首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   781篇
  免费   18篇
  国内免费   3篇
化学   554篇
晶体学   11篇
力学   1篇
数学   166篇
物理学   70篇
  2022年   5篇
  2021年   15篇
  2020年   12篇
  2019年   16篇
  2018年   9篇
  2017年   11篇
  2016年   15篇
  2015年   22篇
  2014年   20篇
  2013年   41篇
  2012年   29篇
  2011年   31篇
  2010年   20篇
  2009年   31篇
  2008年   57篇
  2007年   45篇
  2006年   45篇
  2005年   48篇
  2004年   34篇
  2003年   36篇
  2002年   29篇
  2001年   16篇
  2000年   12篇
  1999年   4篇
  1998年   8篇
  1997年   10篇
  1996年   8篇
  1995年   7篇
  1994年   9篇
  1993年   8篇
  1992年   7篇
  1991年   4篇
  1990年   8篇
  1989年   3篇
  1988年   10篇
  1987年   3篇
  1985年   4篇
  1984年   9篇
  1983年   8篇
  1982年   10篇
  1980年   7篇
  1979年   6篇
  1978年   7篇
  1977年   11篇
  1975年   7篇
  1974年   3篇
  1973年   7篇
  1943年   4篇
  1938年   3篇
  1932年   2篇
排序方式: 共有802条查询结果,搜索用时 15 毫秒
41.
It is known that an n-dimensional convex body, which is typical in the sense of Baire category, shows a simple, but highly non-intuitive curvature behaviour: at almost all of its boundary points, in the sense of measure, all curvatures are zero, but there is also a dense and uncountable set of boundary points at which all curvatures are infinite. The purpose of this paper is to find a counterpart to this phenomenon for typical convex bodies of given constant width. Such bodies cannot have zero curvatures. A main result says that for a typical n-dimensional convex body of constant width 1 (without loss of generality), at almost all boundary points, in the sense of measure, all curvatures are equal to 1. (In contrast, note that a ball of width 1 has radius 1/2, hence all its curvatures are equal to 2.) Since the property of constant width is linear with respect to Minkowski addition, the proof requires recourse to a linear curvature notion, which is provided by the tangential radii of curvature.  相似文献   
42.
Gold nanoparticles reduced by sodium citrate (d ~ 10 nm) and purchased gold colloid particles (d ~ 500 nm) were examined and compared. The properties of both gold particles and their biofunctionalized derivatives with L-cysteine and L-glutathione were studied in the presence of sodium nitrate. The structural investigations indicated an aggregated inner structure.

The isoelectric points of pure gold, citrate reduced gold, and functionalized gold were measured and compared. The low isoelectric point of pure gold/water interface was explained by considering the distribution and accumulation of H+ and OH? ions within the interfacial water layer, being more pronounced for OH? ions.   相似文献   
43.
Journal of Radioanalytical and Nuclear Chemistry - A contribution to the discussion about Vila Nova de São Pedro (VNSP) as a production centre of symbolic lithic artefacts, the origin of raw...  相似文献   
44.
Given a sequence A = (a 1, …, a n ) of real numbers, a block B of A is either a set B = {a i , a i+1, …, a j } where ij or the empty set. The size b of a block B is the sum of its elements. We show that when each a i ∈ [0, 1] and k is a positive integer, there is a partition of A into k blocks B 1, …, B k with |b i ?b j | ≤ 1 for every i, j. We extend this result in several directions.  相似文献   
45.
Tetrakis-, tris-, bis-, and mono salicylic acid derivatives 14 were synthesized by reaction of methyl 2-hydroxy benzoate (methyl salicylate) with 2,2-bis (hydroxymethyl) propane-1,3-diol (pentaerythritol) in the presence of sodium. Yields of different salicyloyloxy derivatives were changed by varying the molar ratios of reactants. For compounds 2 and 3, X-ray structure analysis was performed, as well as molecular energy minimization, to define their conformation in terms of their energy minima. Comparison of crystal and energy minimized structures for these two compounds (2 and 3) revealed that the intramolecular hydrogen bonds play an important role, stabilizing conformation of the most part of the molecule. The antioxidant activity and cytotoxicity of the synthesized derivatives were evaluated in a series of in vitro tests, as well as 17β-hydroxysteroid dehydrogenase type 2 inhibition potency. Tetrakis salicyloyloxy derivative 1 expressed the highest antioxidant potency, tris salicyloyloxy derivative 2 was the best inhibitor of 17βHSD2 enzyme, while bis salicyloyloxy derivative 3 showed strong cytotoxicity against prostate and breast cancer cells with no cytotoxicity against healthy cells.  相似文献   
46.
