全文获取类型
收费全文 | 268篇 |
免费 | 8篇 |
国内免费 | 1篇 |
专业分类
化学 | 196篇 |
晶体学 | 1篇 |
力学 | 5篇 |
数学 | 45篇 |
物理学 | 30篇 |
出版年
2023年 | 2篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 4篇 |
2015年 | 3篇 |
2013年 | 7篇 |
2012年 | 12篇 |
2011年 | 14篇 |
2010年 | 7篇 |
2009年 | 5篇 |
2008年 | 15篇 |
2007年 | 16篇 |
2006年 | 15篇 |
2005年 | 16篇 |
2004年 | 10篇 |
2003年 | 7篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 8篇 |
1967年 | 7篇 |
1966年 | 9篇 |
1916年 | 2篇 |
1912年 | 3篇 |
1902年 | 3篇 |
1901年 | 3篇 |
1897年 | 2篇 |
1896年 | 2篇 |
1883年 | 1篇 |
1882年 | 2篇 |
1870年 | 1篇 |
排序方式: 共有277条查询结果,搜索用时 31 毫秒
271.
Polshin V Popescu DL Fischer A Chanda A Horner DC Beach ES Henry J Qian YL Horwitz CP Lente G Fabian I Münck E Bominaar EL Ryabov AD Collins TJ 《Journal of the American Chemical Society》2008,130(13):4497-4506
The iron(III) complexes of tetra amidato macrocyclic ligands (TAMLs) ([Fe{1-X1-2-X2C6H2-4,5-(NCOCMe2NCO)2CR2}(OH2)]- , 1: X1 = X2 = H, R2 = Me2 (a), R2 = (CH2)2 (b); X1 = X2 = Cl, R2 = F2 (c), etc.), which the proton is known to demetalate at pH < 3, are also subject to catalyzed demetalation by Br?nsted acid buffer components at pH 4-9 such as H2PO4-, HSO3-, and CH3CO2H, HO2CCH2CO2-. Buffers based on pyridine (py) and tris(hydroxymethyl)aminomethane (TRIS) are catalytically inactive. Where reactions proceed, the products are demetalated TAMLs and iron species of variable composition. Pseudo-first-order rate constants for the demetalation (kobs) are linear functions of the acid concentrations, and the effective second-order rate constants k1,eff have a hyperbolic dependence on [H+] (k1,eff = a1[H+]/(b1+[H+]). The rate of demetalation of 1a in H2PO4-/HPO42- buffer is appreciable, but the kobs values for 1b and 1c are immeasurably low, showing that the rates are strongly affected by the CR2 or "tail" fragments, which are known to potently affect the TAML basicity. The reactivities of 1 depend insignificantly on the aromatic ring or "head" group of 1. The proposed mechanism involves precoordination of the acidic buffer species followed by hydrolysis. The demetalating abilities of buffer species depend on their structures and acidities. Thus, although pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation, its 3- and 4-isomers (nicotinic and isonicotininc acids) are inactive. The difference is rationalized to result from the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in an intramolecular manner. The reaction order in picolinic acid equals one for 1a and two for 1b. For 1b, "inactive" pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid, suggesting that the second order arises from the axial binding of two pyridine molecules, one of which must be picolinic acid. The binding of pyridine- and imidazole-type ligands was confirmed by UV/vis equilibrium measurements and X-ray crystallography. The implications of these mechanistic findings for designing superior Fe-TAML oxidation catalysts and catalyst formulations are discussed using the results of DFT calculations. 相似文献
272.
Adje F Lozano YF Meudec E Lozano P Adima A N'zi GA Gaydou EM 《Molecules (Basel, Switzerland)》2008,13(6):1238-1245
Following the development of new applications of pilot plant scale extraction and formulation processes for natural active bioproducts obtained from various underutilized tropical plants and herbs, we have manufactured water-extracts from Delonix regia flowers, grown in Ivory Coast. These extracts, which contain polyphenols, are traditionally home made and used as healthy bioproducts. They are reddish-coloured due to the presence of anthocyanins. The three major anthocyanins in these extracts have been characterized. The molecular structures were confirmed by LC-SM analysis. Amongst them, two are described for the first time in Delonix regia. 相似文献
273.
Charbel Roukoss Jean-Marie Basset Christophe Copret Christine Lucas Emile Kuntz 《Comptes Rendus Chimie》2008,11(6-7):620-627
Grafting of mono- and dinuclear Fe(II) complexes on oxide supports such as silica {SiO2-(700)}, silica–alumina {SiO2–Al2O3-(500)} or alumina {Al2O3-(500)} yields the corresponding mono- and dinuclear surface complexes according to mass balance analysis and IR spectroscopy. 相似文献
274.
Recent experimental data by Mizoguchi and Ohta are analyzed under the assumption of fast equilibrium between the locally excited (LE), radical-ion-pair (RIP), and exciplex states. In the absence of the equilibrium, no magnetic field effect on the LE fluorescence would be observed. Owing to the equilibrium, the relative electric-field-induced changes of the quantum yields of the LE and exciplex emissions as functions of the magnetic field are linearly connected to each other. The electric field shifts the equilibrium from the LE state toward the RIP state. Predictions are made for the magnetic field effect on the fluorescence kinetics. 相似文献
275.
Kervern G Pintacuda G Zhang Y Oldfield E Roukoss C Kuntz E Herdtweck E Basset JM Cadars S Lesage A Copéret C Emsley L 《Journal of the American Chemical Society》2006,128(41):13545-13552
A general protocol for the structural characterization of paramagnetic molecular solids using solid-state NMR is provided and illustrated by the characterization of a high-spin Fe(II) catalyst precursor. We show how good NMR performance can be obtained on a molecular powder sample at natural abundance by using very fast (>30 kHz) magic angle spinning (MAS), even though the individual NMR resonances have highly anisotropic shifts and very short relaxation times. The results include the optimization of broadband heteronuclear (proton-carbon) recoupling sequences for polarization transfer; the observation of single or multiple quantum correlation spectra between coupled spins as a tool for removing the inhomogeneous bulk magnetic susceptibility (BMS) broadening; and the combination of NMR experiments and density functional theory calculations, to yield assignments. 相似文献
276.
Jason England Dr. Marlène Martinho Dr. Erik R. Farquhar Jonathan R. Frisch Emile L. Bominaar Prof. Dr. Eckard Münck Prof. Dr. Lawrence Que Jr. Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(20):3622-3626
High versus low : The high‐yield generation of a synthetic high‐spin oxoiron(IV) complex, [FeIV(O)(TMG3tren)]2+ (see picture, TMG3tren = 1,1,1‐tris{2‐[N2‐(1,1,3,3‐tetramethylguanidino)]ethyl}amine), has been achieved by using the very bulky tetradentate TMG3tren ligand, in order to both sterically protect the oxoiron(IV) moiety and enforce a trigonal bipyramidal geometry at the iron center, for which an S=2 ground state is favored.
277.