全文获取类型
收费全文 | 378篇 |
免费 | 12篇 |
专业分类
化学 | 256篇 |
力学 | 7篇 |
数学 | 30篇 |
物理学 | 59篇 |
无线电 | 38篇 |
出版年
2024年 | 1篇 |
2023年 | 8篇 |
2022年 | 9篇 |
2021年 | 14篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 12篇 |
2017年 | 8篇 |
2016年 | 13篇 |
2015年 | 8篇 |
2014年 | 13篇 |
2013年 | 16篇 |
2012年 | 18篇 |
2011年 | 42篇 |
2010年 | 16篇 |
2009年 | 12篇 |
2008年 | 30篇 |
2007年 | 35篇 |
2006年 | 20篇 |
2005年 | 28篇 |
2004年 | 9篇 |
2003年 | 17篇 |
2002年 | 7篇 |
2001年 | 8篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1985年 | 1篇 |
1977年 | 1篇 |
1858年 | 1篇 |
排序方式: 共有390条查询结果,搜索用时 10 毫秒
381.
382.
Valeria Di Tullio Noemi Proietti Marco Gobbino Donatella Capitani Roberto Olmi Saverio Priori Cristiano Riminesi Elisabetta Giani 《Analytical and bioanalytical chemistry》2010,396(5):1885-1896
As is well known, the deterioration of wall paintings due to the capillary rise of water through the walls is a very widespread
problem. In this paper, a study of microclimate monitoring, unilateral nuclear magnetic resonance (NMR), and evanescent-field
dielectrometry (EFD) was applied to map non-destructively, in situ, and in a quantitative way the distribution of the moisture
in an ancient deteriorated wall painting of the eleventh century. Both unilateral NMR and EFD are quite new, fully portable,
and non-destructive techniques, and their combination is absolutely new. The approach reported here is proposed as a new analytical
protocol to afford the problem of mapping, non-destructively, the moisture in a deteriorated wall painting in a hypogeous
building such as that of the second level of St. Clement Basilica, Rome (Italy), where the use of IR thermography is impaired
due to the environmental conditions, and the gravimetric tests are forbidden due to the preciousness of the artifact. The
moisture distribution was mapped at different depths, from the very first layers of the painted film to a depth of 2 cm. It
has also been shown how the map obtained in the first layers of the artwork is affected by the environmental conditions typical
of a hypogeous building, whereas the maps obtained at higher depths are representative of the moisture due to the capillary
rise of water from the ground. The quantitative analysis of the moisture was performed by calibrating NMR and EFD signals
with purposely prepared specimens. This study may be applied before and after performing any intervention aimed at restoring
and improving the state of conservation of this type of artwork and reducing the dampness or extracting salts (driven by the
variation of moisture content) and monitoring the effectiveness of the performed interventions during the time. This protocol
is applicable to any type of porous material. 相似文献
383.
The four positional isomers of nitrocatechol monobenzyl ether were prepared as intermediates to nitrobenzodioxanes directly from 2-benzyloxyphenol or, through two-four steps, from catechol. These preparations addressed the issue of the certain identification of the nitration products prescinding from chemical correlation to the synthetic precursors because the positional isomers are very similar for some properties and analytical data available from the literature are largely incomplete and not conclusive. The here provided NMR, DSC, and acidity data unequivocally distinguish each nitrocatechol monobenzyl ether from its regioisomers. 相似文献
384.
Cammi R Gennari M Giannetto M Lanfranchi M Marchio L Mori G Paiola C Pellinghelli MA 《Inorganic chemistry》2005,44(12):4333-4345
Dinuclear Cu(I) complexes with bifunctionalized homoscorpionate ligands, hydrotris(thioxotriazolyl)borato [Li(Tr(Me,o)(-)(Py)) (1) and Li(Tr(Mes,Me)) (2)], and the heteroscorpionate ligand hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]borato [K(Br(Mes)pz(o)(-)(Py))] (3) were synthesized and crystallographically characterized. The complexes [Cu(Tr(Mes,Me))](2) (4) and [Cu(Tr(Me,o)(-)(Py))](2) (5) exhibit a similar coordination geometry where every metal is surrounded by three thioxo groups in a trigonal arrangement. The presence of a [B-H...Cu] three-center-two-electron interaction in both compounds causes the overall coordination to become tetrahedrally distorted (S(3)H coordination for each metal). The complex [Cu(Br(Mes)pz(o)(-)(Py))](2) (6) presents a trigonal geometry in which the metals interact with two thioxo groups and a bridging pyrazolyl nitrogen atom. A weak contact with a pyridine nitrogen atom completes the coordination of the metals (S(2)N,N' coordination for each metal). [Cu(Tr(Mes,Me))](2), [Cu(Tr(Me,o)(-)(Py))](2), and [Cu(Br(Mes)pz(o)(-)(Py))](2) exhibit fluxional behavior in solution as evidenced by variable-temperature NMR spectroscopy, and for 5 and 6 two species in equilibrium [in the ratio 2/1 for 5 (CDCl(3)) and 3/2 for 6 (CD(2)Cl(2))] are distinguishable in the (1)H NMR spectra at 270 K. 2D-NOESY spectra recorded at 270 K assisted in the attribution of solution molecular geometries for each isomer of 5 and 6. The free energy of activation (DeltaG()(Tc)) was determined for both equilibria from the evaluation of the coalescence temperature. DFT calculations were performed to describe plausible molecular geometry for the minor isomer of 5 and 6 and to propose a possible mechanism of interconversion between major and minor isomers. Cyclic voltammograms were recorded in CH(2)Cl(2) (3 and 6) or CH(2)Cl(2)/CH(3)CN (1/1, v/v) (2, 4, and 5) solutions using 0.1 M TBAHFP or TBAOTf as supporting electrolytes. [Cu(Tr(Mes,Me))](2), [Cu(Tr(Me,o)(-)(Py))](2), and [Cu(Br(Mes)pz(o)(-)(Py))](2) exhibit a quasi-reversible Cu(I)/Cu(II) redox behavior with E(pa) = +719 mV and E(pc) = +538 mV for 4, E(pa) = +636 mV and E(pc) = -316 mV for 5, and E(pa) = +418 mV and E(pc) = -319 mV for 6. 相似文献
385.
