全文获取类型
收费全文 | 1992篇 |
免费 | 84篇 |
国内免费 | 18篇 |
专业分类
化学 | 1343篇 |
晶体学 | 4篇 |
力学 | 70篇 |
数学 | 428篇 |
物理学 | 249篇 |
出版年
2022年 | 15篇 |
2021年 | 15篇 |
2020年 | 19篇 |
2019年 | 34篇 |
2018年 | 27篇 |
2017年 | 20篇 |
2016年 | 46篇 |
2015年 | 55篇 |
2014年 | 44篇 |
2013年 | 137篇 |
2012年 | 117篇 |
2011年 | 115篇 |
2010年 | 89篇 |
2009年 | 69篇 |
2008年 | 117篇 |
2007年 | 117篇 |
2006年 | 105篇 |
2005年 | 95篇 |
2004年 | 113篇 |
2003年 | 91篇 |
2002年 | 91篇 |
2001年 | 36篇 |
2000年 | 33篇 |
1999年 | 23篇 |
1998年 | 26篇 |
1997年 | 21篇 |
1996年 | 42篇 |
1995年 | 21篇 |
1994年 | 24篇 |
1993年 | 13篇 |
1992年 | 22篇 |
1991年 | 14篇 |
1990年 | 7篇 |
1989年 | 23篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 19篇 |
1984年 | 18篇 |
1983年 | 15篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 18篇 |
1979年 | 13篇 |
1978年 | 15篇 |
1977年 | 16篇 |
1976年 | 9篇 |
1975年 | 13篇 |
1974年 | 15篇 |
1973年 | 17篇 |
排序方式: 共有2094条查询结果,搜索用时 15 毫秒
31.
Phosphines react with butterfly tetranuelear nitrido-iron clusters, [Fe4N(CO)12]− and [Fe4N(CO)11(NO)], to give mono- and di-substituted complexes. X-Ray analyses of the title compounds showed that the phosphine ligands are bound to the wing-tip atoms. 相似文献
32.
The reactivity of different Co(2)(CO)(6)-acetylene complexes in the Pauson-Khand reaction is strongly dependent on the nature of the olefin. Theoretical calculations at both the DFT and ONIOM levels are concordant with experimental observations and suggest how the olefin-cobalt interactions in the complex could have a major effect on the relative reactivity of the olefins. This study rationalizes for the first time the experimentally observed reactivity differences of cyclohexene, cyclopentene, and norbornene. 相似文献
33.
ZINDO semiempirical calculations on the Ni(CN)(4)(2-) ion were performed, and ground-state energies for all 41 valence-orbital-based MOs and orbital transition components of the two lowest energy fully allowed electronic transitions are reported. Gaussian 94 was used to calculate ground-state energies as a comparison. The ground-state energies using ZINDO compare much more favorably with those found through ab initio techniques than with those from a reported INDO calculation. The found electronic transitions agree substantially with earlier assignments with the exception that several orbital transitions are required to adequately model the lowest energy allowed x,y-polarized experimental transition. Calculation parameters were optimized to give excellent agreement with experiment and may serve well for more complex arrangements of this ion. 相似文献
34.
The synthesis of 4-(1-haloalkyl)-2-azetidinones and 4-(2-haloalkyl)-2-azetidinones was investigated with use of the Staudinger reaction between in situ generated ketenes and alpha-haloimines or beta-haloimines. This new class of functionalized 2-azetidinones was further evaluated for its potential use as intermediates in the synthesis of highly functionalized compounds. The reaction of 4-(1-haloalkyl)-2-azetidinones and 4-(2-haloalkyl)-2-azetidinones with sodium methoxide in methanol yielded ring-opened products, i.e., methyl 2-alkoxy-4-(alkylamino)pentenoate and methyl 5-(alkylamino)pentenoate, respectively. Further attention was paid in detail to the reaction mechanism involved in this peculiar transformation. It was proven that these reactions proceeded via intermediate aziridines or azetidines. 相似文献
35.
7-(2-Methoxycarbonylvinyl)-3-hydroxychromones have been synthesized using Heck coupling reaction from the corresponding 7-bromo-3-hydroxychromones. Introduction of the electron acceptor (2-methoxycarbonylvinyl) group at 7-position of 3-hydroxychromone results in a 30-40 nm red shift in absorption and >50 nm red shifts of both bands in emission. This derivatization allowed us to develop dyes with absorption maxima reaching 480 nm and dual emission in the red region of the spectrum. In comparison to the parent dyes, 7-acryl-3-hydroxychromones demonstrate significantly stronger solvatochromism. This is due to the acceptor group at 7-position, which increases the transfer character of the excited state of the dyes. The new dyes are highly prospective for the development of new fluorescent probes in biological research. 相似文献
36.
Theodorus J.M. de Bruin Karoly Vekey Yves Gimbert Anne Milet 《Journal of organometallic chemistry》2006,691(20):4281-4288
The influence of carbon-carbon triple bond polarization on the regiochemistry of the Pauson-Khand reaction has been studied with the B3LYP functional. The regiochemistry determining step of this reaction, i.e., olefin insertion leading to cobaltacycle formation, has been examined with ethylene as the olefin and propyne, methyl 2-butynoate, and methyl propiolate as the acetylenes. From this study, it has been concluded that, in absence of overwhelming steric effects of an acetylene substituent, the regiochemistry is influenced by the polarization of the acetylenic bond, which arises from the different substituents. The initial C-C bond is preferentially formed with the acetylenic carbon that has the greater electron density: with propyne, this leads to a cyclopentenone having the methyl group in the α-position; with methyl 2-butynoate, to a cyclopentenone with the CO2Me in the β-position; with methyl propiolate, which is virtually unpolarized in the complex, to a cyclopentenone with the CO2Me in the α-position (a result of steric effects). These theoretical results are concordant with those observed experimentally with norbornene. The question of axial versus equatorial reactive positions for the coordinated olefin is also addressed and a kinetic simulation is presented. 相似文献
37.
Yves Martin 《Transactions of the American Mathematical Society》1996,348(12):4825-4856
Let be the Dedekind -function. In this work we exhibit all modular forms of integral weight , for positive integers and and arbitrary integers , such that both and its image under the Fricke involution are eigenforms of all Hecke operators. We also relate most of these modular forms with the Conway group via a generalized McKay-Thompson series.
38.
39.
Improved L.C.A.O. method primitively built for conjugated molecules holding only atoms of second row in the periodic classification is extended to molecules holding atoms of the third row. The application is done for thiophen, thioketones, SO2 and SO3. d orbitals are not introduced in this study. Discussion about results shows that d orbitals must be introduced when the sulphur atom is strongly positively charged like in sulphur oxides. 相似文献
40.
Francois Delyon Yves Lévy Bernard Souillard 《Communications in Mathematical Physics》1985,100(4):463-470
We prove that discrete Schrödinger operators on
d
with a random-potential have almost-surely only pure point spectrum and exponentially decaying eigenfunctions for large disorder or large energy. This is the first proof of localization for multi-dimensional Anderson models.Groupe de recherche 048 du CNRS 相似文献