全文获取类型
收费全文 | 1099篇 |
免费 | 42篇 |
国内免费 | 6篇 |
专业分类
化学 | 899篇 |
晶体学 | 6篇 |
力学 | 13篇 |
数学 | 101篇 |
物理学 | 128篇 |
出版年
2023年 | 4篇 |
2022年 | 18篇 |
2021年 | 32篇 |
2020年 | 32篇 |
2019年 | 16篇 |
2018年 | 22篇 |
2017年 | 17篇 |
2016年 | 34篇 |
2015年 | 25篇 |
2014年 | 32篇 |
2013年 | 53篇 |
2012年 | 75篇 |
2011年 | 110篇 |
2010年 | 47篇 |
2009年 | 33篇 |
2008年 | 86篇 |
2007年 | 63篇 |
2006年 | 77篇 |
2005年 | 85篇 |
2004年 | 59篇 |
2003年 | 45篇 |
2002年 | 51篇 |
2001年 | 14篇 |
2000年 | 10篇 |
1999年 | 8篇 |
1998年 | 8篇 |
1997年 | 16篇 |
1996年 | 17篇 |
1995年 | 3篇 |
1994年 | 5篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1987年 | 4篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有1147条查询结果,搜索用时 31 毫秒
81.
Carmen L. Huffman Melissa Lee Williams D. Michelle Benoist Richard E. Overstreet Emily E. Jellen‐McCullough 《Rapid communications in mass spectrometry : RCM》2011,25(16):2299-2306
In collision‐induced dissociation mass spectrometry experiments, the collision energy required for dissociation linearly depends on the degrees of freedom in the precursor ion. The magnitude of the slope of this relationship previously has been shown to qualitatively correlate to the relative binding strength of a noncovalently bound, monovalent complex. The goal of the work presented here is to determine if a similar methodology can be applied for assessing relative binding strengths in multivalent species. We have tested the method on complexes formed from 18‐crown‐6 and a variety of protonated, primary alkylamines, [CnH2n+1NH3]+ (n = 9, 12, 14, 16 and 18) and alkyldiamines, [H3NCnH2nNH3]2+ (n = 3, 5, 6, 9 and 12), and compared our results with dissociation energies calculated using density functional theory at the B3LYP/6‐31G* level. We found that the method correctly assessed the stronger crown ether/headgroup interaction in the two divalent species (1:1 and 2:1 complexes formed from the diaminoalkanes) compared with the weaker interaction in the monovalent species (1:1 complexes formed from mono‐aminoalkanes). However, the experimental method could not distinguish between the binding strengths of the two divalent complexes, perhaps because their calculated dissociation energies were quite similar. Our preliminary results suggest that this method could potentially be used for a quick and simple analysis of binding strengths in multivalent species if the binding strengths of the species are significantly different from one another. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
82.
Ernest W. Della Paul E. Pigou Michelle K. Livett J.Barrie Peel 《Journal of Electron Spectroscopy and Related Phenomena》1984,33(2):163-169
The He(I) photoelectron spectrum of bicyclo[2.1.1]hexane shows excellent correlation with a theoretical spectrum based on the eigenvalues obtained in an ab initio calculation at the theoretical molecular geometry. By contrast, a calculation performed at the experimental geometry exhibits an effect of enhanced ring strain in showing a first ionization energy which is considerably lower than the experimental value. This observation offers experimental support for the recent claim that the published structure for bicyclo [2.1.1]hexane based on electron-diffraction measurements is incorrect. It also accounts for inconsistencies observed in the calculated values of some spin—spin coupling constants for substituted bicyclo[2.1.1]hexanes. 相似文献
83.
Al(OTf)3 was found to be an extremely effective catalyst (at ppm levels) for ring opening reactions of epoxides using a range of alcohols. 相似文献
84.
While the underlying chemistry of enzyme-catalyzed reactions may be almost identical, the actual turnover rates of different substrates can vary significantly. This is seen in the turnover rates for the catalyzed hydrolysis of organophosphates by the bacterial phosphotriesterase OpdA. We investigate the variation in turnover rates by examining the hydrolysis of three classes of substrates: phosphotriesters, phosphothionates, and phosphorothiolates. Theoretical calculations were used to analyze the reactivity of these substrates and the energy barriers to their hydrolysis. This information was then compared to information derived from enzyme kinetics and crystallographic studies, providing new insights into the mechanism of this enzyme. We demonstrate that the enzyme catalyzes the hydrolysis of organophosphates through steric constraint of the reactants, and that the equilibrium between productively and unproductively bound substrates makes a significant contribution to the turnover rate of highly reactive substrates. These results highlight the importance of correct orientation of reactants within the active sites of enzymes to enable efficient catalysis. 相似文献
85.
