全文获取类型
收费全文 | 490篇 |
免费 | 22篇 |
国内免费 | 2篇 |
专业分类
化学 | 297篇 |
晶体学 | 3篇 |
力学 | 24篇 |
数学 | 110篇 |
物理学 | 80篇 |
出版年
2023年 | 5篇 |
2022年 | 11篇 |
2021年 | 27篇 |
2020年 | 15篇 |
2019年 | 18篇 |
2018年 | 19篇 |
2017年 | 5篇 |
2016年 | 25篇 |
2015年 | 22篇 |
2014年 | 28篇 |
2013年 | 35篇 |
2012年 | 35篇 |
2011年 | 38篇 |
2010年 | 21篇 |
2009年 | 32篇 |
2008年 | 22篇 |
2007年 | 36篇 |
2006年 | 19篇 |
2005年 | 24篇 |
2004年 | 14篇 |
2003年 | 7篇 |
2002年 | 14篇 |
2001年 | 6篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1975年 | 1篇 |
1973年 | 3篇 |
排序方式: 共有514条查询结果,搜索用时 156 毫秒
121.
Piraud M Vianey-Saban C Petritis K Elfakir C Steghens JP Morla A Bouchu D 《Rapid communications in mass spectrometry : RCM》2003,17(12):1297-1311
The diagnosis of inherited disorders of amino acids (AA) metabolism is usually performed on automated analysers by ion-exchange chromatography and quantification after ninhydrin derivatisation of about 50 different AA. A single run liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for these molecules can be an alternative to this time-consuming technique. The first step of this development is the infusion study of the fragmentation of 79 molecules of biological interest in electrospray ionisation tandem mass spectrometry (ESI-MS/MS), in positive and in negative ionisation mode. Among them, three molecules can be detected only in negative ionisation mode, 38 only in positive mode and 38 in the two modes. All the most abundant fragmentations are presented, with optimisation of the MS/MS parameters. The positive ionisation mode was retained for the simultaneous analysis of 76 molecules. One sensitive and/or specific transition is proposed for the monitoring of each molecule. Improvement in sensitivity of detection was obtained with the use of an acidic mobile phase. Flow injection analysis studies led us to highlight a number of interferences-due to isobaric molecules, to in-source collision-induced dissociation, or to natural isotopic distribution of the elements-which are listed. For a reliable quantification method, these molecules have to be separated by LC before analysis in the tandem mass spectrometer. Ion-pairing reversed-phase liquid chromatography (RPLC) using perfluorinated carboxylic acids as ion-pairing agents has already been found suitable for analysis of AA in MS/MS positive ionisation mode and is under development. 相似文献
122.
We report a laboratory measurement of the rate coefficient for the collisional removal of O(2)(X(3)Sigma(g) (-),upsilon=1) by O((3)P) atoms. In the experiments, 266-nm laser light photodissociates ozone in a mixture of molecular oxygen and ozone. The photolysis step produces vibrationally excited O(2)(a(1)Delta(g)) that is rapidly converted to O(2)(X(3)Sigma(g) (-),upsilon=1-3) in a near-resonant electronic energy-transfer process with ground-state O(2). In parallel, a large amount of O((1)D) atoms is generated that promptly relaxes to O((3)P). Under the conditions of the experiments, only collisions with the photolytically produced O((3)P) atoms control the lifetime of O(2)(X(3)Sigma(g) (-),upsilon=1), because its removal by molecular oxygen at room temperature is extremely slow. Tunable 193-nm laser light monitors the temporal evolution of the O(2)(X(3)Sigma(g) (-),upsilon=1) population by detection of laser-induced fluorescence near 360 nm. The removal rate coefficient for O(2)(X(3)Sigma(g) (-),upsilon=1) by O((3)P) atoms is (3.2+/-1.0)x10(-12) cm(3) s(-1) (2sigma) at a temperature of 315+/-15 K (2sigma). This result is essential for the analysis and correct interpretation of the 6.3-mum H(2)O(nu(2)) band emission in the Earth's mesosphere and indicates that the deactivation of O(2)(X (3)Sigma(g) (-),upsilon=1) by O((3)P) atoms is significantly faster than the nominal values recently used in atmospheric models. 相似文献
123.
The reaction of Pb and Eu with a molten mixture of A(2)Se/P(2)Se(5)/Se produced the quaternary compounds APbPSe(4), A(4)Pb(PSe(4))(2) (A = Rb,Cs), and K(4)Eu(PSe(4))(2). The red crystals of APbPSe(4) are stable in air and water. The orange crystals of A(4)Pb(PSe(4))(2) and K(4)Eu(PSe(4))(2) disintegrate in water and over a long exposure to air. CsPbPSe(4) crystallizes in the orthorhombic space group Pnma (No. 62) with a = 18.607(4) ?, b = 7.096(4) ?, c = 6.612(4) ?, and Z = 4. Rb(4)Pb(PSe(4))(2) crystallizes in the orthorhombic space group Ibam (No. 72) with a = 19.134(9) ?, b = 9.369(3) ?, c = 10.488(3) ?, and Z = 4. The isomorphous K(4)Eu(PSe(4))(2) has a = 19.020(4) ?, b = 9.131(1) ?, c = 10.198(2) ?, and Z = 4. The APbPSe(4) have a layered structure with [PbPSe(4)](n)()(n)()(-) layers separated by A(+) ions. The coordination geometry around Pb is trigonal prismatic. The layers are composed of chains of edge sharing trigonal prisms running along the b-direction. [PSe(4)](3)(-) tetrahedra link these chains along the c-direction by sharing edges and corners with the trigonal prisms. A(4)M(PSe(4))(2) (M = Pb, Eu) has an one-dimensional structure in which [M(PSe(4))(2)](n)()(n)()(-) chains are separated by A(+) ions. The coordination geometry around M is a distorted dodecahedron. Two [PSe(4)](3)(-) ligands bridge two adjacent metal atoms, using three selenium atoms each, forming in this way a chain along the c-direction. The solid state optical absorption spectra of the compounds are reported. All compounds melt congruently in the 597-620 degrees C region. 相似文献
124.
