全文获取类型
收费全文 | 232篇 |
免费 | 13篇 |
专业分类
化学 | 193篇 |
力学 | 2篇 |
数学 | 29篇 |
物理学 | 21篇 |
出版年
2023年 | 6篇 |
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 10篇 |
2019年 | 5篇 |
2018年 | 6篇 |
2017年 | 3篇 |
2016年 | 7篇 |
2015年 | 7篇 |
2014年 | 6篇 |
2013年 | 15篇 |
2012年 | 14篇 |
2011年 | 17篇 |
2010年 | 7篇 |
2009年 | 9篇 |
2008年 | 16篇 |
2007年 | 22篇 |
2006年 | 20篇 |
2005年 | 13篇 |
2004年 | 13篇 |
2003年 | 13篇 |
2002年 | 6篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 3篇 |
1994年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1974年 | 2篇 |
1965年 | 1篇 |
1933年 | 1篇 |
排序方式: 共有245条查询结果,搜索用时 31 毫秒
61.
The differential solubility of two poly(ethyleneterphthalate) (PET) fibres was determined in one thermally treated substrate
and in another thermally untreated. In both cases, mixtures of phenol/tetrachloroethane with the appropriate composition were
used.
The non-dissolved fractions of the untreated PET fibre display a molecular mass and a crystallinity which increase on increase
of the temperature of the differential solubility test. In the case of the thermally treated substrate, both the molecular
mass and the crystallinity of the residual fraction remain constant on increase of the test temperature.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
62.
The collision-induced dissociation of C(2)X(5)(+) (C(2)Cl(2)F(3)(+), C(2)Cl(3)F(2)(+) and C(2)Cl(4)F(+)) and C(2)X(4)(+.) ions (C(2)ClF(3)(+*), C(2)Cl(2)F(2)(+*), and C(2)ClF(3)(+*)) derived from three chlorofluoroethanes (the isomeric 1,1,1- and 1,1,2-trichlorotrifluoroethane and 1,1,1,2-tetrachlorodifluoroethane) was investigated by means of multi-stage mass spectrometric (MS(n)) experiments in an ion trap mass spectrometer. The observation of a common dissociation pattern for ions of any given elemental composition suggests that the experiments could not differentiate isomeric C(2)X(5)(+) ions formed from different neutral precursors and originally having different structures. For any given elemental composition, a common dissociation pattern was observed, suggesting that energy barriers for isomer interconversion are lower than for dissociation. For ions containing two or more fluorine atoms, the major (in some cases unique) dissociation involves C-C cleavage to form CX(3)(+) and CF(2). Energetically, CF(2) loss is always the most favorable reaction; mechanistically it implies, at least in some cases, rearrangement via halogen transfer from one carbon to the other (for example, in the case of the C(2)Cl(2)F(3)(+) species derived from 1,1,1-trichlorotrifluoroethane, which should have initially the Cl(2)C(+)-CF(3) structure). Similar behavior was observed with C(2)X(4)(+*) ions produced both from the three chlorofluoroethanes and from model alkenes (trifluorochloroethene and tetrachloroethene). The dissociation behavior of these C(2)X(4)(+*) species is characteristic of the ion composition, with no memory of the original neutral precursor structure. Specifically, C(2)Cl(2)F(2)(+*) ions dissociate uniquely via loss of CF(2), C(2)ClF(3)(+*) ions eliminate preferentially CF, with CF(2) loss being only a minor reaction, whereas C(2)Cl(3)F(+*) and C(2)Cl(4)(+*) dissociate exclusively via Cl elimination. 相似文献
63.
Caro E Marcé RM Cormack PA Sherrington DC Borrull F 《Journal of separation science》2005,28(16):2080-2085
A molecularly imprinted polymer (MIP) is synthesised by a noncovalent protocol in which ibuprofen was used as a template molecule. The polymer was evaluated chromatographically and it was seen that the MIP showed cross-reactivity. Subsequently, when this polymer was used as sorbent in SPE it was possible to selectively extract a mixture of nonsteroidal anti-inflammatory drugs from aqueous samples when a cleanup step with dichloromethane was performed. The performance of the MIP was evaluated with river water and water from a wastewater treatment plant, and compared with the performance of a commercial Isolute ENV+ sorbent. 相似文献
64.
Jansat S Gómez M Philippot K Muller G Guiu E Claver C Castillón S Chaudret B 《Journal of the American Chemical Society》2004,126(6):1592-1593
Palladium nanoparticles (4 nm, fcc) were prepared through decomposition of [Pd2(dba)3] by H2 in the presence of a chiral xylofuranoside diphosphite. These particles catalyze the allylic alkylation of rac-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate leading to an almost total conversion of the (R) enantiomer and almost no reaction with the (S). This gives rise to 97% ee for the alkylation product and a kinetic resolution of the substrate recovered with ca. 90% ee. This behavior was compared to that of a molecular catalyst at various dilutions, and the differences between the two systems are discussed. This is the first colloidal system shown to display such a high enantioselectivity besides the well-known Pt/cinchonidine system. 相似文献
65.
