全文获取类型
收费全文 | 231篇 |
免费 | 11篇 |
国内免费 | 10篇 |
专业分类
化学 | 128篇 |
力学 | 8篇 |
综合类 | 3篇 |
数学 | 45篇 |
物理学 | 68篇 |
出版年
2018年 | 3篇 |
2016年 | 2篇 |
2015年 | 2篇 |
2014年 | 9篇 |
2013年 | 39篇 |
2012年 | 6篇 |
2011年 | 10篇 |
2010年 | 4篇 |
2009年 | 1篇 |
2008年 | 7篇 |
2007年 | 7篇 |
2006年 | 7篇 |
2005年 | 10篇 |
2004年 | 9篇 |
2003年 | 6篇 |
2002年 | 17篇 |
2001年 | 7篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 6篇 |
1997年 | 12篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1958年 | 1篇 |
1955年 | 1篇 |
1948年 | 1篇 |
排序方式: 共有252条查询结果,搜索用时 15 毫秒
61.
McCrory CC Devadoss A Ottenwaelder X Lowe RD Stack TD Chidsey CE 《Journal of the American Chemical Society》2011,133(11):3696-3699
A Cu(I) complex of 3-ethynyl-phenanthroline covalently immobilized onto an azide-modified glassy carbon surface is an active electrocatalyst for the four-electron (4-e) reduction of O(2) to H(2)O. The rate of O(2) reduction is second-order in Cu coverage at moderate overpotential, suggesting that two Cu(I) species are necessary for efficient 4-e reduction of O(2). Mechanisms for O(2) reduction are proposed that are consistent with the observations for this covalently immobilized system and previously reported results for a similar physisorbed Cu(I) system. 相似文献
62.
Silva SA DE Castro JC Da Silva TG Da-Cunha EV Barbosa-Filho JM Da Silva MS 《Natural product letters》2001,15(5):323-329
A new phenylcoumarone type trinorlignan, krametosan (1), along with the known norlignans, ratanhiaphenol I (2) and 2-(2'-hydroxy-4',6'-dimethoxyphenyl)-5-(E)-propenylbenzofuran (3), the lignan conocarpan (4) and dinorlignan decurrenal (5), were isolated from the CHCl3 extract of the roots of Krameria tomentosa. The structure of these compounds were elucidated by the spectroscopic methods. 相似文献
63.
Kang P Bobyr E Dustman J Hodgson KO Hedman B Solomon EI Stack TD 《Inorganic chemistry》2010,49(23):11030-11038
N,N,N',N'-tetramethylethylenediamine (TMED), the simplest and most extensively used peralkylated diamine ligand, is conspicuously absent from those known to form a bis(μ-oxo)dicopper(III) (O) species, [(TMED)(2)Cu(III)(2)(μ(2)-O)(2)](2+), upon oxygenation of its Cu(I) complex. Presented here is the characterization of this O species and its reactivity toward exogenous substrates. Its formation is complicated both by the instability of the [(TMED)Cu(I)](1+) precursor and by competitive formation of a presumed mixed-valent trinuclear [(TMED)(3)Cu(III)Cu(II)(2)(μ(3)-O)(2)](3+) (T) species. Under most reaction conditions, the T species dominates, yet, the O species can be formed preferentially (>80%) upon oxygenation of acetone solutions, if the copper concentration is low (<2 mM) and [(TMED)Cu(I)](1+) is prepared immediately before use. The experimental data of this simplest O species provide a benchmark by which to evaluate density functional theory (DFT) computational methods for geometry optimization and spectroscopic predictions. The enhanced thermal stability of [(TMED)(2)Cu(III)(2)(μ(2)-O)(2)](2+) and its limited steric demands compared to other O species allows more efficient oxidation of exogenous substrates, including benzyl alcohol to benzaldehyde (80% yield), highlighting the importance of ligand structure to not only enhance the oxidant stability but also maintain accessibility to the nascent metal/O(2) oxidant. 相似文献
64.
