全文获取类型
收费全文 | 249篇 |
免费 | 9篇 |
专业分类
化学 | 163篇 |
晶体学 | 8篇 |
力学 | 11篇 |
数学 | 36篇 |
物理学 | 40篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 2篇 |
2020年 | 4篇 |
2019年 | 9篇 |
2018年 | 5篇 |
2017年 | 10篇 |
2016年 | 15篇 |
2015年 | 6篇 |
2014年 | 11篇 |
2013年 | 19篇 |
2012年 | 17篇 |
2011年 | 14篇 |
2010年 | 12篇 |
2009年 | 12篇 |
2008年 | 17篇 |
2007年 | 17篇 |
2006年 | 9篇 |
2005年 | 15篇 |
2004年 | 10篇 |
2003年 | 7篇 |
2002年 | 5篇 |
2001年 | 4篇 |
1999年 | 4篇 |
1996年 | 3篇 |
1994年 | 1篇 |
1992年 | 1篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有258条查询结果,搜索用时 15 毫秒
61.
Isolation and concentration of mercury by reduction of mercury(II) with acidic tin(II) chloride solution and absorption of mercury vapour in acidic permanganate solution is combined with potentiometric titration with dithiooxamide. The simple procedure is applicable to 5–100 ppb mercury in 1-dm3 samples. Recoveries from organomercurials are discussed. 相似文献
62.
Miros?aw Florjańczyk Carlos Alonso-Ramos Przemek Bock Alexei Bogdanov Pavel Cheben í?igo Molina-Fernández Siegfried Janz Boris Lamontagne Alejandro Ortega-Mo?ux Alan Scott Kenneth Sinclair Brian Solheim Dan-Xia Xu 《Optical and Quantum Electronics》2012,44(12-13):549-556
We describe the development of a waveguide Fourier-transform spectrometer for space-borne high-resolution sensing. A prototype device is designed to monitor the water vapor absorption band near 1,364?nm with a resolution of 0.05?nm. It has no moving parts and is based on a unique concept of arrayed interferometers implemented in silicon-on-insulator planar waveguide chip. The optical input is formed by many independent waveguides, providing a significantly increased light gathering capability (étendue) compared to single-waveguide input configurations. Enhancements of the spectrometer capabilities are achieved by stacking planar waveguide layers and by using surface gratings to couple light into the waveguides. 相似文献
63.
Nonlinear Dynamics - In this study, the solution to the kinematically optimal control problem of the mobile manipulators is proposed. Both dynamic equations are assumed to be uncertain, and... 相似文献
64.
The substituent effect is usually considered by means of various Hammett-like substituent constants and is most often related to aromatic systems. Unlike this, we present results of our research on the influence of 27 substituents spanning a wide range of electronic properties, from strongly electron-withdrawing to strongly electron-donating, on the electron structure of X-substituted acetylenes and diacetylenes – thus the systems which until now have practically not been subject of any deeper studies. It is shown that the interaction through triple bond(s) is associated with a significant advantage of resonance effects and that the substituent effect transmitted by the C≡C−C≡C unit is about half of that transmitted by the C≡C unit alone. Substituent X mainly affects the closest carbon atom by means of proximity effect, hence changes of charge on this atom do not follow any substituent constants. The effect on further carbon atoms is much smaller. The presence of the C≡C−C≡C unit withdraws more charge from X than a triple bond alone, and hinders communication between X and the terminal H atom. Comparison of substituent effects to those present in X-substituted benzene derivatives shows that the electronic properties of the terminal hydrogen atom in acetylenes and diacetylenes are most similar to the electronic properties of ortho and para hydrogen atoms in X-substituted benzene derivatives. 相似文献
65.
Agnieszka Czylkowska Danuta Czakis-Sulikowska Rafał Kruszyński Mirosława Markiewicz 《Structural chemistry》2010,21(2):415-423
A novel mixed-ligand complexes of Er(III), Yb(III) and Lu(III) with title ligands were prepared and characterized by chemical
and elemental analysis and IR spectroscopy, conductivity (in methanol, dimethyloformamide and dimethylsulphoxide). The thermal
properties of complexes in the solid state were studied. The mode of metal–ligand coordination was discussed. The title compounds
are isomorphic and isostructural in solid state. All atoms in studied compounds lie in general positions but occurrence of
inversion on the midpoint of the bond linking two pyridine rings leads to existence in asymmetric unit one complex molecule
and half of outer coordination sphere 4-bpy molecule. All chelating carboxylate groups are symmetrically bonded to the metal
cations. The molecules of studied compounds are connected to the three dimensional network via O–H···O and O–H···N intermolecular
hydrogen bonds. In the structures also exist C–H···O, C–H···Cl weak hydrogen bonds and π····π stacking interactions. 相似文献
66.