The scope of the enantioselective Mukaiyama–Michael reactions catalyzed by trans‐2,5‐diphenylpyrrolidine has been expanded to include both α‐ and β‐substituted enals. However, the rationalization of the observed enantioselectivity is far from obvious since the catalyst is not very sterically hindered. DFT calculations were carried out to rationalize the observed stereoselectivities. Transition states of the C?C bond formation between iminium intermediates and silyloxyfurans were located and their relative energies were used to estimate the stereoselectivity data. We find excellent agreement between the predicted and observed stereoselectivities. The analysis of intermolecular forces reveals that the enantioselectivity is mostly due to stabilizing noncovalent interactions between the reacting partners, not due to steric hindrance. The role of attractive noncovalent interactions in enantioselective catalysis may be underappreciated.  相似文献   
47.
A new pyclen-3,9-diacetate derivative ligand (H23,9-OPC2A) was synthesized possessing an etheric O-atom opposite to the pyridine ring, to improve the dissociation kinetics of its Mn(II) complex (pyclen = 3,6,9,15-tetraazabicyclo(9.3.1)pentadeca-1(15),11,13-triene). The new ligand is less basic than the N-containing analogue (H23,9-PC2A) due to the non-protonable O-atom. In spite of its lower basicity, the conditional stability of the [Mn(3,9-OPC2A)] (pMn = −log(Mn(II)), cL = cMn(II) = 0.01 mM. pH = 7.4) remains unaffected (pMn = 8.69), compared to the [Mn(3,9-PC2A)] (pMn = 8.64). The [Mn(3,9-OPC2A)] possesses one water molecule, having a lower exchange rate with bulk solvents (kex298 = 5.3 ± 0.4 × 107 s−1) than [Mn(3,9-PC2A)] (kex298 = 1.26 × 108 s−1). These mild differences are rationalized by density-functional theory (DFT) calculations. The acid assisted dissociation of [Mn(3,9-OPC2A)] is considerably slower (k1 = 2.81 ± 0.07 M−1 s−1) than that of the complexes of diacetates or bisamides of various 12-membered macrocycles and the parent H23,9-PC2A. The [Mn(3,9-OPC2A)] is inert in rat/human serum as confirmed by 52Mn labeling (nM range), as well as by relaxometry (mM range). However, a 600-fold excess of EDTA (pH = 7.4) or a mixture of essential metal ions, propagated some transchelation/transmetalation in 7 days. The H23,9-OPC2A is labeled efficiently with 52Mn at elevated temperatures, yet at 37 °C the parent H23,9-PC2A performs slightly better. Ultimately, the H23,9-OPC2A shows advantageous features for further ligand designs for bifunctional chelators.  相似文献   
48.
Cloudpoint data for polystyrene/methylcyclohexane solutions extending over moderate ranges of pressure and molecular weight are available in the literature. Those data are supplemented with new results from this laboratory to fill in gaps and extend the MW range (to 761 ≤ MW/amu ≤ 2 × 107). The resulting data net is discussed and reasons to extend studies to higher pressure are presented. © 1996 John Wiley & Sons, Inc.  相似文献   
49.
The accurate prediction of the solar diffuse fraction (DF), sometimes called the diffuse ratio, is an important topic for solar energy research. In the present study, the current state of Diffuse irradiance research is discussed and then three robust, machine learning (ML) models are examined using a large dataset (almost eight years) of hourly readings from Almeria, Spain. The ML models used herein, are a hybrid adaptive network-based fuzzy inference system (ANFIS), a single multi-layer perceptron (MLP) and a hybrid multi-layer perceptron grey wolf optimizer (MLP-GWO). These models were evaluated for their predictive precision, using various solar and DF irradiance data, from Spain. The results were then evaluated using frequently used evaluation criteria, the mean absolute error (MAE), mean error (ME) and the root mean square error (RMSE). The results showed that the MLP-GWO model, followed by the ANFIS model, provided a higher performance in both the training and the testing procedures.  相似文献   
50.
Summary The enantioselective hydrogenation of ethyl pyruvate (EtPy) was studied on Pt-alumina catalysts modified by C3-substituted cinchonidines (NC, A, B, C1, C2in Fig. 1) and for comparison by DHCDand MeO-DHCDin AcOH. The effect of the C3-substituent on the reaction rate and the enantioselectivity were examined. Using the Engelhard 4759 catalyst under mild experimental conditions (room temperature, hydrogen pressure 1 bar) such as DHCDthe (R)-ethyl lactate formed in excess (e.e.max: 79-91%)</o:p>  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号