全文获取类型
收费全文 | 439篇 |
免费 | 4篇 |
专业分类
化学 | 275篇 |
晶体学 | 10篇 |
力学 | 3篇 |
数学 | 65篇 |
物理学 | 90篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 2篇 |
2019年 | 3篇 |
2018年 | 6篇 |
2017年 | 8篇 |
2016年 | 4篇 |
2015年 | 6篇 |
2014年 | 4篇 |
2013年 | 16篇 |
2012年 | 22篇 |
2011年 | 20篇 |
2010年 | 9篇 |
2009年 | 6篇 |
2008年 | 23篇 |
2007年 | 20篇 |
2006年 | 20篇 |
2005年 | 24篇 |
2004年 | 15篇 |
2003年 | 11篇 |
2002年 | 13篇 |
1999年 | 8篇 |
1997年 | 3篇 |
1996年 | 4篇 |
1994年 | 2篇 |
1992年 | 3篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 8篇 |
1984年 | 14篇 |
1983年 | 15篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 12篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 17篇 |
1976年 | 6篇 |
1975年 | 8篇 |
1974年 | 7篇 |
1973年 | 14篇 |
1971年 | 8篇 |
1970年 | 5篇 |
1969年 | 7篇 |
1967年 | 3篇 |
1909年 | 2篇 |
1882年 | 2篇 |
排序方式: 共有443条查询结果,搜索用时 15 毫秒
111.
Michael D. Sliger Rachel K. Traylor Sidney H. Young Norris W. Hoffman Robin D. Rogers 《Journal of organometallic chemistry》2005,690(15):3540-3545
Ionic liquid solvents N-hexylpyridinium bistrifylimide ([C6pyr][Tf2N]] and 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) promoted the displacement of anionic ligands by pyridine derivatives in trans-(Ph3P)2Rh(CO)NO3 to a much greater extent than did dichloromethane. Thus, addition of a slight excess of 2-fluoropyridine to trans-(Ph3P)2Rh(CO)NO3 in [C4mim][PF6] gave a 29:71 product mixture of trans-(Ph3P)2Rh(CO)NO3:[trans-(Ph3P)2Rh(CO)(2-fluoropyridine)][NO3], while the ratio was 91:9 in dichloromethane. 相似文献
112.
The phosphorescence of the 4-bromo-l-naphthoyl group is readily quenched by molecular oxygen in homogeneous solvents. However, when this lumophore is complexed with γ-cyclodextrin in aqueous solution at room temperature, its phosphorescence is observed even under 1 atm of oxygen! Phosphorescence decay data indicated that two types of probe/cyclodcxtrin complexes are formed with lifetimes of 600 u,s and 3.5 ms. Oxygen completely quenches the fast decay, but only partially quenches the slow decay. 相似文献
113.
Treatment of an electron-rich benzyl ether with DDQ at ambient temperature followed by addition of a silyl enol ether undergoes a C-C bond-forming reaction to afford 3-alkoxy-3-phenyl-propionyl compound. This is a general reaction and works well with a variety of silyl enol ethers to give carbonyl products in yields ranging from 10 to 85%. 相似文献
114.
Vogt FG Freyer AJ Levinson SH Shu AY Heys JR 《Magnetic resonance in chemistry : MRC》2005,43(2):147-155
A better understanding of the structure of complex 3H-labeled molecules can be obtained by complete assignment of their 1H and 3H solution-state NMR spectra. The assignment process is aided by the detection of heteronuclear chemical shift correlations between 1H and 3H nuclei. Heteronuclear correlation (HETCOR) experiments previously applied to this task exhibit several drawbacks caused by the nature of both the pulse sequences and 1H-3H spin systems. The range of J-couplings involved in 1H-3H coupling networks make it challenging to perform correlation experiments using methods that rely on coherences created during free precession periods and interrupted by transfer pulses. Two alternative HETCOR experiments are demonstrated for 1H-3H systems in the present work and are shown to have advantages over earlier methods. The first experiment is known as hetero-TOCSY and correlates heteronuclear chemical shifts using J-cross polarization. This experiment achieves both homonuclear and heteronuclear mixing and connects the chemical shifts of all 1H and 3H nuclei in a coupling network. A second HETCOR experiment uses the heteronuclear Overhauser effect to obtain through-space correlations between nearby nuclei. The 1H-3H HETCOR experiments are phase sensitive and typically contain more correlations than other methods, which is beneficial for assignment purposes, while being sensitive enough to be applicable to routine analytical samples. The experiments were used to analyze 3H incorporation in sub-milligram quantities of 3H-labeled pharmaceutical derivatives with complex labeling schemes. 相似文献
115.