Carolina Carrillo‐Carrin Raquel Martínez María F. NavarroPoupard Beatriz Pelaz Ester Polo Ana Arenas‐Vivo Alessandro Olgiati Pablo Taboada Mahmoud G. Soliman Úrsula Cataln Sara Fernndez‐Castillejo Rosa Sol Wolfgang J. Parak Patricia Horcajada Ramon A. Alvarez‐Puebla Pablo delPino 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(21):7152-7156
A plasmonic core–shell gold nanostar/zeolitic‐imidazolate‐framework‐8 (ZIF‐8) nanocomposite was developed for the thermoplasmonic‐driven release of encapsulated active molecules inside living cells. The nanocomposites were loaded, as a proof of concept, with bisbenzimide molecules as functional cargo and wrapped with an amphiphilic polymer that prevents ZIF‐8 degradation and bisbenzimide leaking in aqueous media or inside living cells. The demonstrated molecule‐release mechanism relies on the use of near‐IR light coupled to the plasmonic absorption of the core gold nanostars, which creates local temperature gradients and thus, bisbenzimide thermodiffusion. Confocal microscopy and surface‐enhanced Raman spectroscopy (SERS) were used to demonstrate bisbenzimide loading/leaking and near‐IR‐triggered cargo release inside cells, thereby leading to DNA staining. 相似文献
66.
Chelo Ferreira Jos L. Lpez Ester Prez Sinusía 《Journal of Computational and Applied Mathematics》2008,217(1):88-109
It has been shown in Ferreira et al. [Asymptotic relations in the Askey scheme for hypergeometric orthogonal polynomials, Adv. in Appl. Math. 31(1) (2003) 61–85], López and Temme [Approximations of orthogonal polynomials in terms of Hermite polynomials, Methods Appl. Anal. 6 (1999) 131–146; The Askey scheme for hypergeometric orthogonal polynomials viewed from asymptotic analysis, J. Comput. Appl. Math. 133 (2001) 623–633] that the three lower levels of the Askey table of hypergeometric orthogonal polynomials are connected by means of asymptotic relations. In Ferreira et al. [Limit relations between the Hahn polynomials and the Hermite, Laguerre and Charlier polynomials, submitted for publication] we have established new asymptotic connections between the fourth level and the two lower levels. In this paper, we continue with that program and obtain asymptotic expansions between the fourth level and the third level: we derive 16 asymptotic expansions of the Hahn, dual Hahn, continuous Hahn and continuous dual Hahn polynomials in terms of Meixner–Pollaczek, Jacobi, Meixner and Krawtchouk polynomials. From these expansions, we also derive three new limits between those polynomials. Some numerical experiments show the accuracy of the approximations and, in particular, the accuracy in the approximation of the zeros of those polynomials. 相似文献
67.
Development of Selective RabGGTase Inhibitors and Crystal Structure of a RabGGTase–Inhibitor Complex
68.
Manrubia SC Lázaro E Pérez-Mercader J Escarmís C Domingo E 《Physical review letters》2003,90(18):188102
We explore a mean-field model for the evolution of exponentially growing populations of mutating replicators. Motivated by recent in vitro experiments devised to analyze phenotypic properties of bacterial and viral populations subjected to serial population transfers, we allow our in silico individuals to undergo unrestricted growth before applying bottleneck events. Different dynamical regimes of our model can be mapped to different experimental situations. Numerical and analytical results for fitness distributions calculated at the statistically stationary states of the dynamics compare favorably with available experimental data. Our model and results provide a common framework to better understand populations evolving under different selection pressures. 相似文献
69.
Bennasar ML Zulaica E Juan C Alonso Y Bosch J 《The Journal of organic chemistry》2002,67(21):7465-7474
Several 4-substituted dihydropyridones or 2-pyridones have been prepared by nucleophilic addition of alpha-(methylsulfanyl)ester enolates to N-alkyl-2-fluoropyridinium salts, followed by acid hydrolysis or oxidation with concomitant hydrolysis, of the intermediate 2-fluoro-1,4-dihydropyridine adducts, respectively. Addition of the enolate derived from isopropyl alpha-(methylsulfanyl)butyrate to N-(quinolylmethyl)-2-fluoropyridinium triflate 21 followed by DDQ treatment gave pyridone 29, from which (+/-)-20-deoxycamptothecin (31), a known precursor of camptothecin, was synthesized by a radical cyclization-desulfurization, with subsequent elaboration of the lactone E ring by chemoselective reduction. A similar sequence starting from the enolate of a chiral 2-hydroxybutyric acid derivative (33) provides access to natural (+)-camptothecin (37). 相似文献
70.