The effect of adsorbed S at different coverages on the adhesion of Fe(110) surfaces in match and mismatch is examined using density functional theory (DFT). S is adsorbed in atop, bridge, and 4-fold hollow sites on one side of the interface in c(2 x 2) and p(1 x 1) arrangements, corresponding to coverages of 1/2 and 1 monolayer, respectively. The calculated adhesion energy values at different interfacial separations are fitted to the universal binding energy relation, and the effect of the S coverages on the adhesive strength is analyzed. The effect of relaxation of the interfaces at equilibrium is also investigated, and the resulting interfacial structures and related magnetic and charge density properties are compared. 相似文献
86.
Snowden M Bermudez A Kelly DR Radkiewicz-Poutsma JL 《The Journal of organic chemistry》2004,69(21):7148-7156
Explanations for stereoselectivity in the Baeyer-Villiger reaction have relied on the assumption that the antiperiplanar (app) group migrates. However, the magnitude of the preference for app-migration over gauche migration is unknown. To investigate this, the energy differences between the two were estimated from ab initio calculations. App-migration was found to be the preferred pathway since no transition structure could be located for gauche migration. Barriers for gauche migration were estimated by performing constrained optimizations. App-migration was found to be strongly favored with a barrier that is at least 3.8 kcal/mol and as much as 58.0 kcal/mol lower in energy than the gauche migration barrier. 相似文献
87.
Detection of hypophosphite, phosphite, and orthophosphate in natural geothermal water by ion chromatography 总被引:3,自引:0,他引:3
McDowell MM Ivey MM Lee ME Firpo VV Salmassi TM Khachikian CS Foster KL 《Journal of chromatography. A》2004,1039(1-2):105-111
Current doctrine states that phosphorus is incorporated into cells in the pentavalent(V) oxidation state as orthophosphate. However, recent studies show that microorganisms contain enzymes used to metabolize reduced forms of phosphorous, including phosphite(III) and hypophosphite(I), which suggests that there is a natural source for these chemical species. This paper will discuss suppressed conductivity ion chromatography methods developed to detect hypophosphite, phosphite, and orthophosphate in a geothermal water matrix containing fluoride, chloride, bromide, nitrate, hydrogen carbonate and sulfate. All peaks were clearly resolved, and calibrations were linear with estimated 3sigma detection limits of 0.83, 0.39, and 0.35 microM for hypophosphite, phosphite, and orthophosphate, respectively. 相似文献
88.
[reaction: see text] New anionic oxygen to alpha- and beta-vinyl carbamoyl migration reactions, 17a and 26a-c --> 18 and 30a-c, proceed under LDA-mediated conditions leading stereoselectively to highly substituted stilbenes bearing electron-donating and -withdrawing substituents. Compounds 17a and 26a-c are prepared by combination of efficient, directed ortho metalation, Sonogashira, and Suzuki-Miyaura cross-coupling procedures. 相似文献
89.
Lubbad I Mayinda JP Chatelut M Vittori O 《Analytical and bioanalytical chemistry》2004,380(3):551-555
The contamination of drinking water and industrial wastewaters is a critical environmental problem. The nitrophenol, dinitrophenol, cadmium, and lead contaminants are classified as hazardous compounds. Their rapid determination may be obtained using differential pulse polarography with concentrated electrolyte. CaCl2, which is very soluble to levels exceeding 5 mol l–1, allows separation of coalescent peaks at 0.1 mol l–1. A systematic study undertaken from 0.1 to 5 mol l–1 shows good separation of lead and cadmium from the organic compounds, and optimization of the electrolyte concentration according to the objective is described. Preconcentration of real samples is necessary because pollution levels are usually very low. 相似文献
90.
Evidence is provided that nucleophilic attack on five-membered ring oxocarbenium ions occurs from the inside face of the envelope. An eight-five fused-bicyclic system in which two substituents are constrained to pseudoequatorial positions underwent nucleophilic addition with selectivity that was comparable to an unconstrained monocyclic system. On the other hand, a bicyclic six-five analogue underwent reaction with low selectivity. This observation indicates that minimization of eclipsing interactions by attacking inside the envelope is not enough to control selectivity, but that the changes in the overall three-dimensional structure of the ring must be favorable as well. In the bicyclic six-five series, the six-membered ring is accommodated in the cation, but it destabilizes the transition state structure leading to the first-formed product of inside attack. 相似文献