A. Cruz‐Torres F. DE L. Castillo‐Alvarado J. Ortíz‐López J. S. Arellano 《International journal of quantum chemistry》2010,110(13):2495-2508
Ab initio density functional calculations were performed for a toroidal carbon C120 nanostructure. Hydrogen molecules, n = 1–15, were added inside the nanotorus and for each one of these systems a geometry optimization was obtained. The cohesive energy shows that these structures are energetically stable. For example, the binding energies are ?34.95 and ?36.19 Hartrees and the interatomic distances H? H are 0.753 and 0.772 Å for 1 and 14 molecules, respectively. Considering only molecular hydrogen, we have always seen so far weak physisorption into the C120 nanotorus. There is no chemisorption until the number oh hydrogen molecules are increased to 14. In this case, four hydrogen atoms are chemisorbed. With 15 molecules, there are 10 hydrogen atoms chemisorbed just at the inner nanotorus surface forming 10 H? C bondings with bond length close to that in methane. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2495–2508, 2010 相似文献
65.
66.
M.V. Stack Helen D. Donoghue J.E. Tyler 《Journal of Analytical and Applied Pyrolysis》1982,3(3):221-227
Curie-point pyrolysis with a relatively low resolution gas—liquid chromatographic system (packed Carbowax 20M columns) provided data on washed cells of oral streptococci. When the data were ordered into a hierarchy of comparisons the steps were matched not only by the increase in variability between sets but also by the proportion of the output showing relatively high variability. It was possible to segregate strains of Streptococcus mutans serotype c from those of different serotypes by use of a discriminant function derived from only three pyrogram peaks. 相似文献
67.
The movement of a particular component along an HPLC column is studied on the ba-sis of equilibrium equations. Numerical simulation of multi--step gradient elution HPLC isperformed by means o? a personal computer program. The location x, as a fraction of thecolumn length, for a given time t o? each compound is calculated, the velocity and corre-sponding acceleration are given as well. Diagrams for x,dx/dt and d~2x/dt~2 versus t display themovement process of the different components along the HPLC column during gradient elu-tion. The prediction of the retention time and peak width, and the optimization for the multi-step gradient elution HPLC are all based on such simulation and the molecular structure ofthe. components is separated as well. 相似文献
68.
[reaction: see text] A mu-oxo-iron(III) dimer, [((phen)(2)(H(2)O)Fe(III))(2)(mu-O)](ClO(4))(4), is an efficient epoxidation catalyst for a wide range of alkenes, including terminal alkenes, using peracetic acid as the oxidant. Low catalyst loadings, in situ catalyst preparation from common reagents, fast reaction times (<5 min at 0 degrees C), and enhanced reaction performance at high substrate concentrations combine to create a temporally and synthetically efficient procedure for alkene epoxidation. 相似文献
69.
Abstract— Photoreactivation in the extreme halophilic archaebacterium Halobacterium cutirubrum was studied both in vivo and in vitro. Cells irradiated with ultraviolet (UV)-fluences up to 350 J/m2 could be completely photoreactivated, indicating very efficient repair of pyrimidine dimers in UV-irradiated DNA. Dark repair is apparently absent in Halobacterium since liquid holding under non-growth conditions did not influence the survival of UV-irradiated cells, while cells remained completely photoreactivable with no change in the kinetics of photoreactivation. Experiments with Halobacterium isolates of different carotenoid content indicated that carotenoids do not influence either UV-inactivation or photoreactivation. Small differences in the rates of UV-inactivation and photoreactivation could be assigned to the occurrence of gas vesicles. Flash experiments and the temperature dependence of photoreactivation indicated an enzymatical reaction. This was confirmed by in vitro experiments with partially purified photoreactivating enzyme. The in vivo action spectrum of photoreactivation showed a main band in the 400-470 nm region with a maximum at 440 nm. Comparison with action spectra of other microorganisms classified the Halobacterium enzyme as a 8-hydroxy-5-deazaflavin type photoreactivating enzyme. 相似文献
70.
本文研究了系列金属卟啉对苯基氯(phCH~2Cl)和CO~2电羧化反应的催化活性.用熔点,MS,IR,UV鉴别产物为苯乙酸卟脂,利用高效液相色谱(HPLC)定量分析了羧化产物,选出最佳电羧化电解电位为-1.6v(相对于饱和甘汞电极,vs.SCE,下同). 并对催化活性较高的钴卟啉系列配合物进行了深入研究. 探讨了金属卟啉具有催化活性的原因,认为能形成M(Ⅰ)中间体的金属卟啉配合物.具有较高的催化活性. 相似文献