全文获取类型
收费全文 | 426篇 |
免费 | 67篇 |
国内免费 | 33篇 |
专业分类
化学 | 476篇 |
综合类 | 1篇 |
物理学 | 37篇 |
综合类 | 12篇 |
出版年
2023年 | 3篇 |
2022年 | 5篇 |
2021年 | 7篇 |
2020年 | 12篇 |
2019年 | 9篇 |
2018年 | 17篇 |
2017年 | 15篇 |
2016年 | 23篇 |
2015年 | 19篇 |
2014年 | 19篇 |
2013年 | 31篇 |
2012年 | 27篇 |
2011年 | 27篇 |
2010年 | 16篇 |
2009年 | 21篇 |
2008年 | 24篇 |
2007年 | 22篇 |
2006年 | 28篇 |
2005年 | 30篇 |
2004年 | 23篇 |
2003年 | 28篇 |
2002年 | 12篇 |
2001年 | 8篇 |
2000年 | 12篇 |
1999年 | 6篇 |
1998年 | 8篇 |
1997年 | 5篇 |
1996年 | 11篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 12篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1984年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有526条查询结果,搜索用时 46 毫秒
351.
Valentin A. Semenov Dmitry O. Samultsev Leonid B. Krivdin 《Magnetic resonance in chemistry : MRC》2015,53(6):433-441
A combined theoretical and experimental study revealed that the nature of the upfield (shielding) protonation effect in 15N NMR originates in the change of the contribution of the sp2‐hybridized nitrogen lone pair on protonation resulting in a marked shielding of nitrogen of about 100 ppm. On the contrary, for amine‐type nitrogen, protonation of the nitrogen lone pair results in the deshielding protonation effect of about 25 ppm, so that the total deshielding protonation effect of about 10 ppm is due to the interplay of the contributions of adjacent natural bond orbitals. A versatile computational scheme for the calculation of 15N NMR chemical shifts of protonated nitrogen species and their neutral precursors is proposed at the density functional theory level taking into account solvent effects within the supermolecule solvation model. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
352.
353.
Volchkov V. V. Gromov S. P. Uzhinov B. M. Alfimov M. V. 《Russian Chemical Bulletin》2001,50(7):1186-1189
The protonation of some 2-quinolylthiazoles by trichloroacetic, trifluoroacetic, and sulfuric acids in the ground and excited states was studied in ethanol. The formation of the cations was found in the reaction with sulfuric acid. The formation of a mixture of cations and ionic pairs was found in the reactions with trichloroacetic and trifluoroacetic acids. The spectral-luminescence parameters of the reaction products obtained and the kinetic characteristics of the processes were determined. 相似文献
354.
355.
Agnieszka Iwan Bozena Kaczmarczyk Janusz Kasperczyk Jan Jurusik Henryk Janeczek Danuta Sek Zbigniew mazurak 《Journal of polymer science. Part A, Polymer chemistry》2006,44(19):5645-5660
New luminescent materials—aromatic polyketanils (PKs) containing p‐phenylene and biphenylene linkages in the backbone and electron‐donating, side‐group substitutions—were synthesized via melt polycondensation with the goal of studying the interactions between the polymer chains (PKs), primary dopant [1,2‐(di‐2‐ethylhexyl)ester of 4‐sulfophthalic acid (PSA)], and secondary dopant [m‐cresol (MC)] with 1H NMR, 13C NMR, Fourier transform infrared (FTIR), ultraviolet–visible, and photoluminescence measurements. Spectral changes were observed for the PKs after primary and secondary doping, and they supported ionic‐ and hydrogen‐bond formation between the PKs and the PSA and MC, respectively. This specific interaction of the dopant with the host polymer influenced the PK properties, and the following observations were made: (1) HOMO/LUMO energy gap of the protonated PKs; (2) bathochromic/hypsochromic photoluminescence shifts observed after protonation with PSA; (3) a downfield chemical shift of the labile proton in the protonating agent OH group in the 1H NMR spectra of the protonated polymers; (4) a protonation decrease in the intensity of the C?N stretching band in the FTIR spectrum; and (5) a spherulite‐like morphology, detected by atomic force microscopy and polarized optical microscopy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5645–5660, 2006 相似文献
356.
Wei Guan Li‐Kai Yan Zhong‐Min Su En‐Bo Wang Xiao‐Hong Wang 《International journal of quantum chemistry》2006,106(8):1860-1864
Density functional theory (DFT) calculations were carried out to characterize the optimal site of the protons and the precise protonation state in the polyoxometalate (POM) anions [V13O40]15? and [H12V13O40]3?. Six kinds of possible protonated stable isomers with the whole Keggin anion units are discussed. The calculations reveal that the preferred protonation site corresponds to bridging oxygens that belong to the same trimetallic group (isomers B and C). Both isomers B and C are comparatively stable in the gas phase, but only isomer B could exist stably in aqueous solution because of being stabilized by the electrostatic interaction. The solvent effects and protonation are also discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
357.
Alternating copolymers of α-methylstyrene (α-MeSt) and maleimide (MI) were prepared by free-radical initiated polymerization at different monomer-to-monomer concentrations in the feed in CHCl3, as solvent. The equilibrium constant of -MeSt and MI was determined by the transformed Benesi-Hildebrand NMR method in CDCl3, and has a value of 0.03 L/mol. From the equation Rp = Rp(f) + Rp(CT) proposed by Shirota and coworkers, Rpf) and RpCT) were calculated, and it was found that the copolymerization of -MeSt with MI proceeds predominantly through participation of the CT complex. Alternating copolymers have a glass transition temperature of 567 K (DSC method). Alternating copolymer decomposes via a one-step reaction at 350°C. 相似文献
358.
359.
360.
核磁共振研究大叶蛇葡萄提取物蛇葡萄素 总被引:3,自引:0,他引:3
通过质子检测的13C-1H异核多键化学位移相关谱、13C-1H异核多量子化学位移相关谱和质子相敏交换谱等NMR二维方法确定了传统中草药大叶蛇葡萄提取物的结构为蛇葡萄素(3,3',4',5,5',7-六羟基双氢黄酮醇),得到了更加准确的1H和13C化学位移归属,并观测和讨论了这一体系中溶剂二甲亚砜的质子化现象. 相似文献