全文获取类型
收费全文 | 1498篇 |
免费 | 53篇 |
国内免费 | 1篇 |
专业分类
化学 | 1195篇 |
晶体学 | 6篇 |
力学 | 34篇 |
数学 | 170篇 |
物理学 | 147篇 |
出版年
2023年 | 17篇 |
2022年 | 38篇 |
2021年 | 70篇 |
2020年 | 37篇 |
2019年 | 40篇 |
2018年 | 37篇 |
2017年 | 35篇 |
2016年 | 45篇 |
2015年 | 55篇 |
2014年 | 46篇 |
2013年 | 97篇 |
2012年 | 112篇 |
2011年 | 122篇 |
2010年 | 80篇 |
2009年 | 77篇 |
2008年 | 97篇 |
2007年 | 110篇 |
2006年 | 77篇 |
2005年 | 74篇 |
2004年 | 58篇 |
2003年 | 45篇 |
2002年 | 46篇 |
2001年 | 11篇 |
2000年 | 12篇 |
1999年 | 13篇 |
1998年 | 9篇 |
1997年 | 3篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1987年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1965年 | 2篇 |
1964年 | 3篇 |
1963年 | 2篇 |
1960年 | 3篇 |
1959年 | 2篇 |
1958年 | 3篇 |
1955年 | 2篇 |
1932年 | 2篇 |
1929年 | 2篇 |
1911年 | 2篇 |
1899年 | 1篇 |
排序方式: 共有1552条查询结果,搜索用时 31 毫秒
21.
Queiroz MJ Abreu AS Ferreira PM Oliveira MM Dubest R Aubard J Samat A 《Organic letters》2005,7(22):4811-4814
[structure: see text] A new photochromic thieno-2H-chromene alpha-amino acid derivative was prepared by C-N palladium-catalyzed cross-coupling of a bromothieno-2H-chromene with the aminated aromatic side chain of the methyl ester of a N,N-diprotected amino acid. Its good photochromic properties demonstrated by flash photolysis and continuous irradiation indicate a possible application in ophthalmic lenses. It may also be inserted into peptides to give photoinduced reversible structural changes. 相似文献
22.
The equation of state of hard hyperspheres in four and five dimensions is calculated from the value of the pair correlation function at contact, as determined by Monte Carlo simulations. These results are compared to equations of state obtained by molecular dynamics and theoretical approaches. In all cases the agreement is excellent. 相似文献
23.
Alvarez-Bercedo P Bond AD Haigh R Hopkins AD Lawson GT McPartlin M Moncrieff D Mosquera ME Rawson JM Woods AD Wright DS 《Chemical communications (Cambridge, England)》2003,(11):1288-1289
The reaction of CyPHNa with Sn(NMe2)2 in the presence of PMDETA (= (Me2NCH2CH2)2NMe) gives the title compound [(Sn(mu-PCy))3(Na x PMDETA)2] (1), containing an electron-deficient [(Sn(mu-PCy))]3(2-) dianion with a novel two-electron, three centre (2e-3c) bonding arrangement. 相似文献
24.
Recently, a new and efficient method for total homocysteine (tHcy) quantitation in plasma using trap and release membrane introduction mass spectrometry (T&R-MIMS) with a versatile removable direct introduction membrane probe (DIMP) was described [R. Haddad, M. A. Mendes, N. F. Hoehr and M. N. Eberlin, Analyst, 2001, 126, 1212]. Herein we report on the use of the DIMP-T&R-MIMS technique for total cysteine (tCys) quantitation; hence combined tCys and tHcy quantitation in plasma or serum can be achieved. The method employs Cys and Hcy derivatization with ethyl chloroformate (after disulfide bond reduction with dithiothreitol and protein precipitation with trichloroacetic acid), preconcentration in a capillary silicone membrane, and their thermal desorption to the gas phase inside the ion source region of a mass spectrometer, at a point exactly between the two ionization filaments. Thermal desorption uses the uniform heat radiation provided by the two ionization filaments. The analytes are then ionized by electron ionization and both Cys and Hcy are quantitated by mass spectrometry using selected ion monitoring. For tCys quantitation, good linearity and reproducibility was observed for concentrations ranging from 5 to 350 microM, recovery was near 95%, and the limit of detection (LOD) was of 2 microM. This LOD is well below the mean Cys concentration in plasma, and serum samples from a large group of healthy people showed a mean tCys concentration of 132 +/- 45 microM. 相似文献
25.
