全文获取类型
收费全文 | 2990篇 |
免费 | 116篇 |
国内免费 | 21篇 |
专业分类
化学 | 2005篇 |
晶体学 | 20篇 |
力学 | 117篇 |
数学 | 524篇 |
物理学 | 461篇 |
出版年
2023年 | 24篇 |
2022年 | 22篇 |
2021年 | 45篇 |
2020年 | 50篇 |
2019年 | 77篇 |
2018年 | 44篇 |
2017年 | 28篇 |
2016年 | 85篇 |
2015年 | 95篇 |
2014年 | 109篇 |
2013年 | 156篇 |
2012年 | 215篇 |
2011年 | 248篇 |
2010年 | 157篇 |
2009年 | 136篇 |
2008年 | 170篇 |
2007年 | 185篇 |
2006年 | 192篇 |
2005年 | 189篇 |
2004年 | 177篇 |
2003年 | 113篇 |
2002年 | 117篇 |
2001年 | 45篇 |
2000年 | 41篇 |
1999年 | 25篇 |
1998年 | 27篇 |
1997年 | 24篇 |
1996年 | 22篇 |
1995年 | 24篇 |
1994年 | 17篇 |
1993年 | 24篇 |
1992年 | 10篇 |
1991年 | 15篇 |
1990年 | 6篇 |
1989年 | 13篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 11篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1982年 | 17篇 |
1981年 | 12篇 |
1980年 | 21篇 |
1979年 | 10篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 11篇 |
1975年 | 8篇 |
1974年 | 9篇 |
1973年 | 9篇 |
排序方式: 共有3127条查询结果,搜索用时 951 毫秒
91.
Polleux L Labbé E Buriez O Périchon J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(16):4678-4686
The electrochemical behaviour of CoBr(2)bpy (bpy=2,2'-bipyridine) catalyst precursor in acetonitrile has been studied, revealing its possible reduction into the corresponding Co(I) and Co(0) complexes. These low-valent cobalt species appear to be stable on the time scale of cyclic voltammetry. In the presence of aromatic halides, both complexes undergo oxidative addition, the latter Co(0) species allowing the activation of poorly reactive aromatic chlorides. The arylcobalt(III) and arylcobalt(II) obtained are reduced at the same potential as the original Co(II) and Co(I) complexes, respectively, resulting in the observation of overall ECE mechanisms in both cases. The electrochemical study shows that vinylic acetates competitively react with electrogenerated Co(0) species, leading to a labile complex. Preparative scale electrolyses carried out from solutions containing aromatic halides (ArX), vinyl acetate (vinylOAc) and a catalytic amount of CoBr(2)bpy lead to a mixture of biaryl (Ar-Ar) and arene (ArH) as long as the potential is set on the plateau of the Co(II) right arrow over left arrow Co(I) reduction wave. The coupling product (Ar-vinyl) is formed only if the electrolysis is performed on the plateau of the Co(I)/Co(0) reduction wave. A mechanism is proposed for the overall cobalt-catalyzed coupling reaction between aromatic halides and allylic acetates. 相似文献
92.
Raimund Schwarz Günter Gliemann Laurent Chassot Philippe Jolliet Alex von Zelewsky 《Helvetica chimica acta》1989,72(2):224-236
The polarized optical absorption and emission (spectra, decay times) of single crystals of [Pd(thpy)2] and [Pt(thpy)2] (thpy ≡ C(3′)-deprotonated form of 2-(2-thienyl)pyridine) at temperatures 1.9 K ? T ? 80 K are reported. The emission of [Pt(thpy)2] can be influenced strongly by applied magnetic fields (0 ? H ? 6 T). Depending on the central ions Pd and Pt, the lowest excited electronic states of the single complexes are ligand-centered (LC) states and metal-to-ligand charge transfer (MLCT) states, respectively. This difference leads to distinctly dissimilar properties of the emission of both compounds. The experimental data show that the emission of single crystals of [Pd(thpy)2] and [Pt(thpy)2] at T ? 30 K originates from several types of traps (defect states of symmetry 3B2?stabilized below the exciton band) with LC and MLCT character, respectively. In the Pt compound, the 3B2 is split by spin-orbit coupling into three states. The states B and A, which determine the emission properties, are separated by Δv ~ 13 cm?1. Both states can mix under the influence of an applied magnetic field yielding an increase of the emission intensity by a factor of ~ 1.5 at H = 6 T. 相似文献
93.
