全文获取类型
收费全文 | 1570篇 |
免费 | 58篇 |
国内免费 | 4篇 |
专业分类
化学 | 1116篇 |
晶体学 | 11篇 |
力学 | 48篇 |
数学 | 159篇 |
物理学 | 298篇 |
出版年
2023年 | 7篇 |
2022年 | 11篇 |
2021年 | 18篇 |
2020年 | 20篇 |
2019年 | 23篇 |
2018年 | 14篇 |
2017年 | 11篇 |
2016年 | 29篇 |
2015年 | 33篇 |
2014年 | 70篇 |
2013年 | 80篇 |
2012年 | 81篇 |
2011年 | 114篇 |
2010年 | 48篇 |
2009年 | 52篇 |
2008年 | 112篇 |
2007年 | 103篇 |
2006年 | 92篇 |
2005年 | 88篇 |
2004年 | 71篇 |
2003年 | 44篇 |
2002年 | 46篇 |
2001年 | 16篇 |
2000年 | 27篇 |
1999年 | 20篇 |
1998年 | 17篇 |
1997年 | 10篇 |
1996年 | 17篇 |
1995年 | 14篇 |
1994年 | 14篇 |
1993年 | 17篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 14篇 |
1989年 | 15篇 |
1988年 | 15篇 |
1987年 | 10篇 |
1985年 | 14篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 14篇 |
1981年 | 18篇 |
1980年 | 17篇 |
1979年 | 18篇 |
1978年 | 12篇 |
1977年 | 19篇 |
1976年 | 7篇 |
1975年 | 8篇 |
1974年 | 19篇 |
1973年 | 13篇 |
排序方式: 共有1632条查询结果,搜索用时 15 毫秒
51.
52.
53.
Ly Dieu Ha Poul Erik Hansen Fritz Duus Hung Dinh Pham Lien‐Hoa D. Nguyen 《Magnetic resonance in chemistry : MRC》2012,50(3):242-245
Four new prenylated depsidones, oliveridepsidones A–D, were isolated from the bark of Garcinia oliveri collected in Vietnam. Their structures were elucidated using mainly NMR techniques (1H and 13C NMR, HMQC, HMBC and NOE experiments). Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
54.
Robert Tchitnga Hilaire Bertrand Fotsin Bonaventure Nana Patrick Hervé Louodop Fotso Paul Woafo 《Chaos, solitons, and fractals》2012,45(3):306-313
This paper shows an experimental evidence of chaos in one of the simplest imaginable autonomous implicit Hartley’s oscillator made simply of a junction field effect transistor (JFET) and a tapped coil. The experimental setup is implemented. The variations of amplitude of the oscillations through the control element are obtained showing the domain of existence of chaos. Phase portraits of the PSpice simulation, of the numerical integration and of the experiment are displayed, confirming a good agreement between theory and praxis. 相似文献
55.
56.
57.
Rik H. Verschueren Julie Schmauck Dr. Michael S. Perryman Dr. Hui-Lan Yue Julian Riegger Dr. Bertrand Schweitzer-Chaput Dr. Martin Breugst Dr. Martin Klussmann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(38):9088-9097
In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ+) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions. 相似文献
58.
Poul Erik Hansen Bahjat A. Saeed Rita S. Rutu Teobald Kupka 《Magnetic resonance in chemistry : MRC》2020,58(8):750-762
1J(15N,H) coupling constants for enaminones and NH-forms of intramolecularly hydrogen-bonded Schiff bases as model compounds for sp2-hybridized nitrogen atoms are evaluated using density functional theory (DFT) to find the optimal functionals and basis sets. Ammonia is used as a test molecule and its one-bond coupling constant is compared with experiment. A methylamine Schiff base of a truncated molecule of gossypol is used for checking the performance of selected B3LYP, O3LYP, PBE, BHandH, and APFD density functionals and standard, modified, and dedicated basis sets for coupling constants. Both in vacuum and in chloroform, modeled by the simple continuum model of solvent, the modified basis sets predict significantly better the 1J(15N,H) value in ammonia and in the methylamine Schiff base of a truncated molecule of gossypol than the standard basis sets. This procure is then used on a broad set of intramolecularly hydrogen-bonded molecules, and a good correlation between calculated and experimental one-bond NH coupling constants is obtained. The 1J(15N,H) couplings are slightly overestimated. The calculated data show for hydrogen-bonded NH interatomic distances that the calculated values depend on the NH bond lengths. The shorter the bond lengths, the larger the 1J(15N,H). A useful correlation between 1J(15N,H) and NH bond length is derived that enables realistic predictions of one-bond NH coupling constants. The calculations reproduce experimentally observed trends for the studied molecules. 相似文献
59.
We consider a pricing and advertising dynamic-optimization problem where the goodwill dynamics evolve à la Nerlove–Arrow. The firm maximizes its profit over a finite-planning horizon corresponding to the product’s lifespan, and it turns out that the Hamiltonian is non-concave. We show the existence and uniqueness of an optimal solution under some mild conditions. 相似文献
60.
Camille Piovesan Anne-Laurence Dupont Isabelle Fabre-Francke Odile Fichet Bertrand Lavédrine Hervé Chéradame 《Cellulose (London, England)》2014,21(1):705-715
Two di-alkoxysilanes, with (AMDES, aminopropylmethyldiethoxysilane) or without (DMDES, dimethyldiethoxysilane) an amine function, and a tri-alkoxy aminosilane (APTES, aminopropyltriethoxysilane) as well as their mixtures were introduced in paper as fiber strengthening agents. The polymerization and copolymerization of these polysiloxanes in the paper were investigated. In all the cases where APTES was present, the formation of networks was established by measuring the soluble fraction amount extracted from the treated papers. A slight decrease of the opacity of the paper sheets when AMDES was part of the treatment was noted. The presence of APTES reduced this opacity loss. The study of the physicochemical properties of the treated paper (mechanical strength and alkalinity) demonstrated that, besides the required deacidification feature, the different treatments allowed an efficient strengthening of the cellulose fibers to various extents. Contact angle measurements indicated a decrease of the hydrophilic character of papers treated with the mixture APTES/AMDES and the occurrence of a hydrophobic character of the papers treated with APTES alone. These results were consistently obtained for both spray and immersion treatment processes. 相似文献