全文获取类型
收费全文 | 670篇 |
免费 | 18篇 |
专业分类
化学 | 489篇 |
晶体学 | 1篇 |
力学 | 18篇 |
数学 | 56篇 |
物理学 | 124篇 |
出版年
2023年 | 5篇 |
2022年 | 38篇 |
2021年 | 29篇 |
2020年 | 22篇 |
2019年 | 14篇 |
2018年 | 13篇 |
2017年 | 13篇 |
2016年 | 26篇 |
2015年 | 17篇 |
2014年 | 12篇 |
2013年 | 47篇 |
2012年 | 65篇 |
2011年 | 52篇 |
2010年 | 26篇 |
2009年 | 36篇 |
2008年 | 42篇 |
2007年 | 40篇 |
2006年 | 38篇 |
2005年 | 22篇 |
2004年 | 22篇 |
2003年 | 10篇 |
2002年 | 8篇 |
2001年 | 8篇 |
2000年 | 12篇 |
1999年 | 5篇 |
1998年 | 4篇 |
1997年 | 10篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 7篇 |
1993年 | 1篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 5篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有688条查询结果,搜索用时 0 毫秒
51.
52.
Ftima C. Teixeira Carla Lucas M. Joo M. Curto Antnio P. S. Teixeira M. Teresa Duarte Vnia Andr 《Heteroatom Chemistry》2016,27(1):3-11
Novel tetraethyl ethylene‐1,1‐bisphosphonate esters derived from 1H‐indazole, 1H‐pyrazolo[3,4‐b]pyridine, and 1H‐pyrazolo[3,4‐b]quinoline were synthesized by a Michael addition reaction of tetraethyl ethylidene‐1,1‐bisphosphonate with the corresponding heterocycle, using conventional heating and microwave‐assisted methods. The microwave‐assisted method provides shorter reaction times and better yields. The hydrolysis of bisphosphonates afforded the corresponding bisphosphonic acids or salt, using concentrated hydrochloric acid or TMSBr/collidine, respectively. All new compounds were fully characterized, and their structures were assigned using 1H, 31P, and 13C NMR and IR spectroscopies and mass spectrometry. The molecular structure of compound 6 was confirmed by X‐ray diffraction studies. 相似文献
53.
Regina Luschtinetz Augusto F. Oliveira Hélio A. Duarte Gotthard Seifert 《无机化学与普通化学杂志》2010,636(8):1506-1512
Density‐functional based calculations were used to investigate self‐assembled monolayers of different alkylphosphonic acids on corundum α‐Al2O3 (0001), bayerite β‐Al(OH)3 (001) and boehmite γ‐AlOOH (010) surface models. Mono‐, bi‐, and tridentate adsorption modes were considered. In addition, the organization of single adsorbed molecules was compared to the organization at full surface coverage. The height (thickness) of the self‐assembled monolayers is always shorter than the length of the phosphonic acid molecules due to tilting of the alkyl chains. Tilt angles at full surface coverage are very similar to the tilt angle of a single adsorbed molecule, which indicates that the density of the self‐assembled monolayers is limited by the density of adsorption sites. The lateral interactions between alkyl chains are evidenced by small torsions of the adsorbed molecules, which may serve to minimize the repulsion forces between interchain hydrogen atoms. Similar tilt angles were obtained for mono‐, bi‐, and tridentate adsorptions. Hence, the coordination mode cannot be characterized by the molecule tilting. 相似文献
54.
Batalioto F Martins OG Duarte AR Figueiredo Neto AM 《The European physical journal. E, Soft matter》2011,34(1):10
In this work we investigate the influence of the adsorption of ions on the impedance spectroscopy of an electrolytic cell.
We consider that the positive and negative ions present in a dielectric liquid are adsorbed in the electrode surfaces with
different adsorption energies. This difference in adsorption energies causes an additional plateaux in the limit of the low-frequency
range of the real part of the impedance Z . In the same frequency range, a second minimum in the imaginary part of Z is predicted. The theory is illustrated with measurements of the impedance of an electrolytic solution in the frequency range
from 10-2Hz to 1KHz. A comparison between the present model and others from the literature to describe the experimental results is
also made. 相似文献
55.
