全文获取类型
收费全文 | 1077篇 |
免费 | 33篇 |
国内免费 | 3篇 |
专业分类
化学 | 709篇 |
晶体学 | 20篇 |
力学 | 5篇 |
数学 | 259篇 |
物理学 | 120篇 |
出版年
2022年 | 7篇 |
2021年 | 18篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 16篇 |
2017年 | 21篇 |
2016年 | 38篇 |
2015年 | 49篇 |
2014年 | 29篇 |
2013年 | 69篇 |
2012年 | 58篇 |
2011年 | 53篇 |
2010年 | 42篇 |
2009年 | 39篇 |
2008年 | 47篇 |
2007年 | 52篇 |
2006年 | 39篇 |
2005年 | 35篇 |
2004年 | 23篇 |
2003年 | 17篇 |
2002年 | 23篇 |
2001年 | 10篇 |
2000年 | 18篇 |
1999年 | 15篇 |
1997年 | 15篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 15篇 |
1992年 | 11篇 |
1991年 | 12篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 10篇 |
1984年 | 8篇 |
1983年 | 14篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 13篇 |
1975年 | 16篇 |
1974年 | 11篇 |
1966年 | 9篇 |
1958年 | 23篇 |
1957年 | 10篇 |
1956年 | 6篇 |
排序方式: 共有1113条查询结果,搜索用时 15 毫秒
971.
Gordana Ćirić‐Marjanović Budimir Marjanović Patrycja Bober Zuzana Rozlívková Jaroslav Stejskal Miroslava Trchová Jan Prokeš 《Journal of polymer science. Part A, Polymer chemistry》2011,49(15):3387-3403
The oxidative polymerization of p‐phenylenediamine with silver nitrate by using various oxidant/monomer mole ratios in aqueous solutions of both acetic and nitric acid was studied experimentally and computationally. The produced micro/nanostructured conducting poly(p‐phenylenediamine)–silver composites, reaching the conductivities >104 S/cm, were characterized by conductivity and density measurements, gel permeation chromatography, transmission electron microscopy, UV–visible, FTIR, and Raman spectroscopies. The highest conductivity was 31,700 S/cm for poly(p‐phenylenediamine) base–silver (81.4 wt % Ag). The unexpected increase of conductivity after deprotonation of polymer component is discussed on the basis of interfacial electrical barriers and their removal. Theoretical study of the mechanism of p‐phenylenediamine oxidation has been based on the AM1 semi‐empirical quantum chemical computations of the heat of formation of the reaction intermediates, taking into account the influence of pH and solvation effects. Quantum chemical predictions of molecular structure of poly(p‐phenylenediamine) were correlated with spectroscopic findings. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 相似文献
972.
Jaroslav Kví?ala Michal Beneš Old?ich Paleta Vladimír Král 《Journal of fluorine chemistry》2010,131(12):1327-1337
2,3,4,5,6-Pentafluorobiphenyl (PFBi) was modified by the nucleophilic substitution of one fluorine using a series of O-, S- and N-nucleophiles, viz. alkaline salts of 2,2,2-trifluoro-ethanol, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctanol, 1,2;3,4-di-O-isopropylidenexylitol, allylsulfane, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctane-1-thiol, 3-aminopropan-1-ol (7), and tert-butyl N-(3-aminopropyl)carbamate (8). All the substitutions took place exclusively at the position para to the phenyl group. (3-Amino-propyl)amino derivative of PFBi (15) was further modified at the terminal amino group by acylation or fluoroalkylation. The reaction of 8 was applied to meso-5,10,15,20-tetrakis-(pentafluorophenyl)porphyrin (20) to afford tris- (21) and tetrakis-substituted (22) products with complete para-regioselectivity. Theoretical studies of the reaction pathways of PFBi with ammonia, microsolvated lithium fluoride or lithium hydroxide revealed that no Meisenheimer-type intermediates are formed in the course of the simulated reactions: instead, tetrahedral SN2 mechanism was found. Significant regioselectivity of the nucleophilic aromatic substitution, leading to 4-substituted products, was predicted based on relative transition state energies in agreement with the observed experimental results. 相似文献
973.
