全文获取类型
收费全文 | 569篇 |
免费 | 23篇 |
国内免费 | 5篇 |
专业分类
化学 | 408篇 |
晶体学 | 7篇 |
力学 | 14篇 |
数学 | 45篇 |
物理学 | 123篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 8篇 |
2021年 | 4篇 |
2020年 | 13篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 14篇 |
2016年 | 26篇 |
2015年 | 15篇 |
2014年 | 23篇 |
2013年 | 31篇 |
2012年 | 48篇 |
2011年 | 46篇 |
2010年 | 33篇 |
2009年 | 28篇 |
2008年 | 45篇 |
2007年 | 38篇 |
2006年 | 46篇 |
2005年 | 40篇 |
2004年 | 22篇 |
2003年 | 12篇 |
2002年 | 15篇 |
2001年 | 7篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 7篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1991年 | 1篇 |
1989年 | 1篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1963年 | 1篇 |
1931年 | 1篇 |
1930年 | 1篇 |
1926年 | 1篇 |
1900年 | 1篇 |
排序方式: 共有597条查询结果,搜索用时 15 毫秒
51.
Duce S Jorge M Alonso I García Ruano JL Cid MB 《Organic & biomolecular chemistry》2011,9(24):8253-8260
The presence of a p-nitrophenyl group converts acetone into an excellent and versatile nucleophile in organocatalytic processes, able to react with α,β-unsaturated aldehydes affording β-substituted α-arylcyclohexenones via a Michael reaction/aldol reaction/dehydration sequence, which occurs in good yields, ee up to 96% and complete diastereoselectivity. The resulting compounds are excellent synthons for the diastereoselective preparation of a variety of synthetically useful polysubstituted cyclohexanones and derivatives. 相似文献
52.
Martínez-Castañeda Á Poladura B Rodríguez-Solla H Concellón C del Amo V 《Organic letters》2011,13(12):3032-3035
The combined activity of (S)-proline and an achiral cocatalyst (a TBD-derived guanidinium salt) allow direct aldol reactions to be carried out with high diastereoselectivity and enantioselectivity under solvent-free conditions with a rather simple reaction setup where stirring is not required. 相似文献
53.
Alves L Ballesteros B Boronat M Cabrero-Antonino JR Concepción P Corma A Correa-Duarte MA Mendoza E 《Journal of the American Chemical Society》2011,133(26):10251-10261
Small gold nanoclusters in a very narrow size distribution (1.1 ± 0.5 nm) have been stabilized onto multiwalled carbon nanotubes (MWCNT). Theoretical studies supported by XPS and (16)O(2)/(18)O(2) isotopic exchange experiments have shown that, on small gold nanoparticles (0.9-1.5 nm), dissociation of molecular O(2) and formation of a surface oxide-like layer is energetically favorable and occurs at room temperature, while O(2) recombination and desorption involves a larger activation barrier. CO titration experiments and theoretical studies demonstrate that the reactivity of the oxidized particles toward CO does not only depend on particle size but also on oxygen coverage. The oxidation-reduction process described is reversible, and the oxidized nanoparticles are active in the epoxidation of styrene with air. 相似文献
54.
55.
Bain AD Baron M Burger SK Kowalewski VJ Rodríguez MB 《The journal of physical chemistry. A》2011,115(33):9207-9216
Cyclohexane is an extremely flexible molecule that oscillates, at room temperature, between two clearly distinct and extreme conformations that cannot be distinguished at room temperature; so much so that the NMR spectrum is a single line that includes all 12 protons be they axial or equatorial. This raises the interesting question as to what happens when there are equal substituents at the 1 and 4 carbon atoms of the ring. Therefore substitution in the 1,4-positions in the cyclohexane ring has been the subject of considerable interest because some form of interconversion between extreme conformations could lead to the existence of a rather unusual behavior. To study this problem, the interconversion in (di- or tetra-1,4)-substituted six-membered cyclohexane-type rings, trans-1,4-dibromo-1,4-dicyanocyclohexane, was found to be a particularly suitable candidate. Although X-ray diffraction studies on the crystalline solid found the molecule to be centrosymmetric, it still shows a significant dipole moment μ in solution, as determined with a procedure that leads to the vapor phase values of μ. Furthermore, the low magnetic field proton NMR spectrum at ambient temperature appears as a single line, a situation that changes with increasing field intensity and different solvents. Both these effects are attributed to dynamics, because small distortions can easily disrupt the exact cancellation of the individual dipoles (which are quite strong) associated with each end of the molecule. The molecule can exist in two forms, with both the bromines in an axial geometry or both in an equatorial position. Interconversion between these forms is observed, as in the parent cyclohexane. The single NMR line observed at low magnetic fields is due to fast exchange and requires that the two forms have roughly equal populations. Spectra obtained at low temperature confirm this, and variable-temperature studies allow measurement of the rates, leading to an enthalpy of activation of 62 kJ mol(-1). More details of the interconversion are provided by some new calculation methods. Even for a relatively small molecule like this, calculation of a full potential energy surface is prohibitive. However, methods are now available to follow the molecule along the reaction coordinate in quite an efficient way. The results of these calculations lead to an extremely detailed picture of chair-chair interconversion in a di- and tetrasubstituted six-membered ring of the cyclohexane family. 相似文献
56.
A novel iridium-catalysed tandem isomerisation/C-F bond formation from allylic alcohols and Selectfluor? to prepare α-fluorinated ketones as single constitutional isomers is reported. 相似文献
57.
Rodrigo E Morales S Duce S Ruano JL Cid MB 《Chemical communications (Cambridge, England)》2011,47(40):11267-11269
Heteroarylvinyl sulfone 1 has been successfully used as a new sulfonyl Michael acceptor in aminocatalytic reactions with branched aldehydes. Subsequent one-pot Julia-Kocienski olefination allows the challenging preparation of enantiomerically pure α-allylated aldehydes bearing C-α quaternary carbons. 相似文献
58.
Costa EV Pinheiro ML de Souza AD Barison A Campos FR Valdez RH Ueda-Nakamura T Filho BP Nakamura CV 《Molecules (Basel, Switzerland)》2011,16(11):9714-9720
Phytochemical investigation of the branches of Annona foetida Mart. led to isolation from the CH(2)Cl(2) extract of four alkaloids: Atherospermidine (1), described for the first time in this species, liriodenine (2), O-methylmoschatoline (3), and annomontine (4). Their chemical structures were established on the basis of spectroscopic data from IR, MS, NMR (1D and 2D), and comparison with the literature. Compounds 2-4 showed potent trypanocidal effect when evaluated against epimastigote and trypomastigote forms of Trypanosoma cruzi. 相似文献
59.
Storm processes and stochastic geometry 总被引:1,自引:0,他引:1
This paper is devoted to a prototype of max-stable models called the storm process. At first its spatial distribution is given
in association with different observation supports. Then the compatibility relationships between extremal coefficients at
various supports are completely characterized. Particular attention is paid to the special case where the storms are indicator
functions of Poisson polytopes. Explicit formulae are found for the extremal coefficients with finite or convex supports.
A new algorithm for exactly simulating the Poisson storm process in continuous space is also provided. Overall, the storm
process can be used as a benchmark for comparing the performances of several estimators of extremal coefficients, or for model
selection. 相似文献
60.
E. A. Doisy R. D. Bel W. C. Ball und E. Tschopp 《Fresenius' Journal of Analytical Chemistry》1926,69(1-2):63-67
Ohne Zusammenfassung 相似文献