Eduardo de Almeida Paulo Rogério Massoni Amauri Antonio Menegário Laudi Cunha Leite Dante Pazzanese Duarte Lanna Virgílio Franco do Nascimento Filho 《X射线光谱测定》2011,40(6):424-426
This article describes a comparison of conventional energy‐dispersive X‐ray fluorescence (EDXRF) and synchrotron radiation total‐reflection X‐ray fluorescence (SRTXRF) for Co determination in ruminal fluid from Holstein cow. This element is used as marker for animal nutrition studies. For EDXRF, 200 µl of the sample were dried on 6.35 µm Mylar film at 60 °C. The excitation was carried out using an X‐ray tube with Mo target and Zr filter operated at 30 kV/20 mA. For SRTXRF, 10 µl of the sample were pipetted on a Lucite carrier and dried at 60 °C. In both the techniques, Ga was used as internal standard and the acquisition time was 200 s. The trueness of both techniques was evaluated through the standard addition method, the recoveries obtained by SRTXRF and EDXRF were 76 and 99%, and the limits of detection, 13 and 240 µg l?1, respectively. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
56.
Vicente Campos Rafael Martí Jesús Sánchez-Oro Abraham Duarte 《The Journal of the Operational Research Society》2014,65(12):1800-1813
In this paper, we address an optimization problem resulting from the combination of the well-known travelling salesman and knapsack problems. In particular, we target the orienteering problem, originated in the context of sport, which consists of maximizing the total score associated with the vertices visited in a path within the available time. The problem, also known as the selective travelling salesman problem, is NP-hard and can be formulated as an integer linear program. Since the 1980s, several solution methods for this problem have been developed and applied to a variety of fields, particularly in routing and tourism. We propose a heuristic method—based on the Greedy Randomized Adaptive Search Procedure (GRASP) and the Path Relinking methodologies—for finding approximate solutions to this optimization problem. We explore different constructive methods and combine two neighbourhoods in the local search of GRASP. Our experimentation with 196 previously reported instances shows that the proposed procedure obtains high-quality solutions employing short computing times. 相似文献
57.
R. S. Corrêa S. R. Souza e Silva L. P. Duarte G. D. F. Silva L. C. A. Barbosa J. Ellena A. C. Doriguetto 《Journal of Structural Chemistry》2012,53(1):156-163
The structural study of two (C30H48O2) pentacyclic triterpene (PCTT) isomers is presented. These terpenes, known as 30-hydroxy-lup-20(29)-en-3-one (1) and (11α)-11-hydroxy-lup-20(29)-en-3-one (2), were isolated from Maytenus imbricata Mart. Ex Reissek (Celastraceae). The molecular structure of 1 and 2 differs in the position of the hydroxyl group. Both compounds crystallize in non-centrosymmetric space groups with two molecules
in the asymmetric unit. The crystal structure of 1 shows a triclinic P1 space group (a = 9.5518(1) ?, b = 9.7083(1) ?, c = 14.4696(2) ?, α = 93.832(1)°, β = 102.833(1)°, and γ = 103.307(1)°), while compound 2 crystallizes in a monoclinic P21 one (a = 13.4439(16) ?, b = 14.4463(14) ?, c = 13.5224(9) ? and β = 99.703(8)°). The two molecules independent by symmetry of 1 differ slightly due to the presence of static disorder in oxygen atoms. In addition, the intermolecular geometries of 1 and 2 were analysed, and in each isomer the crystal packing is stabilized by O-H…O intermolecular hydrogen bonds and van der Waals
forces. 相似文献
58.
59.
Mónica S. Estevão Filipe J.S. Duarte Eduarda Fernandes A. Gil Santos M. Manuel B. Marques 《Tetrahedron letters》2012,53(16):2132-2136
Several trifluoromethylated compounds were reacted with indole sodium salt, leading to monofluorinated compounds. The unexpected products formation was rationalized by DFT calculations. 相似文献
60.
The Heck-Matsuda arylation of 2-aza and 2-oxo-substituted acrylates is described. Several reaction conditions were evaluated including the influence of solvents, temperature, catalysts, and stoichiometry. While the oxygenated system was successfully arylated in benzonitrile with Pd2(dba)3 as catalyst, the aza-acrylate furnished methoxylated products. The methoxylated products were subjected to an elimination/reduction protocol to obtain the corresponding N,O-protected phenylalanine derivatives. A one-pot procedure for the preparation of phenylalanine derivatives from 2-aza-substituted acrylates is presented. 相似文献