Libor Dostál Roman Jambor Zdeňka Padělková Jaroslav Hole?ek 《Journal of organometallic chemistry》2010,695(3):392-8934
The reactions of organoantimony chloride LSbCl2 (1) (L = [2,6-(Me2NCH2)2C6H3]−) with the silver salts of selected carboxylic acids (1:2 molar ratio) resulted to the corresponding organoantimony carboxylates LSbX2, where X = CH3COO for (2); CF3COO for (3). Similar reactions of 1 with the silver salt of the low nucleophilic anion (1:0.5 and 1:1 molar ratio) gave the ionic compounds [LSb(Cl)--Cl-Sb(Cl)L]+[CB11H12]− (4), and [LSbCl]+[CB11H12]− (5). All compounds were characterized by the help of the elemental analysis, ESI-MS, 1H, 11B, 13C NMR spectroscopy and IR spectroscopy. The solid state structure investigation using single crystal X-ray diffraction technique (3-5) revealed the presence of the strong Sb-N intramolecular dative connections in all cases and also significant differences in the shapes of the coordination polyhedra around the central antimony atoms was observed, i.e. a tetragonal pyramidal environment in 3 (CF3COO groups are placed mutually in trans positions), an unusual chlorine bridged dinuclear cation consisting of one apex (Cl atom) sharing square pyramids in 4, and finally a vacant ψ-trigonal bipyramid around the central antimony atom in 5. Even more, crystallization of 5 from THF provided the single crystals of a THF aduct of 5 [LSbCl(THF)]+[CB11H12]−5a, where the central antimony atom is located in a tetragonal pyramidal environment. The solid state structures of 3-5 are retained in solution. Solution structure of the compound 2 was determined by the help of VT-1H NMR spectroscopy and IR spectroscopy showing, that both carboxylates (CH3COO) are unidentate and are placed mutually in cis positions in the coordination polyhedron around the central antimony atom. The whole coordination polyhedron in 2 might be best described as a biccaped - trigonal pyramid, due to the additional Sb-N intramolecular interactions. 相似文献
974.
Richard Paout Jitka Houskov Michal Duek Jaroslav Maixner Jana Cibulkov Petr Ka
er 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(10):m273-m275
The title compound, cis‐diacetonitrile[(1R,2R)‐1,2‐diaminocyclohexane‐κ2N,N′]platinum(II) dinitrate monohydrate, [Pt(C2H3N)2(C6H14N2)](NO3)2·H2O, is a molecular salt of the diaminocyclohexane–Pt complex cation. There are two formula units in the asymmetric unit. Apart from the two charge‐balancing nitrate anions, one neutral molecule of water is present. The components interact via N—H...O and O—H...O hydrogen bonds, resulting in supramolecular chains. The title compound crystallizes only from acetonitrile with residual water, with the acetonitrile coordinating to the molecule of cis‐[Pt(NO3)2(DACH)] (DACH is 1,2‐diaminocyclohexane) and the water forming a monohydrate. 相似文献
975.
Detection and quantitative characterization of the internal cavities in proteins remain an important topic in studying protein structure and function. Here we propose a new analytical method for detecting the existence of cavities in proteins. The method is based on constructing the special enveloping triangulation enclosing the cavities. Based on this method, we develop an algorithm and a fortran package, CAVE, for computing volumes and surface areas of cavities in proteins. We first test our method and algorithm in some artificial systems of spheres and find that the calculated results are consistent with exact results. Then we apply the package to compute volumes and surface areas of cavities for some protein structures in the Protein Data Bank. We compare our calculated results with those obtained by some other methods and find that our approach is reliable. 相似文献
976.