An enantioselective gas chromatography (GC) method has been developed and validated for determination of the enantiomers of citronellol in kinetic resolution experiments. S-(-)-beta-Citronellol is a precursor of rose oxide. After solid-phase extraction (SPE) with ethyl acetate, the enantiomers of R-(+)-beta-citronellol and S-(-)-beta-citronellol and their corresponding acetate- and butyrate esters were separated through enantioselective GC respectively. The method was validated and found to be reproducible, specific, accurate, and precise. Analyte recoveries and detection limits were also determined. The applicability of this method was shown in a kinetic resolution experiment using lipase A of Bacillus subtilis. 相似文献
26.
We report the synthesis and radioligand binding analysis of a series of naphthalenic melatonin receptor ligands, N-[2-(7-alkoxy-2-methoxy-1-naphthyl)ethyl]propionamide. This series of ligands exhibits subpicomolar binding affinity to both MT1 and MT2 melatonin receptors expressed in chinese hamster ovary (CHO) cells. 相似文献
27.
Ruifeng Liu Paula R. Moody Alex S. Vanburen Jeffrey A. Clark Joel A. Krauser Dennis R. Tate 《Vibrational Spectroscopy》1996,10(2):325-329
Results of ab initio and density functional theory calculations on the structure and vibrational frequencies of hypophosphite anion indicate earlier experimental assignments of the fundamental vibrational modes are correct while the recent reassignments of several modes proposed by Bickley et al. are inconsistent with the calculated results. 相似文献
28.
An enhanced etched electrochemical (EC) detection technique has been developed for CE in micron inner diameter capillaries. The design improvements allow for better alignment between the capillary bore and the electrode. This new method involves utilizing a carbon fiber microelectrode and etching both the carbon fiber and the detection end of a micrometer-sized inner diameter capillary to limit dead volume and analyte diffusion at the amperometric EC detector. To understand the factors affecting enhanced detector efficiency, a detailed examination of the relationship between detector design and performance has been completed by exploring the effects of varying electrode diameter, tip shape, and size, in addition to the etch length of the capillary outlet. The enhanced detection provides peak efficiencies as high as 75000 theoretical plates and estimated detection limits as low as 40 nM for dopamine. This etched detection method should further facilitate volume-limited sample analysis by CE. 相似文献
29.
Nitroazobenzene films have been grafted to pyrolyzed photoresist films by electrochemical reduction of the corresponding diazonium salt in acetonitrile solution. Two component films were also prepared by electrochemically grafting methylbenzene layers to preformed NAB films. Voltammetric investigation of the films in aqueous acid medium and the measurement of film thickness using atomic force microscopy (AFM) lead to new insights into film structure. In aqueous acid solution, the azobenzene groups have no detectable electroactivity and not all nitro groups in the films can be reduced. These findings point to a compact film structure in which proton diffusion is limited. There may also be spatial inhibition of the conformational changes that accompany azobenzene reduction. For increasingly thick NAB films, the peak for reduction of the nitro groups moves to more negative potentials and the peaks become more asymmetric in shape. These changes are interpreted in terms of the dielectric properties and the rate of proton diffusion in the films. Film thickness was measured by ploughing through the film with an AFM tip. When an NAB film prepared in acetonitrile solution is reduced in aqueous acid, the film thickness decreases by more than 50%. The changes can be partially reversed by treatment in acetonitrile-electrolyte solution and hence are attributed to ion-solvent induced swelling and shrinking. Thus, the large decrease in thickness detected by AFM after treatment of the film in aqueous acid is consistent with the compact film structure revealed by electrochemistry. 相似文献
30.
Paula Messina Marcela A. Morini Pablo C. Schulz Gerardo Ferrat 《Colloid and polymer science》2002,280(4):328-335
We used a battery of different methods to study the association in aqueous sodium dehydrocholate (NaDHC) solutions. This
salt associates by a stepwise mechanism. Below (9.6 ± 4.2) × 10−4 mol dm−3 there is a molecular solution with some strongly insoluble dehydrocholic acid produced by hydrolysis. Between (9.6 ± 4.2) × 10−4 and (5.2 ± 2.2) × 10−3 mol dm−3, an aggregate similar to acid soap (NaDHC.HDHC) appears and its amount and the aggregate's size increase with concentration.
At =(2.20 ± 0.85) × 10−2 mol dm−3 the aggregates formed have properties usually associated with true micelles, such as solubilisation of water-insoluble dyes.
These aggregates increase in size with concentration and change their shape at 8 × 10−2 mol dm−3, giving nonsymmetrical aggregates. The changes in the solution physicochemical properties at these concentrations may be
misinterpreted and this explains the different values of the critical micelle concentration reported in the literature for
substances with similar structure, such as bile salts.
Received: 14 May 2001 Accepted: 10 August 2001 相似文献