全文获取类型
收费全文 | 649篇 |
免费 | 15篇 |
国内免费 | 4篇 |
专业分类
化学 | 407篇 |
晶体学 | 9篇 |
力学 | 39篇 |
数学 | 76篇 |
物理学 | 137篇 |
出版年
2021年 | 5篇 |
2020年 | 14篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 5篇 |
2016年 | 19篇 |
2015年 | 13篇 |
2014年 | 14篇 |
2013年 | 29篇 |
2012年 | 32篇 |
2011年 | 45篇 |
2010年 | 19篇 |
2009年 | 17篇 |
2008年 | 31篇 |
2007年 | 42篇 |
2006年 | 27篇 |
2005年 | 22篇 |
2004年 | 23篇 |
2003年 | 21篇 |
2002年 | 14篇 |
2001年 | 12篇 |
2000年 | 9篇 |
1999年 | 7篇 |
1998年 | 6篇 |
1997年 | 6篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1993年 | 4篇 |
1992年 | 14篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1988年 | 13篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1975年 | 6篇 |
1974年 | 9篇 |
1973年 | 6篇 |
1972年 | 8篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1968年 | 4篇 |
1935年 | 10篇 |
排序方式: 共有668条查询结果,搜索用时 15 毫秒
21.
22.
Promode C. Bardalaye Willis B. Wheeler 《International journal of environmental analytical chemistry》2013,93(1-3):105-113
Abstract A facile and efficient method is described for the determination of trace quantities of triazine herbicides, terbutryn, prometryn and ametryn in water. The procedure involved preconcentration of water samples by sorption on chromatographic grade silica gel particles with chemically modified surface, being covalently bonded with a nonofunctional C8H17 group. This was followed by solvent desorption with 2-propanol. The determinative step was achieved by capillary gas chromatography on Supelcowax-10 fused silica column using a nitrogen-phosphorus detector. The limit of detection was 0.1 μg-10 μgL?1. 相似文献
23.
The production of track-spots in CR-39 detectors for alpha particles and the subsequent optical counting of the detectors are often influenced by foreign substances deposited on the surface of the detectors. The removal of these substances can be achieved using chemicals for pre-irradiation treatment. Two methods of pre-irradiation treatment were studied in the present experiments. One of them involves the dipping of the detectors in ethanol. In the other method, the detectors were treated in 6N KOH etchant prior to exposure. It was found that prolonged treatment could only produce a reduction in the track density of less than 20%. 相似文献
24.
25.
Pingli Lv Rongxiu Zhu Tongxiang Lu Steven E. Wheeler Dongju Zhang Ruoxi Wang Chengbu Liu 《Tetrahedron: Asymmetry》2013,24(24):1598-1604
Computational studies to determine the origin of enantioselectivity in the (1R,2R)-1,2-diphenylethane-1,2-diamine (DEPN)–Brønsted acid catalyzed epoxidation of 2-cyclohexen-1-one have been performed using density functional theory. Transition states for conjugate addition and ring closure steps of the epoxidations catalyzed by three different catalyst systems were characterized. Our calculations show that the Csp2H?O H-bond interaction between the benzene ring of the catalyst and H2O is mainly responsible for the chiral discrimination observed. The Brønsted acid counterion plays a very important role in ensuring high enantioselectivity by improving the rigidity of the transition state structures to allow the efficient formation of the Csp2H?O H-bond. Moreover, we explain why these two diamine catalysts (1S,2S)-DACH and (1R,2R)-DPEN display consistent enantioselectivities in the catalytic epoxidation of 2-cyclohexen-1-one when combining with three different cocatalysts; achiral TFA, and chiral (R)- and (S)-TRIP. 相似文献
26.
In this paper we give sufficient conditions that guarantee the mean curvature flow with free boundary on a pinched cylinder develops a Type 2 curvature singularity. We additionally prove that Type 0 singularities may only occur at infinity. 相似文献
27.
For the work on the synthesis of 2-acyl-5-methoxynaphthoquinones as tricyclic analogues of daunomycinone1, we wanted to develop an efficient regioselective synthesis of 5-methoxy-2-acyl-1-naphthols without using Lewis acids. This requirement precluded the use of the thermal Fries but not the photo-Fries rearrangement. Although the mechanistic aspects of the photo reaction have received much study2, the reaction has been little used preparatively3 because it usually gives poor yields of hydroxy ketones even when only one product is possible. 相似文献
28.
James P. Best Johannes Zechner Jeffrey M. Wheeler Rachel Schoeppner Marcus Morstein 《哲学杂志》2016,96(32-34):3552-3569
For the implementation of thin ceramic hard coatings into intensive application environments, the fracture toughness is a particularly important material design parameter. Characterisation of the fracture toughness of small-scale specimens has been a topic of great debate, due to size effects, plasticity, residual stress effects and the influence of ion penetration from the sample fabrication process. In this work, several different small-scale fracture toughness geometries (single-beam cantilever, double-beam cantilever and micro-pillar splitting) were compared, fabricated from a thin physical vapour-deposited ceramic film using a focused ion beam source, and then the effect of the gallium-milled notch on mode I toughness quantification investigated. It was found that notching using a focused gallium source influences small-scale toughness measurements and can lead to an overestimation of the fracture toughness values for chromium nitride (CrN) thin films. The effects of gallium ion irradiation were further studied by performing the first small-scale high-temperature toughness measurements within the scanning electron microscope, with the consequence that annealing at high temperatures allows for diffusion of the gallium to grain boundaries promoting embrittlement in small-scale CrN samples. This work highlights the sensitivity of some materials to gallium ion penetration effects, and the profound effect that it can have on fracture toughness evaluation. 相似文献
29.
Tvaskis V Christy ME Arrington J Asaturyan R Baker OK Blok HP Bosted P Boswell M Bruell A Cochran A Cole L Crowder J Dunne J Ent R Fenker HC Filippone BW Garrow K Gasparian A Gomez J Jackson HE Keppel CE Kinney E Lapikás L Liang Y Lorenzon W Lung A Mack DJ Martin JW McIlhany K Meekins D Milner RG Mitchell JH Mkrtchyan H Moreland B Nazaryan V Niculescu I Opper A Piercey RB Potterveld DH Rose B Sato Y Seo W Smith G Spurlock K van der Steenhoven G Stepanyan S Tadevosian V Uzzle A Vulcan WF Wood SA 《Physical review letters》2007,98(14):142301
We report on a study of the longitudinal to transverse cross section ratio, R=sigmaL/sigmaT, at low values of x and Q2, as determined from inclusive inelastic electron-hydrogen and electron-deuterium scattering data from Jefferson Laboratory Hall C spanning the four-momentum transfer range 0.06相似文献
30.
Stereoelectronic Basis for the Kinetic Resolution of N‐Heterocycles with Chiral Acylating Reagents
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Sheng‐Ying Hsieh Benedikt Wanner Dr. Philip Wheeler Prof. Dr. André M. Beauchemin Prof. Dr. Tomislav Rovis Prof. Dr. Jeffrey W. Bode 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(24):7228-7231
The kinetic resolution of N‐heterocycles with chiral acylating agents reveals a previously unrecognized stereoelectronic effect in amine acylation. Combined with a new achiral hydroxamate, this effect makes possible the resolution of various N‐heterocycles by using easily prepared reagents. A transition‐state model to rationalize the stereochemical outcome of this kinetic resolution is also proposed. 相似文献