全文获取类型
收费全文 | 421篇 |
免费 | 17篇 |
国内免费 | 4篇 |
专业分类
化学 | 293篇 |
晶体学 | 2篇 |
力学 | 3篇 |
数学 | 45篇 |
物理学 | 99篇 |
出版年
2023年 | 3篇 |
2022年 | 4篇 |
2021年 | 8篇 |
2020年 | 7篇 |
2019年 | 6篇 |
2018年 | 4篇 |
2017年 | 3篇 |
2016年 | 11篇 |
2015年 | 12篇 |
2014年 | 10篇 |
2013年 | 16篇 |
2012年 | 35篇 |
2011年 | 25篇 |
2010年 | 18篇 |
2009年 | 26篇 |
2008年 | 38篇 |
2007年 | 28篇 |
2006年 | 24篇 |
2005年 | 22篇 |
2004年 | 15篇 |
2003年 | 12篇 |
2002年 | 20篇 |
2001年 | 9篇 |
2000年 | 17篇 |
1999年 | 14篇 |
1998年 | 9篇 |
1997年 | 1篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 4篇 |
1986年 | 1篇 |
1985年 | 6篇 |
1984年 | 2篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有442条查询结果,搜索用时 15 毫秒
51.
A flow-injection spectrofluorimetric determination of paracetamol is reported, based on the oxidation of the analyte with potassium hexacyanoferrate(III) immobilized on an anion-exchange resin, the fluorescence being enhanced with N,N′-dimethylformamide. Concentrations of paracetamol in the range 0.04–17.60 mg l?1 are determined with a relative standard deviation of 1.5%. The injection rate is 25 samples h?1. The influence of foreign species and the determination of paracetamol in several pharamaceutical formulations are also reported. 相似文献
52.
AM Jayannavar 《Pramana》2002,58(2):173-181
We present a simple model of transmission across a metallic mesoscopic ring. In one of its arm an electron interacts with
a single magnetic impurity via an exchange coupling. We show that entanglement between electron and spin impurity states leads
to reduction of Aharonov-Bohm oscillations in the transmission coefficient. The spin-conductance is asymmetric in the flux
reversal as opposed to the two-probe electrical conductance which is symmetric. In the same model, in contradiction to the
naive expectation of a current magnification effect, we observe enhancement as well as suppression of this effect depending
on the system parameters. The limitations of this model to the general notion of dephasing or decoherence in quantum systems
are pointed out. 相似文献
53.
Benito Alcaide Gerardo Escobar Rafael Pérez-Ossorio Joaquin Plumet 《Journal of heterocyclic chemistry》1984,21(3):919-921
The carbonyl group of substituted 5-benzoyl-4,5-dihydro-1,2,4-oxadiazoles can be reduced by lithium aluminium hydride at low temperature without ring opening. Mixtures of the two related diastereoisomeric benzylalcohols are obtained. Configurations were assigned by 1H-nmr. 相似文献
54.
Recent advances in vibrational spectroscopy have greatly enhanced the possibilities of research of highly excited states in molecular systems of moderate size. At sufficiently high level of excitation the correspondence principle holds, and classical mechanical arguments constitute a useful interpretative tool. The corresponding dynamics often become very complex specially in systems with floppy degrees of freedom, and periodic motion plays an important role for its understanding. In this paper, we present a computational procedure to systematically calculate periodic orbits of LiNCLiCN with a given symmetry, that has the additional advantage of providing a useful insight into the onset of chaos in this system. 相似文献
55.
J. Fdez Ferreras C. Pesquera F. González I. Benito C. Blanco J. Renedo 《Reaction Kinetics and Catalysis Letters》1994,53(1):1-6
Nickel catalysts prepared using two different clays as support, natural bentonite and palygorskite, and with different nickel contents have been tested in the hydrogenation of a sunflower oil. The influence of the clays, and of the nickel-bentonite ratio on the activity, selectivity and trans-isomer formation was investigated. Previously it was establised that no diffusional limitations were controlling the rate. Bentonite with the highest nickel content was the most active and selective catalysts. 相似文献
56.
57.
58.
The leading asymptotic term for the function that counts theeigenvalues of the Stokes operator is determined for fairlygeneral underlying bounded domains. Moreover, the remainderis estimated in terms of the fractality of the boundary of thedomain. The results obtained resemble corresponding ones forthe Dirichlet Laplacian. 1991 Mathematics Subject Classification:35P20. 相似文献
59.
Noemí?Andrade-LópezEmail author José?G.?Alvarado-Rodríguez Benito?Flores-Chávez 《Journal of chemical crystallography》2005,35(3):191-196
Reaction of 2-aminomethylpyridine with diphenylborinic acid in a mixture of CHCl3-diethyl ether gives diphenyl-(2-aminomethylpyridine-N,N)borane, a cyclic adduct that in presence of environmental moisture yields the N-8-(diphenyl-hydroxy-2-aminomethylpyridine)borane. The two compounds were characterized by NMR and infrared spectroscopy. The structure of N-8-(diphenyl-hydroxy-2-aminomethylpyridine)borane was confirmed by X-ray diffraction. This adduct crystallizes in monoclinic P21/n space group with unit cell dimensions: a = 13.193(2) (Å), b = 5.913(7) (Å), c = 19.604(2) (Å), = 90.295(3), V = 1529.3(3) Å3, Z = 4. The BN distance is 1.645(3) Å and lies in the range of a dative bond. The NH and OH groups are involved in intermolecular hydrogen bonds formation and they add to the stabilization of the acyclic adduct in the solid state. 相似文献
60.
Stereoselective NaN3-catalyzed halonitroaldol-type reaction of azetidine-2,3-diones in aqueous media
Azetidine-2,3-diones (alpha-oxo-beta-lactams) and bromonitromethane undergo coupling in aqueous media in the presence of catalytic amounts of sodium azide. The stereoselectivity of the process was generally good, proceeding with reasonable anti : syn ratios under substrate control. On this basis, a simple and fast protocol for the synthesis of the potentially bioactive 3-substituted 3-hydroxy-beta-lactam moiety has been developed. Besides, 2-azetidinone-tethered 1-halo-1-nitroalkan-2-ols are quite useful building blocks; for example, reactions of the above nitrobromohydrins provided spiranic and fused bicyclic-beta-lactams. 相似文献