全文获取类型
收费全文 | 1805篇 |
免费 | 67篇 |
国内免费 | 6篇 |
专业分类
化学 | 1320篇 |
晶体学 | 14篇 |
力学 | 11篇 |
数学 | 251篇 |
物理学 | 282篇 |
出版年
2024年 | 3篇 |
2023年 | 13篇 |
2022年 | 31篇 |
2021年 | 40篇 |
2020年 | 37篇 |
2019年 | 47篇 |
2018年 | 37篇 |
2017年 | 25篇 |
2016年 | 66篇 |
2015年 | 60篇 |
2014年 | 63篇 |
2013年 | 133篇 |
2012年 | 129篇 |
2011年 | 152篇 |
2010年 | 101篇 |
2009年 | 103篇 |
2008年 | 140篇 |
2007年 | 112篇 |
2006年 | 110篇 |
2005年 | 90篇 |
2004年 | 78篇 |
2003年 | 50篇 |
2002年 | 57篇 |
2001年 | 21篇 |
2000年 | 16篇 |
1999年 | 17篇 |
1998年 | 12篇 |
1997年 | 14篇 |
1996年 | 15篇 |
1995年 | 13篇 |
1994年 | 4篇 |
1993年 | 10篇 |
1992年 | 11篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 7篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 8篇 |
1981年 | 2篇 |
1980年 | 8篇 |
1978年 | 5篇 |
1974年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
1970年 | 3篇 |
排序方式: 共有1878条查询结果,搜索用时 31 毫秒
931.
Complete assignments of 1H and 13C NMR spectral data for 7,7′‐dihydroarylnaphthalene lignan lactones
Rosangela da Silva João Henrique Carvalho Batista Carine da Silva Maringolo Paulo Marcos Donate 《Magnetic resonance in chemistry : MRC》2009,47(6):523-526
In this article we present a complete 1H and 13C NMR spectral analysis of three 7,7′‐dihydroarylnaphthalene lignan lactones using modern NMR techniques such as COSY, HSQC, HMBC and NOE experiments. Complete assignment and homonuclear hydrogen coupling constant measurements were performed. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
932.
Marcos L. Dias Luciana P. da Silva Geraldo L. Crossetti Griselda B. Galland Carlos A. L. Filgueiras Cludio M. Ziglio 《Journal of polymer science. Part A, Polymer chemistry》2006,44(1):458-466
DADNiX2 nickel–diimine complexes [DAD = 2,6‐iPr2? C6H3? N?C(Me)? C(Me)?N? 2,6‐iPr2? C6H3] containing nonchelating pseudohalide ligands [X = isothiocyanate (NCS) for complex 1 and isoselenocyanate (NCSe) for complex 2 ] were synthesized, and the propylene polymerization with these complexes and also with the Br ligand (X = Br for complex 3 ) activated by methylaluminoxane (MAO) were investigated (systems 1 , 2 , and 3 /MAO). The polypropylenes obtained with systems 1 , 2 , and 3 were amorphous polymers and had high molecular weights and narrow molecular weight distributions. Catalyst system 1 showed a relatively high activity even at a low Al/Ni ratio and reached the maximum activity at the molar ratio of Al/Ni = 500, unlike system 3 . Increases in the reaction temperature and propylene pressure favored an increase in the catalytic activity. The spectra of polypropylenes looked like those of propylene–ethylene copolymers containing syndiotactic propylene and ethylene sequences. At the same temperature and pressure, system 2 presented the highest number of propylene sequences, and system 3 presented the lowest. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 458–466, 2006 相似文献
933.
Carla C Crispino Kelly G Fernandes Marcos Y Kamogawa Joaquim A Nóbrega Ana Rita A Nogueira Márcia M C Ferreira 《Analytical sciences》2007,23(4):435-438
In this paper we describe our study on the characterization of cigarette samples according to their mineral content. Acid digestion assisted by microwaves was employed, and inductively coupled plasma optical emission spectrometry was the analytical technique used for the determination of Al, Ba, Ca, Cu, Fe, K, Mg, Mn, Na, P and Sr in conventional, light, and flavorized cigarettes. Multivariate techniques, such as hierarchical clusters analysis (HCA) and principal-component analysis (PCA), were applied to discriminate among different types of cigarettes. Cluster analysis and principal-component analysis showed differences in cigarettes according to the type and mineral composition. The cigarette samples were divided within the 3 groups according to their mineral composition. Ca, Sr, Cu, K and Na were the most important elements for cigarette classification, and only these 5 variables were sufficient for the classification and discrimination of the evaluated types of cigarettes. 相似文献
934.
Joaquí n Barber Raquel Gim nez N lida Gimeno Mercedes Marcos Maria Del Carmen Pina Jos Luis Serrano 《Liquid crystals》2003,30(6):651-661
Copper and palladium complexes of new salicylaldimines derived from 3,4,5-tridecyloxyaniline, 2,3,4-tridecyloxyaniline and 4-decyloxyaniline have been synthesized and characterized. All the ligands bear four or more aliphatic chains with the aim of inducing columnar mesophases at low temperatures. In particular, metal complexes derived from 4-(3,4,5-tridecyloxybenzoyloxy)-salicyliden-3,4,5-tridecyloxyaninile display rectangular columnar mesophases at (or near) room temperature. These mesophase assignments have been confirmed by X-ray diffraction. A significant decrease of the melting points of the compounds is observed in the tridecyloxyaniline derivatives compared with those of similar complexes derived from mono- or di-decyloxyaniline. 相似文献
935.
