全文获取类型
收费全文 | 2799篇 |
免费 | 86篇 |
国内免费 | 9篇 |
专业分类
化学 | 1887篇 |
晶体学 | 48篇 |
力学 | 56篇 |
数学 | 443篇 |
物理学 | 460篇 |
出版年
2023年 | 12篇 |
2022年 | 50篇 |
2021年 | 70篇 |
2020年 | 63篇 |
2019年 | 59篇 |
2018年 | 46篇 |
2017年 | 54篇 |
2016年 | 85篇 |
2015年 | 83篇 |
2014年 | 94篇 |
2013年 | 188篇 |
2012年 | 153篇 |
2011年 | 177篇 |
2010年 | 144篇 |
2009年 | 116篇 |
2008年 | 173篇 |
2007年 | 150篇 |
2006年 | 126篇 |
2005年 | 127篇 |
2004年 | 111篇 |
2003年 | 91篇 |
2002年 | 84篇 |
2001年 | 30篇 |
2000年 | 37篇 |
1999年 | 33篇 |
1998年 | 27篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 26篇 |
1994年 | 23篇 |
1993年 | 18篇 |
1992年 | 25篇 |
1991年 | 34篇 |
1990年 | 27篇 |
1989年 | 19篇 |
1988年 | 21篇 |
1987年 | 14篇 |
1986年 | 15篇 |
1985年 | 26篇 |
1984年 | 24篇 |
1983年 | 13篇 |
1982年 | 18篇 |
1981年 | 13篇 |
1980年 | 20篇 |
1979年 | 11篇 |
1978年 | 14篇 |
1977年 | 19篇 |
1976年 | 16篇 |
1975年 | 7篇 |
1969年 | 6篇 |
排序方式: 共有2894条查询结果,搜索用时 15 毫秒
21.
The overall stability constantsK
1 andK
2 of NdNO
3
2+
and Nd(NO3)
2
+
complexes were determined (K
1=1.77;K
2=1.28) using an extraction method with tri-n-butyl phosphate as the extractant. The ratio 1/2 of the stepwise stability constants is discussed. It was established that the Nd(NO3)
2
+
complex was an outer-sphere ion pair.
Die Assoziation von Nitrat-Ion mit Nd3+
Zusammenfassung Mittels einer Extraktionsmethode wurden die StabilitätskonstantenK 1 undK 2 von NdNO 3 2+ - und Nd(NO3)+-Komplexen bestimmt (K 1=1.77;K 2=1.28; Tri-n-butylphosphat als Extraktionsmittel). Das Verhältnis 1/2 der stufenweisen Stabilitätskonstanten wird diskutiert. Es stellte sich heraus, daß der Nd(NO3)+-Komplex als ein Outer-Shere-Ionenpaar vorliegt.相似文献
22.
M. Bokor T. Marek K. Süvegh K. Tompa A. Vértes Zs. Nemes-vetéssy K. Burger 《Journal of Radioanalytical and Nuclear Chemistry》1996,211(1):247-253
The complexes [Zn(1-propyltetraole)6](BF4)2 (Zn-ptz) and [Fe(methyltetrazole)6](BF4)2 (Fe-mtz) were investigated by positron annihilation and solid-state NMR spectroscopy. For Zn-ptz signs of structural rearrangements were found between 70 and 145 K. For the spin-crossover complex Fe-mtz a transition affecting the high-spin stat Fe2+ ions at lattice site B occurs below 15 K. 相似文献
23.
Using nano- and microsize extrusion, a simple synthetic procedure of preparing hollow monodispersed colloidal particles dispersed in an aqueous phase was developed. Hydrophobic styrene monomer containing 2-hydroxy-2-methyl propiophenone photoinitiator was forced into desired diameter membrane channels and stabilized by the hydrophobic regions of a liposome obtained from 1,2-dilauroyl-phosphocholine phospholipid in an aqueous phase. Such moieties exposed to 254-nm UV radiation polymerize monomers in the hydrophobic zone of the liposome, thus resulting in reinforced hollow vesicles. The size of such particles is controlled by the size of the membrane channels in the extruder and may vary from a few nanometers to micrometers, thus allowing the synthesis of monodisperse hollow colloidal spheres. 相似文献
24.
This report describes a new type of intra-intermolecular criss-cross cycloadditions. Thermal reactions of unsymmetrical allenylazines in the presence of alkynes led to three fused five-membered heterocycles in some cases. In the case of unsymmetrical substituted alkynes, a regioselectivity was observed. The molecular structures of all products are discussed. One X-ray crystal structure is also reported. 相似文献
25.
