全文获取类型
收费全文 | 338篇 |
免费 | 18篇 |
国内免费 | 7篇 |
专业分类
化学 | 233篇 |
力学 | 5篇 |
数学 | 66篇 |
物理学 | 59篇 |
出版年
2022年 | 2篇 |
2021年 | 7篇 |
2020年 | 9篇 |
2019年 | 8篇 |
2018年 | 9篇 |
2017年 | 4篇 |
2016年 | 11篇 |
2015年 | 15篇 |
2014年 | 11篇 |
2013年 | 13篇 |
2012年 | 24篇 |
2011年 | 33篇 |
2010年 | 8篇 |
2009年 | 16篇 |
2008年 | 23篇 |
2007年 | 20篇 |
2006年 | 26篇 |
2005年 | 12篇 |
2004年 | 11篇 |
2003年 | 13篇 |
2002年 | 14篇 |
2001年 | 8篇 |
2000年 | 5篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有363条查询结果,搜索用时 31 毫秒
91.
Carlier PR Zhao H MacQuarrie-Hunter SL DeGuzman JC Hsu DC 《Journal of the American Chemical Society》2006,128(47):15215-15220
Benzodiazepines are privileged scaffolds in medicinal chemistry, but enantiopure examples containing quaternary stereogenic centers are extremely rare. We demonstrate that installation of the di(p-anisyl)methyl (DAM) group at N1 of 1,4-benzodiazepin-2-ones and 1,4-benzodiazepine-2,5-diones derived from enantiopure proteinogenic amino acids allows retentive replacement of the C3-proton via a deprotonation/trapping protocol. A wide variety of carbon and nitrogen electrophiles function well in this reaction, providing the corresponding quaternary benzodiazepines with excellent enantioselectivity. Deprotonation/trapping experiments on a pair of diastereomeric 1,4-benzodiazepine-2,5-diones provide evidence for a key role of conformational chirality in these enantioselective reactions. Acidic removal of the DAM group is fast and high-yielding and can be performed selectively in the presence of a N-Boc indole. Thus the synthesis of quaternary benzodiazepines with diverse N1 functionality can now be accomplished. 相似文献
92.
Fox OD Cookson J Wilkinson EJ Drew MG MacLean EJ Teat SJ Beer PD 《Journal of the American Chemical Society》2006,128(21):6990-7002
Polymetallic nanodimensional assemblies have been prepared via metal directed assembly of dithiocarbamate functionalized cavitand structural frameworks with late transition metals (Ni, Pd, Cu, Au, Zn, and Cd). The coordination geometry about the metal centers is shown to dictate the architecture adopted. X-ray crystallographic studies confirm that square planar coordination geometries result in "cagelike" octanuclear complexes, whereas square-based pyramidal metal geometries favor hexanuclear "molecular loop" structures. Both classes of complex are sterically and electronically complementary to the fullerenes (C(60) and C(70)). The strong binding of these guests occurred via favorable interactions with the sulfur atoms of multiple dithiocarbamate moieties of the hosts. In the case of the tetrameric copper(II) complexes, the lability of the copper(II)-dithiocarbamate bond enabled the fullerene guests to be encapsulated in the electron-rich cavity of the host, over time. The examination of the binding of fullerenes has been undertaken using spectroscopic and electrochemical methods, electrospray mass spectrometry, and molecular modeling. 相似文献
93.
Danny Calegari 《Transactions of the American Mathematical Society》2006,358(8):3473-3491
In this paper we introduce and study the notion of dynamical forcing. Basically, we develop a toolkit of techniques to produce finitely presented groups which can only act on the circle with certain prescribed dynamical properties.
where varies over countable groups, are exactly the set of closed subsets which contain and are invariant under . Moreover, we show that every such subset can be approximated from above by for finitely presented .
As an application, we show that the set consisting of rotation numbers which can be forced by finitely presented groups is an infinitely generated -module, containing countably infinitely many algebraically independent transcendental numbers. Here a rotation number is forced by a pair , where is a finitely presented group and is some element, if the set of rotation numbers of as varies over is precisely the set .
