全文获取类型
收费全文 | 515篇 |
免费 | 26篇 |
国内免费 | 8篇 |
专业分类
化学 | 212篇 |
晶体学 | 1篇 |
力学 | 26篇 |
数学 | 146篇 |
物理学 | 73篇 |
无线电 | 91篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 16篇 |
2021年 | 18篇 |
2020年 | 19篇 |
2019年 | 20篇 |
2018年 | 17篇 |
2017年 | 13篇 |
2016年 | 25篇 |
2015年 | 11篇 |
2014年 | 25篇 |
2013年 | 52篇 |
2012年 | 34篇 |
2011年 | 27篇 |
2010年 | 22篇 |
2009年 | 21篇 |
2008年 | 30篇 |
2007年 | 19篇 |
2006年 | 24篇 |
2005年 | 22篇 |
2004年 | 19篇 |
2003年 | 14篇 |
2002年 | 17篇 |
2001年 | 7篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1885年 | 1篇 |
排序方式: 共有549条查询结果,搜索用时 15 毫秒
521.
Bassam Mourad 《Linear and Multilinear Algebra》2013,61(2):99-113
In this article, we study generalized doubly stochastic matrices using the theory of Lie groups and Lie algebras. Applications to the inverse eigenvalue problem for symmetric doubly stochastic matrices are presented. 相似文献
522.
Bassam Mourad 《Linear and Multilinear Algebra》2013,61(9):1234-1243
In this note, we present a generalization of some results concerning the spectral properties of a certain class of block matrices. As applications, we study some of its implications on nonnegative matrices and doubly stochastic matrices as well as on graph spectra and graph energy. 相似文献
523.
524.
Abdoullah Bimoussa Ali Oubella Yassine Laamari Mourad Fawzi Mouhi Eddine Hachim My Youssef Ait Itto Hamid Morjani El Mostafa Ketatni Olivier Mentre Aziz Auhmani 《Journal of heterocyclic chemistry》2021,58(12):2334-2347
A new 1,2,3-triazole sesquiterpenic named (4aR,5aS,7aR,10aR,10bR)-10-benzyl-5,5-dichloro-1,1,4a,7a-tetramethyl-1,2,3,4,4a,5,7a,10,10a,10b decahydrocyclopropa[2′,3′]cyclohepta [1′,2′:3,4]benzo[1,2-d][1,2,3]triazol-7(6H)-one was synthesized by a dipolar cycloaddition 1,3 between benzyl azide and α,β-unsaturated sesquiterpene ketone catalyzed by titanium tetrachloride TiCl4. It was obtained with a good yield and characterized using single-crystal X-ray diffraction technique, infra red (IR) spectroscopy, High Resolution Mass Spectrometry (HRMS), UV-Visible, and 1H and 13C NMR spectroscopies. The optimization by the density functional theory (DFT) method was carried out to study the formation mechanism of the derivative. The selected compounds 1 – 4 exhibited anticancer activities in vitro against namely fibrosarcoma (HT-1080), lung carcinoma (A-549), and breast (MCF-7 and MDA-MB-231 cell lines with IC50 in the range of 10.02–44.71 μM). In addition to using flow cytometry and the apoptosis assay, cell cycle arrest at the S phase (10 μM) and G2/M arrest in (5 and 10 μM) was performed for compound 4 in fibrosarcoma (HT-1080) cancer cells. Results suggested that compound 4 test with good activity against HT-1080 cells with IC50 value from 10.02 μM and induced the apoptotic effect. Theoretical studies were realized to estimate the molecular proprieties that lead to a high potential of cytotoxic effect, which is in agreement with the experimental results. 相似文献
525.
