全文获取类型
收费全文 | 3864篇 |
免费 | 103篇 |
国内免费 | 22篇 |
专业分类
化学 | 2720篇 |
晶体学 | 31篇 |
力学 | 99篇 |
数学 | 503篇 |
物理学 | 636篇 |
出版年
2022年 | 22篇 |
2021年 | 32篇 |
2020年 | 40篇 |
2019年 | 41篇 |
2018年 | 30篇 |
2017年 | 34篇 |
2016年 | 69篇 |
2015年 | 62篇 |
2014年 | 64篇 |
2013年 | 194篇 |
2012年 | 185篇 |
2011年 | 226篇 |
2010年 | 106篇 |
2009年 | 90篇 |
2008年 | 218篇 |
2007年 | 221篇 |
2006年 | 202篇 |
2005年 | 191篇 |
2004年 | 196篇 |
2003年 | 156篇 |
2002年 | 147篇 |
2001年 | 43篇 |
2000年 | 39篇 |
1999年 | 40篇 |
1998年 | 39篇 |
1997年 | 45篇 |
1996年 | 59篇 |
1995年 | 54篇 |
1994年 | 56篇 |
1993年 | 43篇 |
1992年 | 38篇 |
1991年 | 40篇 |
1990年 | 50篇 |
1989年 | 38篇 |
1988年 | 51篇 |
1987年 | 52篇 |
1986年 | 45篇 |
1985年 | 78篇 |
1984年 | 57篇 |
1983年 | 57篇 |
1982年 | 69篇 |
1981年 | 63篇 |
1980年 | 66篇 |
1979年 | 46篇 |
1978年 | 42篇 |
1977年 | 42篇 |
1976年 | 27篇 |
1975年 | 28篇 |
1974年 | 36篇 |
1973年 | 37篇 |
排序方式: 共有3989条查询结果,搜索用时 31 毫秒
111.
Barnes JH Schilling GD Stone SF Sperline RP Denton MB Young ET Barinaga CJ Koppenaal DW Hieftje GM 《Analytical and bioanalytical chemistry》2004,380(2):227-234
The use of a separation step, such as liquid chromatography, prior to inductively coupled plasma mass spectrometry (ICP–MS) has become a common tool for highly selective and sensitive analyses. This type of coupling has several benefits including the ability to perform speciation analysis or to remove isobaric interferences. Several limitations of conventional instruments result from the necessity to scan or pulse the mass spectrometer to obtain a complete mass spectrum. When the instrument is operated in such a non-continuous manner, duty cycle is reduced, resulting in poorer absolute limits of detection. Additionally, with scanning instruments, spectral skew can be introduced into the measurement, limiting quantitation accuracy. To address these shortcomings, a high-performance liquid chromatograph has been coupled to an ICP–MS capable of continuous sample introduction and simultaneous multimass detection. These features have been realized with a novel detector array, the focal plane camera. Instrument performance has been tested for both speciation analysis and for the elimination of isobaric interferences. Absolute limits of detection in the sub picogram to tens of picograms regime are obtainable, while the added mass dimension introduced by simultaneous detection dramatically increases chromatographic peak capacity. 相似文献
112.
Dombrowski GW Dinnocenzo JP Zielinski PA Farid S Wosinska ZM Gould IR 《The Journal of organic chemistry》2005,70(10):3791-3800
[reaction: see text] Deprotonation of the radical cations of aromatic amines, such as anilines, generally occurs much more slowly than other fragmentation reactions. Here we report a stereoelectronic effect involving twisting of the anilino group out of the plane of the benzene ring that results in a significantly increased rate of reactivity toward deprotonation. Quantitative studies of the rate constants for deprotonation as a function of aniline radical cation pKa (Br?nsted plots) demonstrate that the effect is not simply due to a change in the reaction thermodynamics. By combining this stereoelectronic effect with covalent attachment of carboxylate as a base, aniline radical cations that undergo unimolecular deprotonation with rate constants as high as 10(8) s(-1), even in unfavorable protic media, are described. 相似文献
113.
Antoine R Broyer M Dugourd P Breaux G Hagemeister FC Pippen D Hudgins RR Jarrold MF 《Journal of the American Chemical Society》2003,125(30):8996-8997
Molecular beam electric deflection measurements have been used to determine electric susceptibilities for small unsolvated alanine-based peptides. The electric susceptibility provides information about the charge distribution within the peptide and can be used to distinguish between zwitterionic and canonical forms. Measured electric susceptibilities for WAn peptides (n = 1-5) are similar to those for capped Ac-WAn-NH2 peptides (which cannot form zwitterions). Susceptibilities calculated using a simulated tempering-based approach are substantially larger for the zwitterionic form than for the canonical form. The measured susceptibilities are in good agreement with those calculated for the canonical form. For the larger peptides, the lowest potential energy structure found in the simulations is hairpin-like, while the lowest free energy structure found at room temperature is extended. The zwitterionic form is constrained by intramolecular interactions which make it entropically unfavorable. 相似文献
114.
