全文获取类型
收费全文 | 227篇 |
免费 | 0篇 |
国内免费 | 9篇 |
专业分类
化学 | 71篇 |
晶体学 | 1篇 |
力学 | 3篇 |
数学 | 84篇 |
物理学 | 77篇 |
出版年
2013年 | 7篇 |
2012年 | 4篇 |
2008年 | 3篇 |
2007年 | 3篇 |
2006年 | 3篇 |
2005年 | 6篇 |
2003年 | 5篇 |
2002年 | 6篇 |
2001年 | 7篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 9篇 |
1995年 | 14篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1985年 | 5篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1972年 | 7篇 |
1970年 | 2篇 |
1969年 | 5篇 |
1960年 | 2篇 |
1959年 | 2篇 |
1933年 | 2篇 |
1932年 | 2篇 |
1931年 | 2篇 |
1930年 | 2篇 |
1927年 | 3篇 |
1926年 | 2篇 |
1916年 | 2篇 |
1903年 | 2篇 |
1902年 | 2篇 |
1901年 | 5篇 |
1900年 | 4篇 |
1896年 | 3篇 |
1895年 | 3篇 |
1894年 | 2篇 |
1881年 | 2篇 |
排序方式: 共有236条查询结果,搜索用时 31 毫秒
71.
Spin-state alteration from sterically enforced ligand rotation in bis(indenyl)chromium(II) complexes
Brady ED Overby JS Meredith MB Mussman AB Cohn MA Hanusa TP Yee GT Pink M 《Journal of the American Chemical Society》2002,124(32):9556-9566
The rotational orientation of cyclopentadienyl rings usually has no effect on d-orbital energy levels and splitting in transition metal complexes. With related but less symmetrical carbocyclic ligands, however, the magnetic properties of the associated complexes can be altered by the alignment of the ligands. Examples of this effect are found in substituted organochromium(II) bis(indenyl) complexes. The monosubstituted compounds (1-RC(9)H(6))(2)Cr (R = t-Bu, SiMe(3)) are prepared from the substituted lithium indenides and CrCl(2) in THF; they are high-spin species with four unpaired electrons. Their spin state likely reflects that in the unknown monomeric (C(9)H(7))(2)Cr, which is calculated to have a high-spin (S = 2) ground state in the staggered configuration (180 degrees rotation angle). However, the analogous bis(indenyl) complexes containing t-Bu or SiMe(3) groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R(2)C(9)H(5))(2)Cr) are low-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C(9)H(6)](2)Cr exists in a staggered conformation, with Cr-C (av) = 2.32(4) A. In contrast, the average Cr-C distances in [1,3-(t-Bu or SiMe(3))(2)C(9)H(5)](2)Cr are 2.22(2) and 2.20(2) A, respectively, and the rings are in a gauche configuration, with rotation angles of 87 degrees. The indenyl conformations are sterically imposed by the bulk of the t-Bu and SiMe(3) substituents. The change from a staggered to a gauche indenyl orientation lowers the symmetry of a (C(9)H(7))(2)M complex and allows greater mixing of metal and ligand orbitals. Calculations indicate that previously nonbonding pi orbitals of the indenyl anion are able to interact with the chromium d orbitals, producing bonding and antibonding combinations. The latter remain unpopulated, and the resulting increase in the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of using sterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds a potentially rich source of magnetically adjustable molecules. 相似文献
72.
The placement accuracy and resolution of direct-write patterning tools, in particular the atomic force microscope (AFM), is considered for application to fabricating multi-passband integrated optical niters. Because of its simpler fabrication a grating structure is proposed that consists of identical stripes that are non-periodically spaced. The recently developed pseudorandom encoding method from the field of computer generated holography is modified to effectively assign analog reflectances at each point along the grating by selective withdrawal and offsetting of the stripes from a periodic spacing. An example filter designed by this method has two 1.5 nm bandwidth passbands and –23 dB of rejection for lightly coupled stripes. As with single band filters, the passbands broaden as the coupling increases. A calculation of the coupling coefficient of stripes on a fundamental mode, slab waveguide indicate that stripes on the order of 100 nm in depth and width support low insertion loss, multipassband filtering applications at visible wavelengths. Lines of these dimensions patterned with an AFM on (110) silicon indicates the feasibility of fabricating these filters. These conclusions are specific to current AFM’s that are limited to writing fields of 100 μm. Increased rejection and decreased passband widths will result from incorporating precise field stitching into future AFM’s. 相似文献
73.
Ohne Zusammenfassung 相似文献
74.
P. Cohn und F. Fleissner 《Fresenius' Journal of Analytical Chemistry》1897,36(1):796-797
Ohne Zusammenfassung 相似文献
75.
76.
Fritz Cohn 《Mathematische Annalen》1894,44(4):473-538
Ohne Zusammenfassung 相似文献
77.
Lepore SD Khoram A Bromfield DC Cohn P Jairaj V Silvestri MA 《The Journal of organic chemistry》2005,70(18):7443-7446
[reaction: see text] A study of the role of base in the isomerization of manganese-coordinated conjugated alkynyl carbonyls to the corresponding allenyl carbonyls is described. The use of phosphine additives indicates that manganese requires a ligand prior to isomerization with amine bases. A series of amine bases were also examined for their efficacy in this isomerization reaction revealing a strong dependence on pK(a). By contrast, potassium tert-butoxide led to rapid isomerization in the absence of added manganese ligand. 相似文献
78.
It is proved that a ring (0) is projective-free precisely whenit has invariant basis number and every idempotent matrix notequal to I is non-full. 2000 Mathematics Subject Classification16D40. 相似文献
79.
L. Wielopolski A. G. Meek M. Moskowitz S. H. Cohn 《Journal of Radioanalytical and Nuclear Chemistry》1987,114(1):187-193
High energy gamma-radiation (8 to 30 MeV) is gaining acceptance for radiation therapy of patients with deep cancers. This radiation is of sufficient energy to induce photonuclear activation of the elements in the human body. Our results of measurements of nitrogen and phosphorus in an anthropomorphic phantom, a cadaver, and a cancer patient with bremsstrahlung radiation from 15 MeV electrons demonstrate the feasibility of a method to monitor these two elements in the human body in vivo by measuring the radioactivity induced in these targets by photonuclear reactions. 相似文献
80.
P. M. Cohn 《Israel Journal of Mathematics》1974,19(1-2):109-151