Kikkeri R Traboulsi H Humbert N Gumienna-Kontecka E Arad-Yellin R Melman G Elhabiri M Albrecht-Gary AM Shanzer A 《Inorganic chemistry》2007,46(7):2485-2497
In the quest for fast throughput metal biosensors, it would be of interest to prepare fluorophoric ligands with surface-adhesive moieties. Biomimetic analogues to microbial siderophores possessing such ligands offer attractive model compounds and new opportunities to meet this challenge. The design, synthesis, and physicochemical characterization of biomimetic analogues of microbial siderophores from Paracoccus denitrificans and from the Vibrio genus are described. The (4S,5S)-2-(2-hydroxyphenyl)-5-methyl-4,5-dihydro-1,3-oxazole-4-carbonyl group (La), noted here as an HPO unit, was selected for its potential dual properties, serving as a selective iron(III) binder and simultaneously as a fluorophore. Three tripodal symmetric analogues cis-Lb, cis-Lc, and trans-Lc, which mainly differ in the length of the spacers between the central carbon anchor and the ligating sites, were synthesized. These ferric-carriers were built from a tetrahedral carbon as an anchor, symmetrically extended by three converging iron-binding chains, each bearing a terminal HPO. The fourth chain could contain a surface-adhesive function (Lc). A combination of absorption and emission spectrophotometry, potentiometry, electrospray mass spectrometry, and electrochemistry was used to fully characterize the corresponding ferric complexes and to determine their stability. The quenching mechanism is consistent with an intramolecular static process and is more efficient for the analogue with longer arms. Detection limits in the low nanogram per milliliter range, comparable with the best chemosensors based on natural peptide siderophores, have been determined. These results clearly demonstrate that these tris(phenol-oxazoline) ligands in a tripodal arrangement firmly bind iron(III). Due to their fluorescent properties, the coordination event can be easily monitored, while the fourth arm is available for surface-adhesive moieties. The tripodal system is therefore an ideal candidate for integration with solid-state materials for the development of chip-based devices and analytical methodologies. 相似文献
526.
High resolution vacuum ultraviolet emission spectrum of D2: the B' 1Sigmau+-->X 1Sigmag+ band system
In this work, we have extended our previous high resolution study of the vacuum ultraviolet emission spectrum of the D2 molecule [M. Roudjane, et al. J. Chem. Phys. 125, 214305 (2006)] up to 124.2 nm in order to investigate the B' 1Sigmau+-->X 1Sigmag+ band system. The analysis of the spectrum has been carried out by means of a complex spectrum visual identification code IDEN [V. I. Azarov, Phys. Scr. 44, 528 (1991); 48, 656, (1993)] and supported by theoretical calculations using ab initio data [L. Wolniewicz, J. Chem. Phys. 103, 1792 (1995); 99, 1851 (1993); G. Staszewska and L. Wolniewicz, J. Mol. Spectrosc. 212, 208 (2002); L. Wolniewicz and G. Staszewska, 220, 45 (2003)] which provided level energies and transition probabilities. More than 1480 new emission lines have been observed and 109 bands belonging to the B' 1Sigmau+-->X 1Sigmag+ system have been identified between 84.1 and 121.6 nm. Except for the upsilon'-0 bands that were reported in absorption [I. Dabrowski and G. Herzberg, Can. J. Phys. 52, 1110 (1974)], all the upsilon'-upsilon" bands are reported here for the first time. The analysis led to the determination of 111 rovibronic energy levels in the B' 1Sigmau+ state, of which 31 with higher rotational numbers J are new. Observed perturbations are accounted for through a set of coupled equations involving the four excited electronic states B 1Sigmau+, B' 1Sigmau+, C 1Piu, and D 1Piu and including nonadiabatic couplings. The solution of this set provides the percent contribution of these four states to each of the observed rovibronic level. 相似文献
527.