Aristides I Kechriniotis Christos A Tsonos Konstantinos K Delibasis Georgios N Tsigaridas 《理论物理通讯》2020,72(4):45201-63
In this work we study in detail the connection between the solutions to the Dirac and Weyl equations and the associated electromagnetic four-potentials.First,it is proven that all solutions to the Weyl equation are degenerate,in the sense that they correspond to an infinite number of electromagnetic four-potentials.As far as the solutions to the Dirac equation are concerned,it is shown that they can be classified into two classes.The elements of the first class correspond to one and only one four-potential,and are called non-degenerate Dirac solutions.On the other hand,the elements of the second class correspond to an infinite number of four-potentials,and are called degenerate Dirac solutions.Further,it is proven that at least two of these fourpotentials are gauge-inequivalent,corresponding to different electromagnetic fields.In order to illustrate this particularly important result we have studied the degenerate solutions to the forcefree Dirac equation and shown that they correspond to massless particles.We have also provided explicit examples regarding solutions to the force-free Weyl equation and the Weyl equation for a constant magnetic field.In all cases we have calculated the infinite number of different electromagnetic fields corresponding to these solutions.Finally,we have discussed potential applications of our results in cosmology,materials science and nanoelectronics. 相似文献
125.
Vasiliki Palli Georgios Leonis Nikoletta Zoupanou Nikitas Georgiou Maria Chountoulesi Nikolaos Naziris Demeter Tzeli Costas Demetzos Georgia Valsami Konstantinos D. Marousis Georgios A. Spyroulias Thomas Mavromoustakos 《Molecules (Basel, Switzerland)》2022,27(8)
Losartan potassium salt (LSR) is a well-known antihypertensive drug with proven beneficial effects on human health. Its formulation with the non-toxic 2-hydroxypropyl-β-cyclodextrin (2-HP-β-CD) could improve its pharmacological profile. Thus, its molecular interactions are studied using a combination of Differential Scanning Calorimetry (DSC), Nuclear Magnetic Resonance (NMR) and Molecular Dynamics (MD). First, its complexation is shown through Differential Scanning Calorimetry as lyophilization provided distinct thermal properties in comparison to the mixture. The complexation is further proved by utilizing the chemical shift changes in the complexation and T1 values. Furthermore, the reversible favorable complexation was shown by MD calculations. Such physical chemical properties provide evidence that this formulation must be further explored through biological experiments. 相似文献
126.
127.
Konstantinos Ghirtis Nicole Pouli Panagiotis Marakos Alexios‐Leandros Skaltsounis Stephane Leonce Ghanem Atassi Daniel H. Caignard 《Journal of heterocyclic chemistry》2001,38(1):147-152
As part of a research program directed towards the design and synthesis of pyranoxanthones structurally related to acronycine, we present here the synthesis and cytotoxic activity of the pyranoxanthones 3 and 4 (X= H, Br; R= H, OMe; R'= H, Ac). Some of these compounds inhibit L1210 cell proliferation. 相似文献
128.
The synthesis and structural characterization of the first polymeric M-HDTMP organic-inorganic hybrids are described [M = Zn2+, Ca2+; HDTMP = hexamethylenediaminetetrakis(methylenephosphonate)]. The 3D crystal structure of the Zn2+ analogue [Zn(HDTMP) x H2O] is described. The Zn center is found in a distorted octahedral environment of phosphonate oxygens. There is a long Zn...O interaction (2.622 A) originating from a protonated -P-OH group. Synergistic combinations of Zn2+ and the tetraphosphonate are found to form films that protect against the corrosion of carbon steels. 相似文献
129.
Stylianos Mourmouris Konstantinos Kostakis Marinos Pitsikalis Nikos Hadjichristidis 《Journal of polymer science. Part A, Polymer chemistry》2005,43(24):6503-6514
Well‐defined ω‐cholesteryl poly(n‐hexyl isocyanate) (PHIC–Chol), as well as diblock copolymers of n‐hexyl isocyanate (HIC) with styrene, PS‐b‐PHIC [PS = polystyrene; PHIC = poly(n‐hexyl isocyanate)], and triblock terpolymers with styrene and isoprene, PS‐b‐PI‐b‐PHIC and PI‐b‐PS‐b‐PHIC (PI = polyisoprene), were synthesized with CpTiCl2(OR) (R = cholesteryl group, PS, or PS‐b‐PI) complexes. The synthetic strategy involved the reaction of the precursor complex CpTiCl3 with cholesterol or the suitable ω‐hydroxy homopolymer or block copolymer, followed by the polymerization of HIC. The ω‐hydroxy polymers were prepared by the anionic polymerization of the corresponding monomers and the reaction of the living chains with ethylene oxide. The reaction sequence was monitored by size exclusion chromatography, and the final products were characterized by size exclusion chromatography (light scattering and refractive‐index detectors), nuclear magnetic resonance spectroscopy, and, in the case of PHIC–Chol, differential scanning calorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6503–6514, 2005 相似文献
130.
The absorption maximum at longest wave lengths of the 7-dimethylaminoflavylium perchlorate being shifted by 121 nm to longer wave lengths than the maximum of the flavylium perchlorate and yet the auxochromes in 4′-position having a less strong effect in the 7-dimethylamino-flavylium salts than in the flavylium salts, the absorption maxima of both series converge strongly. In the case of the 4′-dimethylamino derivatives the convergence reaches the high value of 61 nm. 相似文献