N,N'-Dimethylthiourea and 3,4,5,6-tetrahydro-2-pyrimidinethiol were allowed to react with 2-chloro-nicotinonitrile ( 1 ) and their products investigated by standard methods and by carbon-13 nmr. In both instances, displacement of the chlorine occurred by nitrogen not the sulfur of the thioureas. Secondary cyclizations occurred by attack of nitrogen on the nitrile to furnish 3a , and by sulfur on the nitrile to give 4b , a new ring system. Tricyclic 4b was hydrolyzed in dilute acid to give 5 , or alkylated with methyl iodide in the presence of sodium hydride to give the ring opened product 6 . 相似文献
115.
Gary Paul Rabold 《Journal of polymer science. Part A, Polymer chemistry》1969,7(5):1187-1201
The interaction of sodium dodecyl sulfate (NaDS) with styrene—butadiene latexes was investigated by application of the recently developed spin-probe technique. It is possible by this method to detect the presence of micelles in a latex reasonably quantitatively. The effective surface area occupied per molecule of soap was calculated to be 53 Å2. Differences in purity can readily be detected for several NaDS samples. The less pure samples show a lower critical micelle concentration (CMC) and a significantly more ordered micelle structure. The addition of hexane to NaDS solutions above the CMC leads to a greater immobilization of the spin probe. This is interpreted as reflecting a tighter packing of the soap molecules. The addition of ethanol, acetic acid, and acrylic acid leads to a looser association of the probe with the micelles. The micelles catalyze the decomposition of the nitroxide probe in the presence of organic acids. The aggregation of sodium deoxycholate was also conveniently followed by the spin-probe method. 相似文献
116.
Pulsed IR radiation from a CO2 TEA laser has been shown to cause dehydrohalogenation of CF3CH3, CF2ClCH3 and C2H5Cl under collisionless conditions. IR emission has been observed and is attributed to HF2 and CF2CH22 from CF3CH3, and to HF2 and HCl2 from CF2ClCH3. 相似文献
117.
In an extension of recently reported syntheses of the I-azaphenoxathiin nucleus as well as several 7-substituted analogs, the synthesis of several 9-substituted members of this series is now reported. In addition, the first 13C-nmr spectral evidence of an interaction between a sulfur atom and the oxygen of an ortho-nilro group which has been previously observed only in X-ray erystallographic studies is also described. The possible consequences of this interaction on the reaction pathway leading to the cyclization of the 9-substituted J-azaphenoxathiin nucleus is also presented. 相似文献
118.
4-Hydroxybenzofuran-3-ones show a large bathochromic shift in UV spectrum on the addition of sodium acetate. Such a large shift is not observed in 6-hydroxycompounds. 相似文献
119.
Summary A sensitive method has been developed for analysis of trace amounts of arsenic in biological materials using the heteropoly blue method. The method employs a closed apparatus and a nitrogen atmosphere, and allows the detection of arsenic in ppm concentration using samples of 100 mg.
Presented at the 6th Annual Northeast Regional Meeting of the American Chemical Society, Burlington, Vermont, August 19, 1974. 相似文献
Zusammenfassung Eine empfindliche Methode zur Bestimmung von Arsenspuren in biologischem Material im Wege der Molybdänblaumethode wurde ausgearbeitet. Man arbeitet dabei in einer geschlossenen Apparatur in Stickstoffatmosphäre und kann so in 100-mg-Proben Arsenkonzentrationen in der Größenordnung von ppm bestimmen.
Presented at the 6th Annual Northeast Regional Meeting of the American Chemical Society, Burlington, Vermont, August 19, 1974. 相似文献
120.
Salvador Puig-Torres Gary E. Martin Joseph J. Ford M. Robert Willcott Keith Smith 《Journal of heterocyclic chemistry》1982,19(6):1441-1446
The reaction of the dianion of 3-mercaptopyridin-2(1H)-thione with 2-chloronitrobenzene in N,N-dimethylformamide leads to the formation of 1-azathianthrene, the first reported mono-aza analog of the thianthrene ring system. A partial assignment of the 13C-nmr spectrum of the title compound is reported, the assignment based on chemical shift arguments, spin-lattice (T1) relaxation times and 1H-13C spin coupling constants. Amplitude modulated two-dimensional Fourier transform (AM2DFT) techniques were employed for the acquisition of the heteronuclear spin-coupling constants. 相似文献