全文获取类型
收费全文 | 698篇 |
免费 | 34篇 |
专业分类
化学 | 505篇 |
晶体学 | 5篇 |
力学 | 16篇 |
数学 | 73篇 |
物理学 | 133篇 |
出版年
2023年 | 10篇 |
2022年 | 4篇 |
2021年 | 21篇 |
2020年 | 28篇 |
2019年 | 24篇 |
2018年 | 11篇 |
2017年 | 14篇 |
2016年 | 39篇 |
2015年 | 31篇 |
2014年 | 37篇 |
2013年 | 45篇 |
2012年 | 51篇 |
2011年 | 76篇 |
2010年 | 31篇 |
2009年 | 29篇 |
2008年 | 43篇 |
2007年 | 38篇 |
2006年 | 40篇 |
2005年 | 32篇 |
2004年 | 36篇 |
2003年 | 18篇 |
2002年 | 20篇 |
2001年 | 9篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 1篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1970年 | 1篇 |
1928年 | 1篇 |
排序方式: 共有732条查询结果,搜索用时 15 毫秒
41.
Using a density functional approach, we study the structural and magnetic properties of the pyrox-ene compound NaTiSi2O6. While all previous workers are taking that NaTiSi2O6 is a quasi-one-dimensional S=1/2 system, our theoretical results indicate that this is a Haldane S=1 chain compound below the phase transition at 210 K. A good agreement is obtained between the calculated and the measured Ti-Ti distances in the dimerized low temperature phase. We present a simple explanation of the flow of the unusual phase transition which is taking place in this compound. 相似文献
42.
43.
Malgorzata Basko Melania Bednarek Leen Billiet Przemyslaw Kubisa Eric Goethals Filip Du Prez 《Journal of polymer science. Part A, Polymer chemistry》2011,49(7):1597-1604
A straightforward strategy for the synthesis and functionalization of polyurethanes (PUs) via the use of alkyne‐functionalized polytetrahydrofuran (PTHF) diols is described. The alkyne groups have been introduced into the PTHF chains by the cationic ring‐opening copolymerization of tetrahydrofuran and glycidyl propargyl ether. These PTHF prepolymers were combined with 1,4‐butanediol and hexamethylene diisocyanate for the synthesis of linear PUs with latent functionalization sites. The polyether segments of the PUs have then been coupled with several types of functionalized azides by the copper‐catalyzed azide‐alkyne “click” chemistry, for example with phosphonium containing azides for their antibacterial properties. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 相似文献
44.
45.
In this paper we give necessary and sufficient conditions for the block sequence of the set X = {x
1 < x
2 < … < x
n
< …} ⊂ ℕ to have an asymptotic distribution function in the form x
λ. 相似文献
46.
Imidazolium-tagged ferrocenyl diphosphanes are useful ligands in palladium-catalyzed allylic substitutions with heteroatom nucleophiles. Substitution with phthalimide proceeds with high enantioselectivity (up to 92% ee) in various ionic liquids. Reaction with p-cresol as nucleophile affords allylation product in up to 62% ee, while using tolylsulfinate as a nucleophile gives a product with very little or no enantioselectivity. Under these reaction conditions, catalyst recyclability is challenging, and decrease in activity as well as enantioselectivity was observed. 相似文献
47.
Bartosz Mozgawa Dr. Filip Zasada Dr. Monika Fedyna Prof. Kinga Góra-Marek Dr. Edyta Tabor Dr. Kinga Mlekodaj Dr. Jiří Dědeček Prof. Zhen Zhao Dr. Piotr Pietrzyk Prof. Zbigniew Sojka 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(68):17159-17180
NH3 temperature-programmed desorption (NH3-TPD) is frequently used for probing the nature of the active sites in CuSSZ-13 zeolite for selective catalytic reduction (SCR) of NOx. Herein, we propose an interpretation of NH3-TPD results, which takes into account the temperature-induced dynamics of NH3 interaction with the active centers. It is based on a comprehensive DFT/GGA+D and first-principles thermodynamic (FPT) modeling of NH3 adsorption on single Cu2+, Cu+, [CuOH]+ centers, dimeric [Cu-O-Cu]2+, [Cu-O22−-Cu]2 species, segregated CuO nanocrystals and Brønsted acid sites (BAS). Theoretical TPD profiles are compared with the experimental data measured for samples of various Si/Al ratios and distribution of Al within the zeolite framework. Copper reduction, its relocation, followed by the intrazeolite olation/oxolation processes, which are concomitant with NH3 desorption, were revealed by electron paramagnetic resonance (EPR) and IR. DFT/FPT results show that the peaks in the desorption profiles cannot be assigned univocally to the particular Cu and BAS centers, since the observed low-, medium- and high-temperature desorption bands have contributions coming from several species, which dynamically change their speciation and redox states during NH3-TPD experiment. Thus, a rigorous interpretation of the NH3-TPD profiles of CuSSZ-13 in terms of the strength and concentration of the active centers of a particular type is problematic. Nonetheless, useful connections for molecular interpretation of TPD profiles can be established between the individual component peaks and the corresponding ensembles of the adsorption centers. 相似文献
48.
Starting from (R)-camphordiamine, 13 new camphor-annulated imidazoline ligands are synthesized in good yields as two regioisomeric series. Yields of up to 94% and good enantioselectivities up to 67% are achieved for the copper(II)-catalyzed Henry reaction giving the product stereoselectively according to the regioisomer used. 相似文献
49.
50.
Antonín Klásek Filip Křemen Hana Křemenová Antonín Lyčka Michal Rouchal 《Tetrahedron》2017,73(12):1583-1593
The reaction of tertiary α-chloroketones with ethanolamine has not been hitherto described in the literature. Herein, we describe the reaction of tertiary 3-chloroquinoline-2,4-diones with ethanolamine to give novel 3-(2-hydroxyethylamino)quinoline-2,4-diones. These compounds provide 3-(2-oxooxazolidin-3-yl)quinoline-2,4(1H,3H)-diones and new compounds with dimeric character after reaction with triphosgene. Molecular rearrangement proceeds during the reaction of 3-(2-hydroxyethylamino)quinoline-2,4-diones with isocyanic acid. Three types of reaction products arise: 2-(2-hydroxyethyl)imidazo[1,5-c]quinazoline-3,5-diones, 3-(2-hydroxyethyl)-3,3a-dihydro-2H-imidazo[4,5-]quinoline-4(5H)diones and primarily 5-hydroxy-1-(hydroxyethyl)-1′H-spiro[imidazolidine-5,3′-indole]-2,2′-diones. The reaction mechanism and product stereochemistry are discussed. The 1H, 13C and 15N NMR spectra of the prepared compounds were measured, and all resonances were assigned from appropriate two-dimensional experiments. 相似文献