全文获取类型
收费全文 | 333篇 |
免费 | 5篇 |
国内免费 | 30篇 |
专业分类
化学 | 343篇 |
力学 | 1篇 |
物理学 | 24篇 |
出版年
2023年 | 4篇 |
2022年 | 1篇 |
2021年 | 2篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 4篇 |
2015年 | 4篇 |
2014年 | 10篇 |
2013年 | 48篇 |
2012年 | 22篇 |
2011年 | 11篇 |
2010年 | 11篇 |
2009年 | 27篇 |
2008年 | 30篇 |
2007年 | 37篇 |
2006年 | 12篇 |
2005年 | 18篇 |
2004年 | 16篇 |
2003年 | 6篇 |
2002年 | 10篇 |
2001年 | 4篇 |
2000年 | 6篇 |
1999年 | 8篇 |
1998年 | 3篇 |
1997年 | 8篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 1篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1981年 | 2篇 |
排序方式: 共有368条查询结果,搜索用时 15 毫秒
91.
It has been demonstrated that calcium montanate (a mixture of aliphatic hydrocarbons, fatty acid esters of C28–C32 acids, the calcium salts of these fatty acids, and the acids themselves) can be selectively extracted with supercritical fluids. By use of carbon dioxide, carbon dioxide containing 10% methanol, and carbon dioxide containing 10% methanol and citric acid (as a further additive), the various compound classes could be isolated with high selectivity. The addition of citric acid results in the complexation of Ca2+ and the simultaneous formation of the free fatty acids, which are soluble in the extractant. 相似文献
92.
Selective determination of airborne hexavalent chromium using inductively coupled plasma mass spectrometry 总被引:1,自引:0,他引:1
A new method for determining ultra-trace levels of hexavalent chromium in ambient air has been developed. The method involves a 24-h sampling of air into potassium hydroxide solution, followed by silica gel column separation of chromium (VI), then preconcentration by complexation and solvent extraction. The chromium (VI) complex was dissolved in nitric acid. The resultant chromium ions were determined by inductively coupled plasma mass spectrometry (ICP–MS) using a dynamic reaction cell (DRC) with ammonia as the reactive gas to reduce polyatomic interferences. The interconversion of chromium in potassium hydroxide solution and air sample matrix were investigated under ambient conditions. It was found that there was no conversion of chromium (VI) into chromium (III) species. However, it was observed that some chromium (III) species were converted into chromium (VI) species. For a KOH solution containing 100 μg l−1 of chromium (III) species, the rate of conversion was found to be 3% after 24 h exposure, 8% after 48 h, 10% after 72 h and no further conversion was observed thereafter. However, in a solution containing air sample matrix, 9.3% of chromium (III) converted to chromium (VI) within 6 h, and during the course of a 11-day exposure period, 13% (range 8–17%) of chromium (III) converted to chromium (VI). The method detection limit (MDL) for chromium (VI) in potassium hydroxide solution (0.025 M) was found to be 2×10−2 μg l−1. This is equivalent to 0.2 ng m−3 (for 23 m3 air sampled into 200 ml of KOH solution over a 24-h period). The recovery of spiked chromium (VI) from solutions containing air sample matrix was 95±9% (n=8). Matrix related interferences were estimated to be less than 10% based on recovery studies. The concentration of airborne chromium (VI) in Sydney residential areas was found to be less than 0.2 ng m−3, however, in industrial areas the concentrations ranged from 0.2 to 1.3 ng m−3 using this analytical procedure. 相似文献
93.
A new class of Salpen analogues based on phosphorus derivatives where the classical alkylene backbone has been replaced by a N-P-N linkage is described. Such linkage both affords a very good stability in water and an additional (fifth) potentially complexing site. The classical ortho-OH groups have been also replaced by various ortho-substituents, including diphenylphosphino groups. The synthesis of these compounds is easy and their structure can be varied at will at several levels. Several ways of synthesis can be used to combine the various fragments constituting these Salpen analogues. The structure of one of these fragments, an azide, was determined by X-ray crystallography. A preliminary study of the complexation ability of some of these new ligands was carried out with groups 10 (Ni) and 11 (Au) elements. Depending on the type of substituents and the type of metals used, these compounds can act as mono-, or tetra-dentate ligands. 相似文献
94.