Erik Ayala-Bribiesca Monica Araya-Farias Grald Pourcelly Laurent Bazinet 《Journal of membrane science》2006,280(1-2):790-801
The aim of this work was to study the effect of the concentrate solution pH and the composition in calcium, carbonate and protein of the diluate solution to be treated by conventional electrodialysis on the fouling of ion-exchange membranes. Conductivity, system resistance, pH of the diluate and cation migration were monitored to follow the evolution of the demineralization. Total cation migration was similar for all conditions although different forms of fouling were identified after three consecutive 100 min electrodialysis treatments. The nature of fouling and the membrane surface fouled depended on the concentrate pH value, the diluate mineral composition and the intrinsic composition of the whey isolate. Once conditions leading to membrane fouling were identified, an alternative configuration for our electrodialysis stack is proposed to prevent fouling onset. 相似文献
94.
Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale. 相似文献
95.
We study facets of the cut coneC
n
, i.e., the cone of dimension 1/2n(n – 1) generated by the cuts of the complete graph onn vertices. Actually, the study of the facets of the cut cone is equivalent in some sense to the study of the facets of the cut polytope. We present several operations on facets and, in particular, a lifting procedure for constructing facets ofC
n+1 from given facets of the lower dimensional coneC
n
. After reviewing hypermetric valid inequalities, we describe the new class of cycle inequalities and prove the facet property for several subclasses. The new class of parachute facets is developed and other known facets and valid inequalities are presented. 相似文献
96.
97.
In solutions of trifluorotoluene or toluene containing 2,2,2-trifluoroethanol, the beta-phosphate radical heterolyzed to give a detectable ion pair, identified as a solvent-separated species. Rate constants for the radical fragmentation reaction forming the ion pair, for ion pair collapse, and for diffusive escape to free ions were measured. The kinetics and entropy of activation for fragmentation indicate that the rearrangement reaction occurs by a heterolysis pathway in all solvents. [reaction: see text] 相似文献
98.
[reaction: see text] N-Monosubstituted carbamates are cleanly converted to amides under treatment with trialkylaluminum. This reaction involves an aluminum-assisted internal delivery of alkyl groups. It can be applied to new and mild protecting group strategies for alcohols. 相似文献
99.
Laurent De Windt Detlef W. M. Hofman Lorenzo Pisani Enrico Clementi 《International journal of quantum chemistry》1995,53(2):131-147
The Coulomb–Hole–Hartree–Fock method introduced by E. Clementi in the early 1960s and reparametrized more recently by S. Chakraworty and E. Clementi to compute the correlated electronic energy in atomic systems, is here extended to compute molecules. The new parametrization is obtained empirically by fitting first and second atomic ionization potentials from He to Ca and a few diatomic molecules. The present formulation makes use of either one or more determinants in order to ensure proper dissociation products, following the early proposal of G.C. Lie and E. Clementi in the context of density functional computations for molecular systems. The new formulation is tested against the dissociation energies of a large number of molecules and it is found satisfactory. © 1995 John Wiley & Sons, Inc. 相似文献
100.
Alain Valla Benoist Valla Régis Le Guillou Dominique Cartier Laurent Dufossé Roger Labia 《Helvetica chimica acta》2007,90(3):512-520
Since the C15 β‐end‐group aldehyde 10 ((β‐ionylidene)acetaldehyde), an excellent intermediate in the syntheses of retinoids, can be synthesized in many ways from β‐ionone, and since the corresponding acyclic C15 ψ‐end‐group aldehyde 5 can easily be synthesized from citral ( 1 ) (Scheme 3), we applied the C15+C5 route to the syntheses of γ‐retinal ((all‐E)‐ 8 ) (Scheme 3) and retinal ((all‐E)‐ 13 ) (Scheme 4), and therefore, by coupling (2×C20→C40), to the preparation of lycopene ( 14 ) and β‐carotene ( 15 ) (Scheme 5). Our new syntheses of retinal ((all‐E)‐ 13 ) and γ‐retinal ((all‐E)‐ 8 use an extended aldol reaction with a C6 building block that incorporates a C5 unit after decarboxylation. 相似文献