全文获取类型
收费全文 | 1933篇 |
免费 | 52篇 |
国内免费 | 5篇 |
专业分类
化学 | 1455篇 |
晶体学 | 13篇 |
力学 | 89篇 |
数学 | 213篇 |
物理学 | 220篇 |
出版年
2023年 | 13篇 |
2022年 | 45篇 |
2021年 | 44篇 |
2020年 | 23篇 |
2019年 | 26篇 |
2018年 | 16篇 |
2017年 | 20篇 |
2016年 | 52篇 |
2015年 | 55篇 |
2014年 | 43篇 |
2013年 | 83篇 |
2012年 | 104篇 |
2011年 | 110篇 |
2010年 | 72篇 |
2009年 | 54篇 |
2008年 | 131篇 |
2007年 | 136篇 |
2006年 | 136篇 |
2005年 | 126篇 |
2004年 | 120篇 |
2003年 | 87篇 |
2002年 | 73篇 |
2001年 | 42篇 |
2000年 | 27篇 |
1999年 | 20篇 |
1998年 | 26篇 |
1997年 | 14篇 |
1996年 | 42篇 |
1995年 | 24篇 |
1994年 | 19篇 |
1993年 | 12篇 |
1992年 | 11篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 10篇 |
1982年 | 8篇 |
1981年 | 23篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1974年 | 4篇 |
1970年 | 4篇 |
1929年 | 3篇 |
排序方式: 共有1990条查询结果,搜索用时 15 毫秒
191.
álvaro?Reyes-Carmona Ramón?Moreno-Tost Josefa?Mérida-Robles José?Santamaría-González Pedro?J.?Maireles-Torres Antonio?Jiménez-López Elisa?Moretti Maurizio?Lenarda Enrique?Rodríguez-CastellónEmail author 《Adsorption》2011,17(3):527-538
This work presents some applications of ZrO2 supported over SBA-15 silica as promoter of sulfated zirconia and as support from CuO/CeO2 catalytic system for preferential oxidation of CO to CO2 in hydrogen rich streams, used as feed for proton exchange membrane fuel cells (PEMFC). Different amounts of ZrO2, from 10 to 30 wt.% were incorporated. These prepared materials were characterized by powder XRD, adsorption-desorption of
N2 at 77 K, transmission and scanning electron microscopy (TEM and SEM) and X-rays photoelectron spectroscopy (XPS). The acidity
was studied by thermo-programmed desorption of ammonia (NH3-TPD). These materials were tested, after treatment with H2SO4, by 2-propanol dehydration and 1-butene isomerization catalytic tests. The samples were found quite good catalyst with strong
acid sites, the sample with 20 wt.% of ZrO2 being the better performing sample. Finally this material was successfully used as support for a CuO/CeO2 system, with 6 wt.% of Cu and 20 wt.% of Ce. The resulting catalyst was tested in the preferential oxidation of CO (CO-PROX)
attaining conversions close to 100% and high selectivity to CO2. 相似文献
192.
Oelsner C Schmidt C Hauke F Prato M Hirsch A Guldi DM 《Journal of the American Chemical Society》2011,133(12):4580-4586
The complementary use of steady-state and time-resolved spectroscopy in combination with electrochemistry and microscopy are indicative of mutual interactions between semiconducting SWNTs and a water-soluble strong electron acceptor, i.e., perylenediimide. Significant is the stability and the strong electronic coupling of the perylenediimide/SWNT electron donor-acceptor hybrids. Several spectroscopic and spectroelectrochemical techniques, i.e., Raman, absorption, and fluorescence, confirmed that distinct ground- and excited-state interactions occur and that kinetically and spectroscopically well characterized radical ion pair states form within a few picoseconds. 相似文献
193.
194.
We have simulated the photodynamics of azobenzene by means of the Surface Hopping method. We have considered both the trans → cis and the cis → trans processes, caused by excitation in the n → π* band (S(1) state). To bring out the solvent effects on the excited state dynamics, we have run simulations in four different environments: in vacuo, in n-hexane, in methanol, and in ethylene glycol. Our simulations reproduce very well the measured quantum yields and the time dependence of the intensity and anisotropy of the transient fluorescence. Both the photoisomerization and the S(1) → S(0) internal conversion require the torsion of the N═N double bond, but the N-C bond rotations and the NNC bending vibrations also play a role. In the trans → cis photoconversion the N═N torsional motion and the excited state decay are delayed by increasing the solvent viscosity, while the cis → trans processes are less affected. The analysis of the simulation results allows the experimental observations to be explained in detail, and in particular the counterintuitive increase of the trans → cis quantum yield with viscosity, as well as the relationship between the excited state dynamics and the solvent effects on the fluorescence lifetimes and depolarization. 相似文献
195.