The effect of temperature and concentration on the structure of sec-butyl alcohol and isobutyl alcohol/water binary mixtures in the alcohol-rich region (mole fraction of water X(H2O) < 0.3) has been studied using Fourier transform (FT) near-infrared (NIR) spectroscopy. The experimental data were analyzed by a two-dimensional (2D) correlation approach and chemometric methods. It was found that molecules of both alcohols in the mixture with water are in the same environment as those in the pure alcohols. Even at very low water content (X(H2O) = 0.001) we did not observe water free from any specific interactions. The molecules of water are attached to the end free OH groups in the open chain associates of alcohol. In this way the structure of neat alcohol remains intact by addition of water. The water-alcohol interactions in sec-butyl alcohol and isobutyl alcohol/water mixtures are stronger than those in bulk water. The results obtained at higher water content or elevated temperatures indicate the possibility of water-water interaction. In the alcohol-rich region the hydrophobic effects are of minor importance and the structure and properties of these systems are determined by hydrogen bonding through the hydroxyl groups. Both alcohols behave similarly on the temperature or water content variation; the minor difference results from a different degree of self-association for sec-butyl alcohol and isobutyl alcohol. 相似文献
67.
Summary: Cationic ring-opening polymerization of cyclic ethers (also esters and acetals) substituted with hydroxyl groups leads to branched multihydroxyl polymers. When 4-membered hydroxysubstituted oxetanes or 5-membered hydroxysubstuituted oxolanes (or lactone and 1,3-dioxolane) are used as monomers the polymerization products have limited molecular weights (about 1000). Polymerization of these monomers leading in situ to oligomeric products was used for inorganic surface modification. Successful polymerizations of oxetane as well as considered as difficult to polymerize 5-membered hydroxysubstituted cyclic monomers initiated from montmorillonite clay and silica nanoparticles surfaces were performed. 相似文献
68.
This paper deals with the class of spaces which are countable unions of zero-dimensional sets and with the larger class of
Haver’s C-spaces. All spaces are assumed to be separable and metrizable. We are concerned with various aspects of universality
of these classes when they are combined with the covering analogue for σ-compactness defined by Menger and the rational dimension
introduced by Menger and N?beling. A solution of a problem of S. D. Iliadis [16] concerning universal spaces for rational dimension will result.
Received 11 September 1998; in revised form 12 January 1999 相似文献
69.
Cyrański MK Gilski M Jaskólski M Krygowski TM 《The Journal of organic chemistry》2003,68(22):8607-8613
Studies based on ab initio optimized geometries (at B3LYP/6-311+G** and MP2/6-311+G** levels) and on experimental structures retrieved from the Cambridge Structural Database (CSD) reveal that the nucleobases constituting DNA and RNA differ significantly in their aromatic character, as shown by the geometry-based index of aromaticity HOMA that ranges from 0.466 for thymine to 0.917 for adenine, based on B3LYP/6-311+G** calculations, and 0.495-0.926, respectively, if based on the MP2/6-311+G** level. Aromaticity of the bases decreases markedly with an increase of the number of double-bond C=X (X = N, O) substituents at the rings. H-bonds involving C=O groups in Watson-Crick pairs cause an increase of the aromatic character of the rings. 相似文献
70.
Roman Mazurkiewicz Mirosława Grymel 《Monatshefte für Chemie / Chemical Monthly》2002,133(9):1197-1204
Summary. Reactions of N-acyl-α-triphenylphosphonio-α-amino acid methyl esters with organic bases (triethylamine or DBU) were investigated as the crucial step of the base-catalyzed displacement of the triphenylphosphonium group by nucleophiles.
It was proved that N-acyl-α-triphenylphosphonioglycinates are transformed to an equilibrium mixture of the corresponding N-acyliminoacetates and N-acyl-α-triphenylphosphoranylidene glycinates by bases. In the case of N-acyl-α-triphenylphosphonio-α-amino acid esters with quaternary α-carbon, the α-substituted homologues of the N-acyliminoacetates were detected to be the only primary reaction product which, however, can undergo further tautomerization
to the corresponding α,β-dehydro-α-amino acid derivatives. In both these cases the reaction of N-acyl-α-triphenylphosphonio-α-amino acid esters with nucleophiles proceeds via the addition of a nucleophile to the activated C=N double bond of the N-acylimino intermediate.
Corresponding author. E-mail: romanm@zeus.polsl.gliwice.pl
Received October 31, 2001. Accepted (revised) December 17, 2001 相似文献