全文获取类型
收费全文 | 198篇 |
免费 | 9篇 |
专业分类
化学 | 161篇 |
晶体学 | 2篇 |
力学 | 1篇 |
数学 | 28篇 |
物理学 | 15篇 |
出版年
2022年 | 8篇 |
2021年 | 7篇 |
2020年 | 3篇 |
2019年 | 2篇 |
2018年 | 4篇 |
2017年 | 1篇 |
2016年 | 8篇 |
2015年 | 6篇 |
2014年 | 8篇 |
2013年 | 8篇 |
2012年 | 5篇 |
2011年 | 13篇 |
2010年 | 8篇 |
2009年 | 10篇 |
2008年 | 13篇 |
2007年 | 14篇 |
2006年 | 18篇 |
2005年 | 13篇 |
2004年 | 11篇 |
2003年 | 9篇 |
2002年 | 5篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1988年 | 3篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有207条查询结果,搜索用时 15 毫秒
91.
Theoretical study of global and local reactivities of coumarin and its hydroxylated derivatives
下载免费PDF全文
![点击此处可从《International journal of quantum chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Francisco J. Melendez J. Sergio Durand‐Niconoff Miguel A. Domínguez‐Ortiz Oscar García‐Barradas Norma A. Caballero Enrique González 《International journal of quantum chemistry》2016,116(9):663-669
Coumarins are bioactive substances of the benzo‐α‐pyrone family, which have shown antioxidant, antiviral, anti‐inflammatory and antitumor activities, among others. 7‐Hydroxycoumarin and 6,7‐dihydroxycoumarin (esculetin) are two coumarin derivatives that have been reported to exhibit antitumor activity, but the action mechanism underlying this activity remains unknown. In this work, to elucidate this mechanism, a theoretical study of the local and global electronic reactivity properties of a series of hydroxylated and dihydroxylated coumarin derivatives with possible antitumor action is performed using Density Functional Theory in aqueous solution. The substitution by one or two hydroxyl groups in the benzene ring of coumarin produces better charge‐donor than charge‐acceptor compounds. All the studied compounds are generally stable in water and exhibit permanent polarization in the solvent. With one hydroxyl substitution, 7‐hydroxycoumarin is the most polar and polarizable derivative, whereas 5,7‐dihydroxycoumarin is the most polar and polarizable compound with two hydroxyl substitutions. 5,7‐Dihydroxycoumarin is suggested to possess antitumor activity. © 2016 Wiley Periodicals, Inc. 相似文献
92.
93.
94.
Gerard P. O’Sullivan Norma M. Scully Jean-Marie Prat Jeremy D. Glennon Benjamin Dietrich Volker Friebolin Klaus Albert 《Analytical and bioanalytical chemistry》2009,394(5):1261-1272
Supercritical carbon dioxide has attracted attention as a potential replacement for traditional organic solvents due to its
simplified workup procedures and reduced environmental impact—providing a green chemistry approach for organic solvent-free
functionalisation. In addition to the environmental benefits, the enhanced diffusivity observed in supercritical solvents
can often enhance reaction rates. We have applied these valuable features to the preparation of silica-bonded stationary phases
and examined their potential in liquid chromatography. We report the successful preparation and characterisation of polyether
silica based on Frechet dendrimers—this significantly enhances the range of stationary-phase chemistries that can be prepared
in supercritical fluids. First- and second-generation polyether silicas were prepared, characterised, end-capped and evaluated
for use as stationary phases for liquid chromatography.
Figure SRM1649 on 2nd generation polyether silica
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
相似文献
Jeremy D. GlennonEmail: |
95.
Manuel Soriano-García Tania Segura Norma Valencia Eugene Bratoeff Marisa Cabeza 《Journal of chemical crystallography》2010,40(12):1119-1124
Abstract
The structures of 3β-cyclobutyl carbonyloxy-5-androsten-17-one (C24H34O3), compound 1; 3β-cyclopentyl carbonyloxy-5-androsten-17-one (C25H36O3), compound 2; and 3β-cyclohexyl carbonyloxy-5-androsten-17-one (C26H38O3) compound 3 were established by spectral and X-ray diffraction studies. Steroidal derivatives 1–3 exhibited a high antiandrogenic effect and could be considered as potential drugs for the treatment of prostate cancer. Compound 1 crystallizes in the orthorhombic space group P212121 with unit cell parameters a = 6.618(2), β = 14.167(3), c = 22.329(5) ?, Z = 4. Compound 2 crystallizes in the orthorhombic space group P212121 with unit cell parameters a = 6.631(2), b = 13.865(4), c = 23.952(7) ?, Z = 4. Compound 3 crystallizes in the orthorhombic space group P212121 with unit cell parameters a = 6.6100(9), b = 13.896(2), c = 24.491(3) ?, Z = 4. All three structures (1–3) were solved by direct methods and refined to R = 0.0708, 0.0750 and 0.0496, respectively. For compound 2, there is positional disorder of the side chain at C3. All the rings of both steroid skeletons are trans connected. For structures 1–3, the six-membered rings A, B and C have a deformed chair, half chair and deformed chair conformations. The five-membered rings D adopt an intermediate envelope and half-chair conformations. For structure 2, the five-membered rings E and EA have a deformed envelope and an intermediate envelope and half-chair conformations, respectively. For structure 3, the six-membered ring E adopts a deformed chair conformation. Cohesion of the crystals can be attributed to van der Waals and weak C–H⋯O interactions. 相似文献96.
