全文获取类型
收费全文 | 6487篇 |
免费 | 209篇 |
国内免费 | 34篇 |
专业分类
化学 | 4160篇 |
晶体学 | 26篇 |
力学 | 264篇 |
数学 | 1113篇 |
物理学 | 1167篇 |
出版年
2023年 | 31篇 |
2022年 | 74篇 |
2021年 | 94篇 |
2020年 | 90篇 |
2019年 | 110篇 |
2018年 | 98篇 |
2017年 | 91篇 |
2016年 | 194篇 |
2015年 | 180篇 |
2014年 | 205篇 |
2013年 | 373篇 |
2012年 | 450篇 |
2011年 | 515篇 |
2010年 | 295篇 |
2009年 | 299篇 |
2008年 | 397篇 |
2007年 | 384篇 |
2006年 | 392篇 |
2005年 | 406篇 |
2004年 | 303篇 |
2003年 | 265篇 |
2002年 | 256篇 |
2001年 | 108篇 |
2000年 | 100篇 |
1999年 | 64篇 |
1998年 | 83篇 |
1997年 | 85篇 |
1996年 | 88篇 |
1995年 | 60篇 |
1994年 | 70篇 |
1993年 | 57篇 |
1992年 | 44篇 |
1991年 | 31篇 |
1990年 | 30篇 |
1989年 | 20篇 |
1988年 | 27篇 |
1987年 | 17篇 |
1985年 | 32篇 |
1984年 | 30篇 |
1983年 | 19篇 |
1982年 | 21篇 |
1981年 | 21篇 |
1980年 | 28篇 |
1979年 | 33篇 |
1978年 | 23篇 |
1977年 | 20篇 |
1976年 | 20篇 |
1975年 | 16篇 |
1974年 | 16篇 |
1973年 | 12篇 |
排序方式: 共有6730条查询结果,搜索用时 484 毫秒
91.
92.
Philippe Gerbier Daniel Ruiz-Molina Neus Domingo David B. Amabilino José Vidal-Gancedo Javier Tejada David N. Hendrickson Jaume Veciana 《Monatshefte für Chemie / Chemical Monthly》2003,134(2):265-276
Summary. A new modified approach for the synthesis of Mn12 clusters, based on the use of complex [Mn12O12(O2C
t
Bu)16(H2O)4] (2) as starting material to promote the acidic ligand replacement, is presented here. This new synthetic approach allowed us
to obtain complex [Mn12O12(O2CC6H4N(O•)
t
Bu)16(H2O)4] (3), whose preparation remained elusive by direct replacement of the acetate groups of Mn12Ac (1). Complex 3 bearing open-shell radical units, was prepared to increase the total spin number of its ground state, and consequently, to
increase T
B
, with the expectation that the radical ligands may couple ferromagnetically with the Mn12 core. Unfortunately, magnetic measurements of complex 3 revealed that the sixteen radical carboxylate ligands interact antiferromagnetically with the Mn12 core to yield a S = 2 magnetic ground state.
Corresponding author. E-mail: vecianaj@icmab.es
Received March 27, 2002; accepted May 2, 2002 相似文献
93.
This paper presents an in-depth look at the use of capillary electrophoretic (CE) techniques for the fingerprinting and characterization of humic substances and natural organic matter. These materials are highly heterogeneous in structure and show all characteristics of mixtures unliked in analytical chemistry. The electrophoretic approach, however, allows the determination of mobility distributions in different solution conditions, representative of the effective charge and size distribution status of the components present. A tabulated review covers over 50 references on the subject and highlights the possibilities and problems encountered in the analysis of such polydisperse materials with CE methods. In a second part of the article the consequences of experimental and buffer parameters on the behavior of humic materials in CE are presented. 相似文献
94.
Philippe Hennig Eric Raimbaud Christophe Thurieau Jean-Paul Volland André Michel Jean-Luc Fauchère 《Journal of computer-aided molecular design》1996,10(1):83-86
Summary The conformation in dimethylsulfoxide of the somatostatin derivative angiopeptin and of three disulfide-free analogs was estimated by two-dimensional nuclear magnetic resonance spectroscopy at room temperature. The resulting 3D molecular graphics were compared and shown to reflect the observed differences in the inhibition of restenosis after rat aorta balloon injury by these octapeptide inhibitors. Angiopeptin and its active analog 2 displayed a relatively rigid conformation of the cyclic hexapeptide backbone due to the presence of two well-defined hydrogen bonds, further stabilized by a third hydrogen bond outside the ring. No such constraints were detected for the two biologically inactive analogs, which, compared to 2, had a two-atom longer or shorter hexapeptide ring. The well-defined structure of compound 2 may serve as an improved pharmacophore for this new class of drugs. 相似文献
95.
Grotjahn DB Van S Combs D Lev DA Schneider C Incarvito CD Lam KC Rossi G Rheingold AL Rideout M Meyer C Hernandez G Mejorado L 《Inorganic chemistry》2003,42(10):3347-3355
Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions. 相似文献
96.
The structural parameters of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), (CH2NNO2)3, 1,3-dinitro-1,3-diazacyclopentane (DDCP), CH2(CH2NNO2)2, andN-nitropyrrolidine (NP), (CH2)4NNO2, have been determined by electron diffraction.The six-membered ring of RDX has a chair form with axial positions of the nitro groups and close to planar bond geometry of the amine nitrogen atoms. The overallC
3 symmetry of the molecule is in agreement with the experimental data.The conformation of the five-membered ring in DDCP is a half-chair ofC
2 symmetry, while that in NP is an envelope ofC
S symmetry. The nitro groups are in equatorial positions in both molecules. The conformations of pyrrolidine and imidazolidine cycles show interesting features.The pyramidal geometry of the amine nitrogen atom bonds flattens in going from pyrrolidine andN-chloropyrrolidine to NP and DDCP and then to RDX and to dimethylnitramine (DMNA), (CH3)2NNO2. 相似文献
97.
Philippe Choné 《Potential Analysis》1995,4(3):269-296
Let be an open set inR
2 andI be a conformally invariant functional defined onH
1(,R
d
). Letu be a critical point ofI. We show that, ifu is apriori assumed to be bounded, thenu is smooth in , up to (ifu
|
is smooth). This is a partial (positive) answer to a conjecture of S. Hildebrandt [13]. As an application, we establish a regularity result for weak solutions to the equation of surfaces of prescribed mean curvature in a three-dimensional compact riemannian manifold. 相似文献
98.
Finite strains at the tip of a crack in a sheet of hyperelastic material: I. Homogeneous case 总被引:1,自引:0,他引:1
This paper describes an asymptotic analysis of the strain and stress fields at the tip of a crack in a sheet of incompressible hyperelastic material. The investigations are carried out within the framework of finite elastostatics and for the class of Generalized Neo-Hookean materials. Both the symmetric (mode I) and non-symmetric (mixed-mode) cases are considered. It is shown that the latter situation corresponds locally to a rigid body rotation of the symmetric fields. The effect of the hardening parameter on crack tip blunting is investigated analytically and numerically. 相似文献
99.
Information
Congresses, conferences, symposia, meetings, and seminars in the field of chemical sciences held in 1995–1996 相似文献100.