Saladino G Pieraccini S Rendine S Recca T Francescato P Speranza G Sironi M 《Journal of the American Chemical Society》2011,133(9):2897-2903
Understanding the molecular mechanisms that allow some organisms to survive in extremely harsh conditions is an important achievement that might disclose a wide range of applications and that is constantly drawing the attention of many research fields. The high adaptability of these living creatures is related to the presence in their tissues of a high concentration of osmoprotectants, small organic, highly soluble molecules. Despite osmoprotectants having been known for a long time, a full disclosure of the machinery behind their activity is still lacking. Here we describe a computational approach that, taking advantage of the recently developed metadynamics technique, allows one to fully describe the free energy surface of a small β-hairpin peptide and how it is affected by an osmoprotectant, glycine betaine (GB) and for comparison by urea, a common denaturant. Simulations led to relevant thermodynamic information, including how the free energy difference of denaturation is affected by the two cosolvents; unlike urea, GB caused a considerable increase of the folded basin stability, which transposes into a higher melting temperature. NMR experiments confirmed the picture derived from the theoretical study. Further molecular dynamics simulations of selected conformations allowed investigation into deeper detail the role of GB in folded state protection. Simulations of the protein in GB solutions clearly showed an excess of osmoprotectant in the solvent bulk, rather than in the protein domain, confirming the exclusion from the protein surface, but also highlighted interesting features on its interactions, opening to new scenarios besides the classic "indirect mechanism" hypothesis. 相似文献
196.
Sala F Bagnati R Livi V Cereda R D'Incalci M Zucchetti M 《Journal of mass spectrometry : JMS》2011,46(10):1039-1045
E-3810, 6-[[7-[(1-aminocyclopropyl)methoxy]-6-methoxy-4-quinolyl]oxy]-N-methyl-naphthalene-1-carboxamide, is a novel, potent, dual inhibitor of vascular endothelial growth factor and fibroblast growth factor receptors with antiangiogenic properties, now under early clinical evaluation as an anticancer agent. To investigate its clinical pharmacokinetics, a high-performance liquid chromatography-tandem mass spectrometry method was developed and validated to measure the drug in human plasma on the basis of simple protein precipitation with methanol after addition of deuterated E-3810 as internal standard. The method requires a small volume of sample (100 μl) and is rapid and selective, allowing good resolution of peaks in 5 min. It is sensitive, precise, and accurate, with overall precision, expressed as CV%, always ≤7.1%, accuracy in the range 92.7%-104.4%, and high recovery, close to 100%. The limit of detection is 0.01 ng/ml, and the lower limit of quantitation is 2.0 ng/ml. The assay was validated in the range from the lower limit of quantitation up to 500.0 ng/ml. This is the first method developed and validated for analyzing E-3810 in human plasma. The method has been successfully applied to study E-3810 pharmacokinetics in cancer patients with solid tumors who are receiving daily oral doses of the drug during the phase I trial. 相似文献
197.
Puglisi A Benaglia M Raimondi L Lay L Poletti L 《Organic & biomolecular chemistry》2011,9(9):3295-3302
Glucosamine has been selected as a cheap and readily available chiral scaffold for the synthesis of a series of novel enantiomerically pure bifunctional organocatalysts bearing a tertiary amino group in proximity to a (thio)urea group. The catalytic behaviour of these compounds, both as neutral and N-protonated species, was investigated using the addition of acetylacetone to β-nitrostyrene as a model reaction. Under optimized experimental conditions, chemical yields up to 93% and enantioselectivities up to 89% were obtained. Semiempirical (AM1) computational studies allowed to find a theoretical rationale for the chemical and stereochemical behaviour of the catalyst of choice. These catalysts were also preliminarily investigated as promoters in the addition of diethyl malonate to the N-Boc imine of benzaldehyde, affording the product in up to 81% ee. 相似文献
198.
Alberti A Fuochi P Guerra M Macciantelli D Torri G Valerio A Vismara E 《Organic & biomolecular chemistry》2011,9(9):3199-3204
Irradiation of a number of different sutures largely employed in the clinical practice with either high energy electrons or with γ-rays followed by quenching with glycidyl methacrylate (GMA) conveniently led to derivatization through a radical-based process. The radicals involved were detected by means of ESR spectroscopy and were characterized on the basis of their ESR spectral parameters which were also found to be consistent with the hfs constants predicted by DFT calculations. Evidence of the GMA derivatization of the sutures was obtained via(13)C CP-MAS NMR spectroscopy, while its extent was evaluated gravimetrically. 相似文献
199.
Liu X Chen H Laurini E Wang Y Dal Col V Posocco P Ziarelli F Fermeglia M Zhang CC Pricl S Peng L 《Organic letters》2011,13(11):2924-2927
2-Difluoromethylene-4-methylenepentanoic acid (DFMPA), a seemingly deviated analog of 2-oxoglutaric acid (2-OG), could surprisingly mimic its signaling function in cyanobacteria. Computer modeling revealed the favorable binding of DFMPA toward the 2-OG receptor, NtcA, via mutual conformational changes, suggesting that structural alteration of 2-OG is tolerated for it to exercise its signaling role. This extremely useful finding could be exploited for the design of affinity probes with which to study new 2-OG receptors in related signaling pathways. 相似文献
200.
Riela S Bruno M Rosselli S Saladino ML Caponetti E Formisano C Senatore F 《Journal of separation science》2011,34(4):483-492
The essential oil of different parts of Ferulago campestris (Bess.) collected in Sicily has been extracted by microwave-assisted hydrodistillation (MAHD) and by classic hydrodistillation (HD). A comparative qualitative-quantitative study on the composition of the oils was carried out. A total of 100 compounds were identified in the oils obtained by MAHD, whereas 88 compounds characterized the HD oils. The most prominent components were, in all different parts of F. campestris and in both extraction methods, 2,4,5-trimethylbenzaldehyde and 2,4,6-trimethylbenzaldehyde isomers; the latter was not previously found. The attempt to evaluate where the oil components are located in all parts of the plant was carried out by means of a kinetic study. Then, electron microscopy observation on the different parts before and after MAHD and HD was performed. 相似文献