Miroslav Sulc Katerina Peslova Martin Zabka Marian Hajduch Vladimir Havlicek 《International journal of mass spectrometry》2009,280(1-3):162
We applied both matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometric and 1D sodium dodecylsulfate polyacrylamide gel electrophoretic (1D-PAGE) approaches for direct analysis of intact fungal spores of twenty four Aspergillus species. In parallel, we optimized various protocols for protein extraction from Aspergillus spores using acidic conditions, step organic gradient and variable sonication treatment. The MALDI-TOF mass spectra obtained from optimally prepared samples provided a reproducible fingerprint demonstrating the capability of the MALDI-TOF approach to type and characterize different fungal strains within the Aspergillus genus. Mass spectra of intact fungal spores provided signals mostly below 20 kDa. The minimum material amount represented 0.3 μg (10,000 spores). Proteins with higher molecular weight were detected by 1D-PAGE. Eleven proteins were identified from three selected strains in the range 5–25 kDa by the proteomic approach. Hemolysin and hydrophobin have the highest relevance in host–pathogen interactions. 相似文献
977.
Let ${{\mathcal D}}$ be the ordered set of isomorphism types of finite distributive lattices, where the ordering is by embeddability. We study first-order definability in this ordered set. We prove among other things that for every finite distributive lattice D, the set {d, d opp} is definable, where d and d opp are the isomorphism types of D and its opposite (D turned upside down). We prove that the only non-identity automorphism of ${{\mathcal D}}$ is the opposite map. Then we apply these results to investigate definability in the closely related lattice of universal classes of distributive lattices. We prove that this lattice has only one non-identity automorphism, the opposite map; that the set of finitely generated and also the set of finitely axiomatizable universal classes are definable subsets of the lattice; and that for each element K of the two subsets, {K, K opp} is a definable subset of the lattice. 相似文献
978.
The paper presents the analysis, approximation and numerical realization of 3D contact problems for an elastic body unilaterally
supported by a rigid half space taking into account friction on the common surface. Friction obeys the simplest Tresca model
(a slip bound is given a priori) but with a coefficient of friction $
\mathcal{F}
$
\mathcal{F}
which depends on a solution. It is shown that a solution exists for a large class of $
\mathcal{F}
$
\mathcal{F}
and is unique provided that $
\mathcal{F}
$
\mathcal{F}
is Lipschitz continuous with a sufficiently small modulus of the Lipschitz continuity. The problem is discretized by finite
elements, and convergence of discrete solutions is established. Finally, methods for numerical realization are described and
several model examples illustrate the efficiency of the proposed approach. 相似文献
979.
Using dissipative particle dynamics, we investigate the behavior of a binary mixture, exhibiting demixing in a bulk phase, confined in slit-like pores with walls modified by the stripes of tethered brush of chains. Our main interest is to determine possible morphologies that can be formed inside the pore, depending on the geometrical parameters characterizing the system (the size of the pore and the width of the stripes). In order to describe the observed morphologies we calculate several characteristics, as the density and local temperature profiles, the radii of gyration for the attached polymers, and the minimum polymer-polymer distances in the direction parallel and perpendicular to the pore walls. The summary of our findings is presented as a sketch of the diagram of morphologies. 相似文献
980.
Kubinec R Blaško J Górová R Addová G Ostrovský I Amann A Soják L 《Journal of chromatography. A》2011,1218(13):1767-1774
Isomer mixtures of monomethyl branched saturated C7-C23 fatty acid methyl esters (FAME) were prepared by performing a methylene insertion reaction to the straight chain FAME and this study model was completed by using commercially available standards of C4-C7 FAME. The equivalent chain lengths (ECL) of all 220 C4-C23 monomethyl branched FAME on OV-1 stationary phase were measured, achieving an average repeatability of ±0.0004 ECL units. The monomethyl branched FAME was identified by GC on the basis of regularity of the fractional chain lengths (FCL) dependence on the number of carbon atoms (C(z)) of individual homologous series of methyl 2-, 3-, …, 21-FAME. The prediction of retention of the first homologues, having the new position of methyl group beginning at higher carbon atoms number, and analogously for the second, third, fourth, and other members of the homologous series, allowed the dependence FCL=f(C(z)) for the first and subsequent members of beginning homologous of monomethyl derivatives of FAME. The identification was confirmed by mass spectrometry. All of the methyl isomers of FAME, which could not be completely separated by gas chromatography due to having a methyl group in surroundings of the middle of the carbon chain, were resolved by mass spectrometry using deconvolution in a SIM-mode. Measured gas chromatographic and mass spectrometric data were applied for identification of the monomethyl branched saturated FAME in tongue coating. 相似文献