The speciation of 1 mM uranium(VI) in carbonate-free aqueous solutions of 50 mM protocatechuic acid (PCA, 3,4-dihydroxybenzoic acid) was studied in the pH range of 4.0 to 6.8 using EXAFS spectroscopy. The uranium LIII-edge EXAFS spectra were analyzed using a newly developed computer algorithm for iterative transformation factor analysis (FA). Two structural different uranium(VI) complexes were observed. The speciation in the pH range of 4.0 to 4.8 is dominated by a 1:2 or 1:3 uranium(VI)/PCA complex with bidentate coordination of the carboxyl group to the uranium(VI) moiety. Already at pH 4.6 significant amounts of a second species are formed. This uranium(VI) species contains two PCA ligands that are bound to the uranium via their neighboring phenolic hydroxyl groups under formation of five-member rings. 相似文献
95.
F. Pelascini Manuel Wesolek F. Peruch A. De Cian N. Kyritsakas P.J. Lutz J. Kress 《Polyhedron》2004,23(18):3193-3199
Dicationic iron complexes were obtained upon complexation of the ligands 6,6″-di(p-tolyl)-2,2′:6′,2″-terpyridine (L1) or 2,6-bis-(3-mesitylpyrazol-1-yl)pyridine (L2) with iron dichloride or iron trichloride. They were characterized by X-ray diffraction and FT-IR spectroscopy. Single crystal structure determinations of , and all show six-coordinate metal center. These complexes were obtained from L1FeCl2 and L2FeCl2 during recrystallization attempts. (L1)2Fe2+ was shown to be a high-spin complex, whereas (L2)2Fe2+ was shown to be low-spin. For , two independent dications of very similar geometry but with distinctive distortion were observed by X-ray analysis. 相似文献
96.
Kamal A. Idriss Magda S. Saleh Hassan Sedaira M. M. Seleim Elham Y. Hashem 《Monatshefte für Chemie / Chemical Monthly》1991,122(6-7):507-520
Summary The solution equilibria of 2-hydroxynicotinic acid (hyna) complexes with mercury(II) have been studied spectrophotometrically in 50% (v/v) ethanol at 20°C and an ionic strength of 0.1mol dm–3 (NaClO4). Three mercuric complexes are formed in solution in dependence on the acidity of the medium. The basic characteristics of the different complexes are determined and the analytical aspects of the complexation reaction are demonstrated. A critical investigation has also been presented of the solution equilibria and stability of the mixed complex of mercury(II) withhyna and thiosalicylic acid (tsa). The various complex transitions leading to the formation of the 1 : 1 : 1 Hg(tsa)(hyna) ternary complex in solution are investigated. The non-charged mono-ligand complex Hg(hyna) is used for UV-spectrophotometric determination of mercury atpH 4.5–5 (max=325nm, =0.8·104lmol–1cm–1). The system obeyed Beer's law up to 36.1 µg ml–1 of Hg(II). The optimum concentration range (Ringbom) is between 6 and 28.5µg ml–1. Interference caused by a number of ions was masked by the addition of fluoride ions.
Lösungsgleichgewichte und Stabilitätskonstanten von Komplexen der Pyridincarbonsäuren: Die Komplexierungsreaktion von Quecksilber(II) mit 2-Hydroxynikotinsäure
Zusammenfassung Die Lösungsgleichgewichte von 2-Hydroxynikotinsäure (hyna) mit Hg(II) wurde spektrophotometrisch in 50% (v/v) Ethanol bei 20°C und einer Ionenstärke von 0.1 mol dm–3 (NaClO4) untersucht. In Abhängigkeit von der Acidität des Mediums werden drei Quecksilberkomplexe gebildet. Die grundlegenden Charakteristika der Komplexe wurden bestimmt und die analytischen Aspekte aufgezeigt. Die gemischten Komplexe von Hg(II) mithyna und Thiosalicylsäure (tsa), insbesondere die verschiedenen Komplexübergänge zum ternären 1 : 1 : 1 Hg(tsa)(hyna)-Komplex, wurden ebenfalls untersucht. Der ungeladene Monoligandenkomplex Hg(hyna) kann beipH 4.5–5 zur UV-spektroskopischen Quecksilberbestimmung eingesetzt werden (max=325nm, =0.8·104lmol–1cm–1). Das System gehorcht bis zu einer Hg(II)-Konzentration von 36.1µgml–1 dem Beerschen Gesetz. Der optimale Konzentrationsbereich (Ringbom) liegt zwischen 6 und 28.5µgml–1. Interferenzen mit einer Reihe anderer Ionen konnten durch Maskierung mit Fluoridionen umgangen werden.相似文献
97.
