全文获取类型
收费全文 | 1342篇 |
免费 | 33篇 |
国内免费 | 8篇 |
专业分类
化学 | 1008篇 |
晶体学 | 4篇 |
力学 | 26篇 |
数学 | 184篇 |
物理学 | 161篇 |
出版年
2023年 | 8篇 |
2022年 | 14篇 |
2021年 | 28篇 |
2020年 | 26篇 |
2019年 | 15篇 |
2018年 | 18篇 |
2017年 | 19篇 |
2016年 | 44篇 |
2015年 | 47篇 |
2014年 | 32篇 |
2013年 | 65篇 |
2012年 | 98篇 |
2011年 | 101篇 |
2010年 | 63篇 |
2009年 | 59篇 |
2008年 | 90篇 |
2007年 | 81篇 |
2006年 | 85篇 |
2005年 | 77篇 |
2004年 | 78篇 |
2003年 | 52篇 |
2002年 | 46篇 |
2001年 | 33篇 |
2000年 | 27篇 |
1999年 | 14篇 |
1998年 | 13篇 |
1997年 | 12篇 |
1996年 | 19篇 |
1995年 | 11篇 |
1994年 | 10篇 |
1993年 | 14篇 |
1992年 | 12篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 11篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 5篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1959年 | 3篇 |
排序方式: 共有1383条查询结果,搜索用时 46 毫秒
101.
Escuer A Vicente R Mautner FA Goher MA Abu-Youssef MA 《Chemical communications (Cambridge, England)》2002,(1):64-65
Novel 3D topologies combining diazine and azido bridges between MnII magnetic centres have been obtained and characterised by low-temperature magnetic measurements. 相似文献
102.
O'Donnell MJ Drew MD Cooper JT Delgado F Zhou C 《Journal of the American Chemical Society》2002,124(32):9348-9349
Optically active (S)-alpha-amino acids are prepared in 54-95% ee (12 cases) by reaction of the Schiff base acetate of glycine tert-butyl ester with B-alkyl-9-BBN derivatives in the presence of the Cinchona alkaloid, cinchonidine, and base. The enantiomeric (R)-alpha-amino acids are available in 59-92% ee (3 cases) by using cinchonine as the chiral control element. 相似文献
103.
Adam W García H Diedering M Martí V Olivucci M Palomares E 《Journal of the American Chemical Society》2002,124(41):12192-12199
The photochemical denitrogenation of the cyclopentene-annelated DBH-type azoalkanes 1 has been examined in solution as a function of bridgehead substitution and temperature. For all derivatives, namely, the unsubstituted 1a(H/H), monomethyl 1b(Me/H) dimethyl 1c(Me/Me), monophenyl 1d(Ph/H), and diphenyl 1e(Ph/Ph), the temperature-dependent ratio of syn and anti housanes 2 provides experimental support for a competition between the singlet (high temperature) and triplet (low temperature) reaction channels in the direct photolysis. The syn/anti ratio of the housanes 2 depends on the extent and type of bridgehead substitution; the amount of the anti diastereomer (retention) follows the order Ph > Me > H, and double substitution is more effective than single. This stereochemical memory is interpreted in terms of the mass effect (inertia) of the substituents and steric interaction (size) between the substituents at the bridgehead and the methylene bridge during the deazetation step of the transient diazenyl diradical conformations (1)DZ (exo-ax) and (1)DZ (exo-eq). These conformers are impulsively generated upon decay of the (1)(n,pi)-excited azoalkane, a trajectory assessed through computational work. The new mechanistic feature disclosed by the unprecedented anti stereoselectivity (retention) is the intervention of a puckered 1,3-cyclopentanediyl singlet diradical (1)DR as product bifurcation step, whose conformational relaxation to the planar species (loss of stereochemical memory) is encumbered by bridgehead substitution. 相似文献
104.
105.
