全文获取类型
收费全文 | 3760篇 |
免费 | 140篇 |
国内免费 | 26篇 |
专业分类
化学 | 2511篇 |
晶体学 | 21篇 |
力学 | 89篇 |
数学 | 623篇 |
物理学 | 682篇 |
出版年
2023年 | 19篇 |
2022年 | 43篇 |
2021年 | 99篇 |
2020年 | 90篇 |
2019年 | 111篇 |
2018年 | 126篇 |
2017年 | 92篇 |
2016年 | 149篇 |
2015年 | 130篇 |
2014年 | 156篇 |
2013年 | 216篇 |
2012年 | 251篇 |
2011年 | 271篇 |
2010年 | 185篇 |
2009年 | 125篇 |
2008年 | 254篇 |
2007年 | 252篇 |
2006年 | 220篇 |
2005年 | 203篇 |
2004年 | 157篇 |
2003年 | 121篇 |
2002年 | 104篇 |
2001年 | 49篇 |
2000年 | 37篇 |
1999年 | 40篇 |
1998年 | 38篇 |
1997年 | 27篇 |
1996年 | 38篇 |
1995年 | 26篇 |
1994年 | 32篇 |
1993年 | 17篇 |
1992年 | 13篇 |
1991年 | 12篇 |
1990年 | 7篇 |
1989年 | 12篇 |
1988年 | 11篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1977年 | 8篇 |
1976年 | 14篇 |
1975年 | 15篇 |
1973年 | 8篇 |
1972年 | 6篇 |
1954年 | 5篇 |
排序方式: 共有3926条查询结果,搜索用时 31 毫秒
31.
The interaction of Me3SiOTf and (C6F5)3SiOTf with enamines generating α-silyl-substituted iminium ions is investigated. A trimethylsilyl iminium cation is formed as a long-lived species observable by NMR spectroscopy, whilst the tris(pentafluorophenyl)silyl analogue is very labile and prone to the loss of a proton. On the basis of the latter phenomenon, a method for the synthesis of β-silyl enamines is proposed. 相似文献
32.
33.
34.
S. N. Karyagin V. F. Kiselev G. S. Plotnikov Yu. D. Fomin 《Theoretical and Experimental Chemistry》1990,25(5):504-509
Electrophysical methods and ESR measurements have been used to investigate the changes, photosensitized by molecules of rhodamine B (RB), in the charge states of electron traps created by molecules of p-benzoquinone (pBQ) in the Ge/GeO2 system. The results obtained in studying the quenching of fluorescence of RB molecules indicate that the photodestruction of electron traps is due to transfer of electronic excitation energy traps is due to transfer of electronic oxidation energy from the RB molecules to charged complexes formed upon adsorption of the pBQ. In order to select optimal conditions for recharging of such complexes, a study has been made of the relationship between the relative change in ESR signal and the concentration of adsorbed RB molecules.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 5, pp. 545–550, September–October, 1989. 相似文献
35.
Alexander D Dilman Pavel A Belyakov Vladimir A Tartakovsky 《Tetrahedron letters》2004,45(19):3741-3744
Silyl enol ethers bearing three pentafluorophenyl groups at the silicon atom are described. These compounds undergo uncatalyzed aldol reactions with aliphatic, α,β-unsaturated, and aromatic aldehydes. The observed reactivity is analyzed in terms of the Lewis acidity of the silyl fragment. 相似文献
36.
Genosar L Lasitza T Gepshtein R Leiderman P Koifman N Huppert D 《The journal of physical chemistry. A》2005,109(21):4852-4861
The pressure dependence of the excited-state proton dissociation rate constant of four photoacids, 2-naphthol-6,8-disulfonate (2N68DS), 10-hydroxycamptothecin (10-CPT), 5-cyano-2-naphthol (5CN2), and 5,8-dicyano-2-naphthol (DCN2), are studied in methanol. The results are compared with the results of the pressure dependence study we recently conducted for several photoacids in water, ethanol, and propanol. The pressure dependence is explained using an approximate stepwise two-coordinate proton transfer model. The increase in rate, as a function of pressure, manifests a strong dependence of proton tunneling on the distance which decreases with an increase of pressure between the two oxygen atoms involved in the process. The decrease in the proton transfer rate with increasing pressure reflects the dependence of the reaction on the solvent relaxation rate. We found that, for the relatively weak photoacids 2N68DS, 10-CPT, and 5CN2, the proton transfer rate constant increases by a factor of about 5-8 at a pressure of about 1.5 GPa. For a strong photoacid like DCN2, the rate increase was only by a factor of 2. 相似文献
37.
The halothane???acetone and fluoroform???acetone complexes are studied using the second‐order Møller–Plesset (MP2) method with a cc‐pVTZ basis set and the density functional theory (DFT) method with a TZVP basis set. Whereas halothane exhibits a small red shift upon complexation, fluoroform shows a pronounced blue shift. To explain this difference in behavior, we perform symmetry‐adapted perturbation theory (SAPT) and natural bond orbital (NBO) analyses. Although the composition of the total stabilization energy of each complex is different, that alone does not provide a satisfactory explanation for the difference in the spectral shifts. This difference is interpreted as a result of the interplay of the hyperconjugation and rehybridization mechanisms. The small and surprising red shift of the C? H stretching frequency of halothane, which resulted from the complexation of this species with acetone,is explained by the compensation of the two above‐mentioned mechanisms. On the other hand, the fluoroform???acetone complex exhibits a blue shift of the C? H stretching frequency upon complexation, the most likely reason for this shift being a concerted occurrence of the hyperconjugation and rehybridization mechanisms. The calculated shift of the C? H stretching vibration frequencies of halothane (+27 cm?1) agree with the experimental value of +5 cm?1. 相似文献
38.
39.
Double gradient ion chromatography on a short carboxybetaine coated monolithic anion exchanger 总被引:1,自引:0,他引:1
An ultra-short 1.0 x 0.46 cm carboxybetaine coated monolithic anion exchanger has been used with a new double gradient ion chromatography (DG-IC) technique. The novel monolithic column can be used with flow rate gradients giving excellent peak efficiencies, and retention times can be simultaneously reduced through combining flow gradients with an eluent pH gradient. 相似文献
40.
Six new dinuclear or trinuclear FeIII complexes involving tetradentate Schiff bases N,N′-bis(salicylidene)ethylenediamine
(salenH2) or bis(salicylidene)-o-phenylenediamine (salophH2) with 2,5-pyridinedicarboxylic acid, acetylenedicarboxylic acid
or 1,3,5-benzenetricarboxylic acid have been synthesized and characterized by means of elemental analysis, i.r. spectroscopy,
thermal analyses, conductivity measurements and variable-temperature magnetochemical measurements to the temperature of liquid
nitrogen. The complexes can be characterized as high-spin distorted octahedral FeIII bridged by carboxylic acids. The dicarboxylic
or tricarboxylic acids play a role as bridges for weak antiferromagnetic intramolecular exchange. The antiferromagnetic coupling
parameters J vary in the -1.99 to -5.47cm-1 range for the dimers, whilst the values are -2.35 and -1.42cm-1 for the salen
and saloph trimers, respectively. One complex, namely [{Fe(saloph)}2(2,5-dicarpy)]middot H2O, obeys the Curie-Weiss law.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献