The complexation reactions of four water-soluble hydrazones, 2-quinolyl-3-sulphophenylmethanone 5-nitro-2-pyridylhydrazone, 3-sulphophenyl-2-thiazolylmethanone 5-nitro-2-pyridylhydrazone (STNPH), 2-benzothiazolyl-3-sulphophenylmethanone 5-nitro-2-pyridylhydrazone and 2-benzimidazolyl-3-sulphophenylmethanone 5-nitro-2-pyridylhydrazone (BISNPH), with cobalt(II) were studied spectrophotometrically. These hydrazones react with cobalt(II) to form stable 1:2 (metal:ligand) complexes, except for STNPH, which forms a 1:1 complex, with high molar absorptivities. A sensitive and selective spectrophotometric method for the determination of cobalt with BISNPH has been developed. The cobalt(II)-BISNPH complex is formed quantitatively in the pH range 2.7–9.4 and oxidized rapidly to give the corresponding cobalt(III) complex with an absorption maximum at 517 nm. Beer's law is obeyed over the range 0.02–1.0 μg ml?1 and the apparent molar absorptivity of the cobalt(III) complex is 6.65 × 104 l mol?1 cm?1 at 517 nm. The method was applied to the determination of cobalt in iron and steel samples with satisfactory results. The sensitivity is increased 11-fold by use of second-derivative spectrophotometry. 相似文献
98.
C. J. Morin M. Carli N. Mofaddel R. Al Rifaï P. A. Jaffrès D. Villemin P. L. Desbène 《Chromatographia》2005,62(3-4):139-143
Benzyl phosphonic acid, meta and para xylene diphosphonic acid and mesytilene triphosphonic acid were previously synthesized. Now we wanted to evaluate the ability of these compounds to complex selectively potassium and ammonium cations. We based our work on two different strategies. First we used Capillary Electrophoresis for calculating the binding constants. They enabled us to calculate the preferential binding energy, in other words the thermodynamic selectivity of complexation. Secondly, we used molecular mechanics to evaluate the difference of stability of the complexes. The results obtained in Capillary Electrophoresis appeared consistent with those obtained in molecular mechanics and, consequently, we demonstrated that mesytilene triphosphonic acid leads to the best selectivity of complexation towards ammonium and potassium cations. This complexation selectivity was shown, as an example, through the separation of four monovalent cations obtained in capillary zone electrophoresis. The evolution of complexing properties was discussed by referring to the structure of the studied phosphonic acids. 相似文献
99.
Alexander?G.?GrechinEmail author Hans-Jürgen?Buschmann Eckhard?Schollmeyer 《Journal of solution chemistry》2005,34(10):1095-1107
A new method is suggested for estimating the electrostatic and covalent contributions to the standard Gibbs energy, enthalpy
and entropy of complexation reactions upon transfer from water (W) to non-aqueous and mixed aqueous organic solvents (S).
The equations derived for calculation of the electrostatic (temperature dependent) contributions are based on the temperature
dependences of thermodynamic parameters of complexation in aqueous solution and the temperature dependence of the dielectric
constant of water. These contributions correspond to the transfer process in water from T1 = 298.15 K to a higher temperature (T2) at which water has the same dielectric constant as does the solvent S at 298.15 K (εW(T2) = εS(T1)). The covalent (temperature independent) contributions are calculated at isodielectric conditions (transfer from water at
T2 to S at T1) using the corresponding thermodynamic cycle. Application of the model to an analysis of solvent effects is demonstrated
and discussed with an example given of a typical complexation reaction between K+ and 18-crown-6 in pure non-aqueous and water-acetonitrile mixed solvents. 相似文献
100.
We have developed a new method for the three-dimensional modeling of extended X-ray absorption fine structure (EXAFS) spectra
which enables the extraction of the local structure of aqueous metal complexes from spectral mixtures of several components.
The new method combines two techniques: Monte Carlo simulation and target transformation factor analysis (TFA). Monte Carlo
simulation is used to create random arrangements between the X-ray absorbing metal ion and the ligand atoms, and to calculate
the theoretical EXAFS spectrum of each arrangement. The theoretical EXAFS spectrum is then introduced as test spectrum in
the TFA procedure, to test whether or not the test spectrum is likely to be a component of the spectral mixtures. This coupled
procedure is repeated until the error in the test spectrum is minimized. The new method can thus be used to isolate and refine
the structure of complexes from spectral mixtures and to determine their relative concentrations, solely on the basis of an
estimate of a ligand structure. The performance of the proposed method is validated using uranium Liii-edge EXAFS spectra
of binary mixtures of two uranium(VI) 3,4-dihydroxybenzoic acid complexes. 相似文献