Frija LM Reva I Ismael A Coelho DV Fausto R Cristiano ML 《Organic & biomolecular chemistry》2011,9(17):6040-6054
Mechanisms of thermal isomerization of allyl tetrazolyl ethers derived from the carbocyclic allylic alcohols cyclohex-2-enol and 3-methylcyclohex-2-enol and from the natural terpene alcohol nerol were investigated. In the process of the syntheses of the three 1-aryl-5-allyloxytetrazoles, their rapid isomerization to the corresponding 1-aryl-4-allyltetrazol-5-ones occurred. The experiments showed that the imidates rearrange exclusively through a [3,3']-sigmatropic migration of the allylic system from O to N, with inversion. Mechanistic proposals are based on product analysis and extensive quantum chemical calculations at the DFT(B3LYP) and MP2 levels, on O-allyl and N-allyl isomers and on putative transition state structures for [1,3']- and [3,3']-sigmatropic migrations. The experimental observations could be only explained on the basis of the MP2/6-31G(d,p) calculations that favoured the [3,3']-sigmatropic migrations, yielding lower energies both for the transition states and for the final isomerization products. 相似文献
386.
Zuccaccia C Macchioni A Busico V Cipullo R Talarico G Alfano F Boone HW Frazier KA Hustad PD Stevens JC Vosejpka PC Abboud KA 《Journal of the American Chemical Society》2008,130(31):10354-10368
Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Br?nsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N(-),N,C(-)}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf{N(-),N,C(-)}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N(-),N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N(-),N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N(-),C(-),N,C(-)}NR3][B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N(-),N}NMe2C6H4][B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N(-),C(-),N,C(-)}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Br?nsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems. 相似文献
387.
388.
Susana C. S. Bugalho Leszek Lapinski M. Lurdes S. Cristiano Luís M. T. Frija Rui Fausto 《Vibrational Spectroscopy》2002,30(2):213-225
Infrared spectra of 1-phenyltetrazole (C7N4H6) and 5-chloro-1-phenyltetrazole (C7N4H5Cl) isolated in argon matrixes (T=8 K) and in the solid state (at room temperature) were studied. DFT(B3LYP)/6-31G* calculations predict the minimum energy conformation of 1-phenyltetrazole as being non-planar, with the two rings (phenyl and tetrazole) twisted by 29°. For 5-chloro-1-phenyltetrazole, the optimized dihedral angle between the two rings is larger (48°). The theoretically calculated IR spectra of both compounds fit well the spectra observed experimentally. This allowed a reliable assignment of observed IR absorption bands. 相似文献
389.
Javier Plou Isabel García Mathias Charconnet Ianire Astobiza Clara García‐Astrain Cristiano Matricardi Agustín Mihi Arkaitz Carracedo Luis M. Liz‐Marzn 《Advanced functional materials》2020,30(17)
The composition and intercellular interactions of tumor cells in the tissues dictate the biochemical and metabolic properties of the tumor microenvironment. The metabolic rewiring has a profound impact on the properties of the microenvironment, to an extent that monitoring such perturbations could harbor diagnostic and therapeutic relevance. A growing interest in these phenomena has inspired the development of novel technologies with sufficient sensitivity and resolution to monitor metabolic alterations in the tumor microenvironment. In this context, surface‐enhanced Raman scattering (SERS) can be used for the label‐free detection and imaging of diverse molecules of interest among extracellular components. Herein, the application of nanostructured plasmonic substrates comprising Au nanoparticles, self‐assembled as ordered superlattices, to the precise SERS detection of selected tumor metabolites, is presented. The potential of this technology is first demonstrated through the analysis of kynurenine, a secreted immunomodulatory derivative of the tumor metabolism and the related molecules tryptophan and purine derivatives. SERS facilitates the unambiguous identification of trace metabolites and allows the multiplex detection of their characteristic fingerprints under different conditions. Finally, the effective plasmonic SERS substrate is combined with a hydrogel‐based three‐dimensional cancer model, which recreates the tumor microenvironment, for the real‐time imaging of metabolite alterations and cytotoxic effects on tumor cells. 相似文献
390.
Gustavo M. do Nascimento Wdeson P. Barros Yoong Ahm Kim Hiroyuki Muramatsu Takuya Hayashi Morinobu Endo Noriberto A. Pradie Cristiano Fantini Marcos A. Pimenta Mildred S. Dresselhaus Humberto O. Stumpf 《Journal of Raman spectroscopy : JRS》2012,43(12):1951-1956
The electronic interactions between the [Cu(opba)]2− anions (where opba is orthophenylenebis (oxamato)) and single‐wall carbon nanotubes (SWCNTs) were investigated by resonance Raman spectroscopy. The opba can form molecular magnets, and the interactions of opba with SWCNTs can produce materials with very different magnetic/electronic properties. It is observed that the electronic interaction shows a dependence on the SWCNT diameter independent of whether they are metallic or semiconducting, although the interaction is stronger for metallic tubes. The interaction also is dependent on the amount of complex that is probably adsorbed on the carbon surface of the SWCNTs. Some charge transfer can be also occurring between the metallic complex and the SWCNTs. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献