Teruel MA Benitez-Villalba J Caballero N Blanco MB 《The journal of physical chemistry. A》2012,116(24):6127-6133
Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with methyl crotonate and ethyl crotonate have been determined at 298 K and atmospheric pressure. The decay of the organics was monitored using gas chromatography with flame ionization detection (GC-FID), and the rate constants were determined using the relative rate method with different reference compounds. Room temperature rate coeficcients were found to be (in cm(3) molecule(-1) s(-1)): k(1)(OH + CH(3)CH═CHC(O)OCH(3)) = (4.65 ± 0.65) × 10(-11), k(2)(Cl + CH(3)CH═CHC(O)OCH(3)) = (2.20 ± 0.55) × 10(-10), k(3)(OH + CH(3)CH═CHC(O)OCH(2)CH(3)) = (4.96 ± 0.61) × 10(-11), and k(4)(Cl + CH(3)CH═CHC(O)OCH(2)CH(3)) = (2.52 ± 0.62) × 10(-10) with uncertainties representing ±2σ. This is the first determination of k(1), k(3), and k(4) under atmospheric pressure. The rate coefficients are compared with previous determinations for other unsaturated and oxygenated VOCs and reactivity trends are presented. In addition, a comparison between the experimentally determined k(OH) with k(OH) predicted from k vs E(HOMO) relationships is presented. On the other hand, product identification under atmospheric conditions has been performed for the first time for these unsaturated esters by the GC-MS technique in NO(x)-free conditions. 2-Hydroxypropanal, acetaldehyde, formaldehyde, and formic acid were positively observed as degradation products in agreement with the addition of OH to C2 and C3 of the double bond, followed by decomposition of the 2,3- or 3,2-hydroxyalkoxy radicals formed. Atmospheric lifetimes, based on of the homogeneous sinks of the unsaturated esters studied, are estimated from the kinetic data obtained in the present work. 相似文献
97.
Pacheco-Malagón G Pérez-Romo P Sánchez-Flores NA Guzmán-Castillo ML López-Franco C Saniger JM Hernández-Beltrán F Fripiat JJ 《Inorganic chemistry》2006,45(8):3408-3414
As shown recently, the networks of mesoporous high-surface-area silicates and zeolites undergo a deep depolymerization process in glycerol, near 200 degrees C. Within 1 h, X-ray diffraction analysis amorphous gels are obtained. However, some local ordering subsists as demonstrated by a striking similarity between the silicon and aluminum high-resolution solid-state NMR spectra before and after the reaction. The residual organization could be investigated indirectly in studying the recrystallization of these gels in the presence or absence of structure-directing agents. Were this attempt successful, the way should be opened for the synthesis of molecular sieves starting from gels obtained from naturally occurring zeolites. Here, it will be shown that an amorphous gel obtained from HZSM-5 recovers the initial long-range structure of the parent material in a few hours at 85 degrees C in the presence of an aqueous solution of tetrapropyl ammonium (TPA) or NH3. The recrystallization of HY requires the presence of tetramethylammonium, but about 25% of the crystallization is obtained rapidly (approximately = 1 day) at 80 degrees C with ammonia. Hypotheses about the preorganized structural units are presented. The value of the Si-O-Si angle in the silica cluster seems to be of paramount importance. 相似文献
98.
Alejandra Salerno Monica E. Hedrera Isabel A. Perillo Norma B. D'Accorso Miriam Martins Alho 《Journal of heterocyclic chemistry》2000,37(1):57-62
The electron impact mass spectral fragmentation pathways for a series of 1,3‐di‐ and 1,2,3‐trisubstituted imidazolidines were investigated. The data show remarkable differences depending on the location and identity of substituents. The atypical 1,2,2,3‐tetrasubstituted imidazolidine, 2‐cyano‐1‐methyl‐3‐(p‐nitro‐phenyl)‐2‐phenylimidazolidine, was also investigated. 相似文献
99.
The importance of examples and exemplification in mathematical thinking, learning and teaching, is well recognized by mathematicians, epistemologists and mathematics educators. In the collection of papers on these topics presented in this issue, we aim to contribute to the debate on this theme, proposing original studies carried out from different approaches and perspectives, and linked to other relevant topics within mathematics education. 相似文献
100.
We study the effects of linking constraints on stability, efficiency and network formation. An exogenous “link-constraining system” specifies the admissible links. It is assumed that each player may initiate links only with players within a specified set of players, thus restricting the feasible strategies and networks. In this setting, we examine the impact of such constraints on stable/efficient architectures and on dynamics. 相似文献