Stimuli-responsive behavior of phospholipids in the presence of ionic surfactants utilized in synthesis of MMA/nBA colloidal particles was investigated. Utilizing 1-myristoyl-2-hydroxy-sn-glycero-phosphocholine (MHPC) phospholipid, and sodium dioctyl sulfosuccinate (SDOSS) surfactant as dispersing media in H(2)O, narrow unimodal particle size distributions of methyl methacrylate (MMA)/n-butyl acrylate (nBA) copolymers were synthesized. The particle diameters were 154 nm when a SDOSS/MHPC mixture was used and 161 nm using MHPC as the only surface-stabilizing species. When such colloidal dispersions are exposed to 1.7, 3.3, and 6.7 mM aqueous CaCl(2) and KCl electrolyte solutions, surface-localized ionic clusters are generated at the film-air interface that may serve as lipid rafts composed of crystalline phases of MHPC deposited on poly(MMA)/nBA films. These studies illustrate that it is possible to control release and morphology developments of surface phospholipid rafts on artificial surfaces. 相似文献
26.
Marek Da?browski 《Tetrahedron letters》2005,46(24):4175-4178
The metalation of (quasi)alkoxy-substituted dibromobenzenes C6H3(OR)Br2 with lithium diisopropylamide (LDA) has been investigated. For 1-(quasi)alkoxy-3,5-dibromobenzenes (R = Me, TMS), different selectivities were observed depending on reaction conditions and the size of the alkoxy group. The methoxy group was an effective ortho-director whereas this was not the case for the bulky trimethylsilyloxy group. The metalation of related 2,5-dibromoanisole was also examined showing a significant meta-directing effect by the methoxy group. The thermal stability of aryllithium intermediates is significantly lower when lithium is flanked by a bromine and a methoxy group, whereas 4-(quasi)alkoxy-2,6-dibromoaryllithiums are less labile. 相似文献
27.
Cypryk M 《The journal of physical chemistry. A》2005,109(51):12020-12026
Hydrolysis and condensation of simple trifluorosilanes, HSiF3 and MeSiF3, was studied by quantum mechanical methods. Hydrolysis of fluorosilanes is highly endothermic. The Gibbs free energy of the first reaction step in the gas phase is 31.4 kJ/mol, which corresponds to an equilibrium constant of 10(-6). Hydrolysis of the subsequent fluorine atoms in trifluorosilanes is thermodynamically more unfavorable than the first step of substitution. No significant difference in thermodynamics of hydrolysis was found between HSiF3 and MeSiF3. The activation energy for hydrolysis by a water dimer is significantly lower than that for hydrolysis by a water monomer. The former reaction is also less unfavorable thermodynamically, due to a high binding energy of the HF-H2O complex formed as a product of hydrolysis. Self-consistent reaction field (SCRF) calculations show that hydrolysis of trifluorosilanes in aqueous medium has lower activation energy than in the gas phase. It is also thermodynamically less unfavorable, due to better solvation of the products. Homofunctional condensation of HSiF2OH is thermodynamically favored. The equilibrium mixture for hydrolysis/condensation of RSiF3 in water is predicted to contain ca. 2.3% disiloxane (HF2Si)2O, if 100-fold excess of water relative to silane is assumed. Further hydrolysis of (HF2Si)2O is negligible. The thermodynamics of fluorosilane hydrolysis contrasts with that of chlorosilanes, where both hydrolysis and condensation are strongly favorable. Moreover, in the case of trichlorosilanes each subsequent hydrolysis step is more facile, leading to the product of full hydrolysis, RSi(OH)3. 相似文献
28.
29.
[reaction: see text] Secondary alpha-lithiosulfinyl carbanions react either intermolecularly, after transmetalation into an organocopper derivative in an S(N)2-type process with zinc carbenoid, or intramolecularly via higher-order zincate to give, through a tandem zinc homologation-beta-elimination reaction the corresponding alkenes. alpha,alpha-Disubstituted alkenes are only formed from tertiary alpha-lithiosulfinyl carbanions via the 1,2-metalate rearrangement. 相似文献
30.
Local density and generalized gradient approximation time-dependent density functional methods have been used for calculation of the singlet and triplet excited states of nickel-porphine, Ni-tetraphenyloporphine, and Ni-octaethyloporphyrine. Special attention is paid to metal-ligand transitions and d-d transitions. It is shown that the lowest exited singlet states of the three compounds can be described as a transfer of an electron from the porphine ring to the d(x2-y2) orbital of the nickel atom. On the other hand, the lowest excited triplet state arises from promotion of an electron between two nickel d orbitals, an occupied d(z2) and an empty d(x2-y2). It is proposed that a rapid quenching of the excited singlet states is due to an ultrafast intersystem crossing between 1Eg)and 3Eg or 3B1g states. 相似文献