Roy S Mourad MC Rijneveld-Ockers MT 《Langmuir : the ACS journal of surfaces and colloids》2007,23(2):399-401
The polyoxometalates, or POMs (clusters comprising at least two metal and many oxygen atoms), have recently gained significant interest, owing to their versatile architecture and especially catalytic activities. Due to their high catalytic activity but low surface area, there is always a demand for making high-surface-area POMs, particularly high-surface-area Keggins. Our present work demonstrates the 'gluing' of the anionic phosphomolybdate Keggin on gibbsite nanoplatelets with a residual positive charge to form large-surface-area composites. The resulting composite reported here has been characterized using TEM imaging, EDX/TEM (elemental) analysis, FTIR spectroscopy, potentiometric titrations, electrophoretic mobility determination, and XRD. The composite reported here could be useful in catalysis. 相似文献
528.
Bachir BenniniAlbert J. Chulia Mourad KaouadjiPatrice Fondanèche Daovy P. Allais 《Tetrahedron letters》2011,52(14):1597-1600
The reinvestigation of Erica cinerea fresh aerial parts led to the isolation of two new diarylnonanoid aglycones along with their glucosides. From spectroscopic data, their structures were elucidated as rel-(3R,7R)-1,9-bis(p-hydroxyphenyl)-3,7-dihydroxynonan-5-one named ericanone, ericanone 3-β-d-glucoside, (3S)-3,7-anhydro-6,7-dehydroericanone and (3S)-3,7-anhydro-6,7-dehydroericanone 4′-β-d-glucoside. Contrary to the numerous diarylheptanoids more frequently distributed in the plant kingdom, the rare diarylnonanoids were previously restricted to the genus Myristica of the Myristicaceae plant family. 相似文献
529.
Hmadeh M Fahrenbach AC Basu S Trabolsi A Benítez D Li H Albrecht-Gary AM Elhabiri M Stoddart JF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(22):6076-6087
The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY2+) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5‐dioxynaphthalene (DNP) units flanking a central BIPY2+ unit in the dumbbell component and encircled by the cyclobis(paraquat‐p‐phenylene) (CBPQT4+) tetracationic cyclophane, has been synthesized employing a threading‐followed‐by‐stoppering approach. Variable‐temperature 1H NMR spectroscopy reveals that the barrier to shuttling of the CBPQT4+ ring over the central BIPY2+ unit is in excess of 17 kcal mol?1 at 343 K. Further information about the nature of the BIPY2+ unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY2+ moiety or a central DNP unit flanked by a BIPY2+ moiety. The threading and dethreading processes of the CBPQT4+ ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY2+ unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self‐folding (through donor–acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (Δ${G{{{\ne}\hfill \atop {\rm f}\hfill}}}The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY(2+)) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5-dioxynaphthalene (DNP) units flanking a central BIPY(2+) unit in the dumbbell component and encircled by the cyclobis(paraquat-p-phenylene) (CBPQT(4+)) tetracationic cyclophane, has been synthesized employing a threading-followed-by-stoppering approach. Variable-temperature (1)H?NMR spectroscopy reveals that the barrier to shuttling of the CBPQT(4+) ring over the central BIPY(2+) unit is in excess of 17 kcal mol(-1) at 343 K. Further information about the nature of the BIPY(2+) unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY(2+) moiety or a central DNP unit flanked by a BIPY(2+) moiety. The threading and dethreading processes of the CBPQT(4+) ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY(2+) unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self-folding (through donor-acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (ΔG(f)(++)) of the CBPQT(4+) for the case of the thread composed of a DNP flanked by two BIPY(2+) units was found to be as high as 21.7 kcal mol(-1) at room temperature. These results demonstrate that we can effectively employ the BIPY(2+) unit to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT(4+) ring in both mechanically interlocked and supramolecular systems. 相似文献
530.
Sucunza D Samadi A Chioua M Silva DB Yunta C Infantes L Carmo Carreiras M Soriano E Marco-Contelles J 《Chemical communications (Cambridge, England)》2011,47(17):5043-5045
The Sandmeyer reaction of differently C-2 substituted N-(prop-2-yn-1-ylamino)pyridines is an efficient, mild, new and practical method for the stereospecific synthesis of (E)-exo-halomethylene bicyclic pyridones bearing the imidazo[1,2-a]pyridine heterocyclic ring system. 相似文献