We show that the set of subsets of which are of the form
where varies over countable groups, are exactly the set of closed subsets which contain and are invariant under . Moreover, we show that every such subset can be approximated from above by for finitely presented .
As another application, we construct a finitely generated group which acts faithfully on the circle, but which does not admit any faithful action, thus answering in the negative a question of John Franks.
94.
We show that a co-orientable taut foliation of a closed, orientable, algebraically atoroidal 3-manifold is either the weak stable foliation of an Anosov flow, or else there are a pair of very full genuine laminations transverse to the foliation. 相似文献
95.
A group G is -admissible if there exists a G-crossed product division algebra over . The -admissibility conjecture asserts that every group with metacyclic Sylow subgroups is -admissible. We prove that the Mathieu group is -admissible, in contrast to any other sporadic group. 相似文献
96.
Shapir E Cohen H Borovok N Kotlyar AB Porath D 《The journal of physical chemistry. B》2006,110(9):4430-4433
High-resolution scanning tunneling microscopy (STM) imaging of single double-stranded poly(G)-poly(C) DNA molecules, made by a novel synthesis method, shows the molecules morphology. The STM images reveal a periodic structure of approximately 4 nm, seen as repeating "bulbs" along the molecules. These "bulbs" are associated with the molecule helix (the major grooves). "Nicks", two per 100 nm on the average, are observed along the DNA as well. The STM measurements were supported by a morphological statistics of the DNA molecule groove length and apparent height relative to the surface. 相似文献
97.
Prosenjit Bose Vida Dujmović Danny Krizanc Stefan Langerman Pat Morin David R. Wood Stefanie Wuhrer 《Journal of Graph Theory》2008,58(3):191-209
A graph G is a 2‐tree if G = K3, or G has a vertex v of degree 2, whose neighbors are adjacent, and G/ v is a 2‐ tree. A characterization of the degree sequences of 2‐trees is given. This characterization yields a linear‐time algorithm for recognizing and realizing degree sequences of 2‐trees. © 2008 Wiley Periodicals, Inc. J Graph Theory 58:191‐209, 2008 相似文献
98.
99.
Danny P. C. Mcgee John C. Martin Julien P. H. Verheyden 《Journal of heterocyclic chemistry》1985,22(4):1137-1140
Synthesis of 2-amino-7-[(1,3-dihydroxy-2-propoxy)methyl]pyrrolo[2,3-d]pyrimidin-4-one (3) a 7-deaza purine analogue and 2-amino-8-[(1,3-dihydroxy-2-propoxy)methyl]imidazo[1,2-a]-s-triazin-4-one (4) a 5-aza-7-deaza purine analogue of DHPG (2) are reported. 相似文献
100.
Despite the ban of the European Union on use of drugs to improve animal growth, occasionally beta-agonist drugs are still found in samples from cattle. Over time, the specified limits for the detection of these illegal drugs have been lowered. To improve the immunochemical screening of urine samples to detect lower levels of several beta-agonists, immunofiltration (IF) was applied for sample cleanup in combination with a beta-agonist-ELISA. In the applied IF format, free (non-immobilised) anti-salbutamol polyclonal antibodies were mixed with the urine sample in an ultra-filtration device (cut off 30 kDa) and the sample was removed by centrifugation. The antibody bound beta-agonists were freed from the antibodies by the addition of a mixture of methanol and 0.1 M acetic acid (1:1; v/v) and centrifugation. The filtrate, containing the free beta-agonists, was evaporated to dryness and the residue dissolved in buffer, an aliquot of which was analysed with the beta-agonist ELISA. Compared with the direct beta-agonist ELISA, this IF cleanup procedure resulted in a 30-times lower limit of detection (LOD) of 0.14 ng ml(-1) (salbutamol equivalents). The anti-salbutamol antibodies recognised several beta-agonists and the combination of IF with the beta-agonist ELISA was found suitable for the detection of at least ten beta-agonists in urine with comparable LODs. 相似文献