The recent demise of certain global unbroken symmetry generators in the presence of a grand unified magnetic monopole leads us to consider more carefully the notion of charges associated with gauge symmetries. It turns out that global transformations associated with the generators of the gauge group, and their charges, make sense only for extended systems which are sufficiently localized. GUT monopoles fail this criterion. Detailed consideration of the monopole-antimonopole system helps remove apparent paradoxes related to the chromodyon excitations of a single monopole and agrees with the previous result that some, but not all, of the states naively expected do exist. The remaining states ns needed to fill out color multiplets are spread throughout space; they are recovered as long-lived excitations when an antimonopole is brought in from infinity. 相似文献
116.
José Maurício A. Caiut Lucas A. Rocha Fernando A. Sigoli Younes Messaddeq Jeannette Dexpert-Ghys Sidney J.L. Ribeiro 《Journal of Non》2008,354(42-44):4795-4799
Boehmite–GPTS (GPTS-glycidoxypropyltrimethoxysilane) hybrids (BGS) have been prepared by the sol–gel methodology. Spectroscopic properties and structural features have been evaluated from the initial sols to xerogels as a function of composition. Eu3+-containing BGS samples were also prepared, and the Eu3+ 5D0 state emission quantum efficiency was observed to depend on the boehmite–siloxane interactions. Boehmite–siloxane heterocondensation was inferred from vibrational spectroscopy and 13C, 29Si, and 27Al nuclear magnetic resonance results. Unreacted epoxi groups could be identified in the final material being an important characteristic for the future applications. Good optical quality and mechanical properties lead to the preparation of thin films deposited on glass substrates by dip and spin-coating. Refractive index could be tuned by the relative boehmite and GPTS content. Waveguide properties have also been studied by m-lines spectroscopy and guided modes together with attenuation measurements suggest potential application of the prepared films in integrated optics. 相似文献
117.
Emerson A. dos Santos Luciana R.P. Kassab Niklaus U. Wetter Sidney J.L. Ribeiro 《Journal of luminescence》2007,124(2):200-206
A simulation of erbium-doped glass systems, which provides population density for the excited states involved in the 1.5 μm and also for 2.7 μm emissions when pumped around 980 nm, is presented. To describe the diode pump laser processes, a theoretical model based in a coupled system of differential rate equations was developed. The approach used and the obtained spectroscopic parameters are discussed. The materials under study are two oxide glasses, lead fluoroborate (PbO-PbF2-B2O3), and heavy metal oxide (Bi2O3-PbO-Ga2O3) and a fluoride glass (ZrF4-BaF2-LaF3-AlF3-NaF), all of them doped with Er3+. 相似文献
118.
Dias Filho FA Carlos LD Messadeq Y Ribeiro SJ 《Langmuir : the ACS journal of surfaces and colloids》2005,21(5):1776-1783
The interaction between metaphosphate chains and the metal ions Ca2+ and Eu3+ has been studied in water by Eu3+ luminescence, infrared absorption, and 31P NMR spectroscopy. Two main families of sites could be identified for the metal ions in the aqueous polyphosphate colloidal systems: (1) cagelike sites provided by the polyphosphate chain and (2) a family which arises following saturation of cagelike sites. Occupation of this second family leads to supramolecular interactions between polyphosphate chains and the consequent destabilization of the colloidal system. In the polyphosphate-Ca2+ system, this destabilization appears as a coacervation process. Equilibrium existing between colloidal species as a function of the compositions could be reasoned based on the spectroscopic measurements. The determination of coordination numbers and the correlation of the results with the observation of coacervates show that Eu3+ luminescence properties can be used to probe in a unique way the coacervation process. 相似文献
119.
B-mode ultrasound images are characterised by the speckle artefact, which introduces fine-false structures whose apparent resolution is beyond the imaging system capabilities. Speckle presence is due to interference effects between overlapping echoes and its occurrence is related to a great number of randomly distributed structure scatterers within a resolution cell. Basing our analysis on linear system theory, we show that a dense random set of scatterers can be substituted by an equivalent one with a much smaller number of periodic scatterers. This new structure with regularly distributed scatterers is able to give rise to the same B-mode image and the same speckle pattern, for a given ultrasound pulse. This new approach helps the understanding of the deterministic nature of speckle and may reduce drastically the computing time in numerical simulations. Additionally, it can contribute to periodicity analysis used in tissue characterisation. 相似文献
120.
Nakanishi I Nishizawa C Ohkubo K Takeshita K Suzuki KT Ozawa T Hecht SM Tanno M Sueyoshi S Miyata N Okuda H Fukuzumi S Ikota N Fukuhara K 《Organic & biomolecular chemistry》2005,3(18):3263-3265
Photoreduction of pyridine N-oxide, which has a key structure of antitumor agents for hypoxic solid tumors, by 1-benzyl-1,4-dihydronicotinamide in deaerated aprotic media resulted in generation of hydroxyl radical, leading to the oxidation of salicylic acid to 2,3- and 2,5-dihydroxybenzoic acids, and catechol. 相似文献