution and kinetic behavior, a new scheme for the heterogeneous polymerization is proposed which indicates that the polymerization proceeds via propagating radicals in two different physical states, namely, loose and rigid states. 相似文献