García-Jareño JJ Giménez-Romero D Keddam M Vicente F 《The journal of physical chemistry. B》2005,109(10):4593-4598
The graphical analysis of the impedance plots is used in the study of the electrodic systems that take place through two consecutive single electron transfers. The zinc anodic dissolution is studied by using this procedure. The characteristic points easily allow us to explain and to simulate the impedance behavior of this electrodic system according to the steady-state potential and the roughness of the working electrode. The direct procedure for parametrical identification from the graphical analysis allows us to reduce the time needed for an impedance experiment. This graphical analysis is suggested for studying thin coated galvanized steels. 相似文献
106.
Effect of the platinum content on the microstructure and micropore size distribution of Pt/alumina-pillared clays 总被引:1,自引:0,他引:1
Barrera-Vargas M Valencia-Rios J Vicente MA Korili SA Gil A 《The journal of physical chemistry. B》2005,109(49):23461-23465
The aim of this work is to study the effect of the platinum content (0-1.8 wt % Pt) on the microstructure of an alumina-pillared clay. For this purpose, the nitrogen physisorption data at -196 degrees C, the micropore size distributions of the supported platinum catalysts, and the hydrogen chemisorption results at 30 degrees C have been analyzed and compared. The preparation of the catalysts has modified the textural properties of the Al-pillared clay support, giving rise to a loss of surface area and micropore volume. After reduction at 420 degrees C, the presence of dispersed metallic platinum with mean crystallite size in the 22-55 A range has been found by hydrogen adsorption. Comparison of all results reveals that the platinum species block the micropore entrances by steric hindrance to nitrogen access as the platinum content increases. 相似文献
107.
Zúñiga-Pérez J Muñoz-Sanjosé V Palacios-Lidón E Colchero J 《Physical review letters》2005,95(22):226105
The surface electrical properties of ZnO thin films grown along the nonpolar [1120] direction have been investigated by Kelvin probe microscopy on a nanometer scale. Two different charge domains, with a 75 meV work function difference, coexist within the ZnO surface, which is covered by rhombohedral pyramids whose sidewalls are shown to be {1011}-type planes. The presence and relative orientation of the two kinds of charge domains are explained in terms of the atomic arrangement at the {1011} polar surfaces. 相似文献
108.
Xia JS Pan W Vicente CL Adams ED Sullivan NS Stormer HL Tsui DC Pfeiffer LN Baldwin KW West KW 《Physical review letters》2004,93(17):176809
At a very low-temperature of 9 mK, electrons in the second Landau level of an extremely high-mobility two-dimensional electron system exhibit a very complex electronic behavior. With a varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed nu=2+2/5 state separated from the even-denominator nu=2+1/2 state by an insulating phase and a nu=2+2/7 and nu=2+1/5 state surrounded by such phases. A developing plateau at nu=2+3/8 points to the existence of other even-denominator states. 相似文献
109.
De Napoli M Nardis S Paolesse R Vicente MG Lauceri R Purrello R 《Journal of the American Chemical Society》2004,126(19):5934-5935
In aqueous solution meso-tetrakis(4-phosphonatophenyl)porphyrin shows self-aggregation processes controlled by the "sergeant-soldier rule". After partial protonation of the external phosphonic groups, it is possible (i) to further protonate the inner nitrogen atoms of the molecules or (ii) to allow, over time, the system to aggregate. Therefore, the two procedure lead to a different system evolution, producing species with different chemico-physical properties. 相似文献
110.
Anhydrides as acylating agents in the enzymatic resolution of an intermediate of (-)-Paroxetine 总被引:2,自引:0,他引:2
de Gonzalo G Brieva R Sánchez VM Bayod M Gotor V 《The Journal of organic chemistry》2003,68(8):3333-3336
A new chemoenzymatic method for the preparation of an intermediate of (-)-Paroxetine is reported. Cyclic anhydrides are used as acylating agents in the lipase-catalyzed esterification of trans-4-(4'-fluorophenyl)-3-hydroxymethyl-N-phenyloxycarbonylpiperidine in organic solvents. The best enantioselectivities are obtained with two different lipases from Candida antarctica. These two lipases show opposite stereochemical preference in these processes, so that both enantiomers can be obtained in their optically pure forms. The (3S,4R) isomer is an intermediate for the synthesis of (-)-Paroxetine. 相似文献