J. Marcos Sirota Dennis C. Reuter 《Journal of Quantitative Spectroscopy & Radiative Transfer》1993,50(6)
The absolute intensities of four lines, Q15–Q18 in the 0310–1000 band, of N2O have been measured using a tunable diode laser spectrometer at temperatures between 380 and 420 K and pressures between 4 and 15 torr. Even though these transitions are weak and produced only about 2% of absorption at the line center for a pathlength of 52m, they were measured with a signal to noise ratio of about 20 due to the high sensitivity of the instrument. The band strength derived is 1.03 × 10-24cm molec-1 at 296 K. 相似文献
936.
Mina Aganagic Albrecht Klemm Marcos Mariño Cumrun Vafa 《Communications in Mathematical Physics》2005,254(2):425-478
We construct a cubic field theory which provides all genus amplitudes of the topological A-model for all non-compact toric Calabi-Yau threefolds. The topology of a given Feynman diagram encodes the topology of a fixed Calabi-Yau, with Schwinger parameters playing the role of Kähler classes of the threefold. We interpret this result as an operatorial computation of the amplitudes in the B-model mirror which is the quantum Kodaira-Spencer theory. The only degree of freedom of this theory is an unconventional chiral scalar on a Riemann surface. In this setup we identify the B-branes on the mirror Riemann surface as fermions related to the chiral boson by bosonization.Acknowledgement We would like to thank D.-E.Diaconescu, R. Dijkgraaf, J. Gomis, A. Grassi, A. Iqbal, A. Kapustin, S. Katz, V. Kazakov, I. Kostov, C-C. Liu, H. Ooguri, J. Schwarz, S. Shenker and E. Zaslow for valuable discussions (and the cap!). The research of MA and CV was supported in part by NSF grants PHY-9802709 and DMS-0074329. In addition, CV thanks the hospitality of the theory group at Caltech, where he is a Gordon Moore Distinguished Scholar. M.A. is grateful to the Caltech theory group for hospitality during part of this work. A.K. is supported in part by the DFG grant KL-1070/2-1. 相似文献
937.
Fuentes N Álvarez de Cienfuegos L Parra A Choquesillo-Lazarte D García-Ruiz JM Marcos ML Buñuel E Ribagorda M Carreño MC Cárdenas DJ Cuerva JM 《Chemical communications (Cambridge, England)》2011,47(5):1586-1588
The synthesis and anodic oxidation of a variety of 2,5-diaryl or dialkynylaryl substituted 1,4-dialkoxybenzenes to quinone bisketals is described. The study of the X-ray structures and electrochemical and spectroscopic properties evidenced that these pairs constitute a first approach to the concept of a molecular nanofuse. 相似文献
938.
Cell viability depends on the correct folding of the proteins involved in metabolism. Proteins are synthesized on the endoplasmic reticulum and must follow a pathway to a correct, metastable, tridimensional structure. Changes in structure or in environmental conditions can drive an instability of the folding conditions and produce non-active aggregates that in principle are proteolysed by the cellular mechanisms. However, these aggregates can be even more stable than the native proteins, escaping the cellular control. They can be classified as amorphous, if there is not a well-organized structural pattern, or ordered if a repetitive pattern is produced. These ordered structures, known as fibrils, are involved in many diseases. Infrared spectroscopy is a method of choice to study its formation because it is not affected by turbidity or the formation of high molecular weight aggregates. Moreover, in both cases, two bands characteristic of intermolecular β-sheets allow the monitoring of the aggregate formation. In both cases, the appearance of these bands involves a non-reversible path in protein folding. It has been suggested that a difference in the ordered structures involves an increasing in band intensity. This change can be the origin in variations on the 2DCOS maps. The synchronous map gives an overall idea of the process involved. The asynchronous is more informative because reflects the kinetic changes produced. The outcome of both processes, amorphous or ordered is that 2DCOS can provide a further insight to the knowledge of the kinetic processes giving rise to aggregated structures. This outcome could consist on the order in which the different secondary structures are prone to form the aggregates. 相似文献
939.
Lima Ade F de Moura GF de Lima MA de Souza PM da Silva CA Takaki GM do Nascimento AE 《Molecules (Basel, Switzerland)》2011,16(3):2486-2500
This study concerns the metabolism of polyphosphate in Trichoderma harzianum, a biocontrol agent with innate resistance against most chemicals used in agriculture, including metals, when grown in the presence of different concentrations of cadmium. The biomass production was affected by the concentration of metal used. Control cultures were able to accumulate polyphosphate under the conditions used. Moreover, the presence of cadmium induced a reduction in polyphosphate content related to the concentration used. The morphological/ultrastructural aspects were characterized by using optical and scanning electron microscopy, and were affected by the heavy metal presence and concentration. The efficiency of cadmium removal revealed the potential of the microorganism for use in remediation. The data indicate the potential for polyphosphate accumulation by the fungus, as well as its degradation related to tolerance/survival in the presence of cadmium ions. 相似文献
940.
Santos ML Toyama DO Oliveira SC Cotrim CA Diz-Filho EB Fagundes FH Soares VC Aparicio R Toyama MH 《Molecules (Basel, Switzerland)》2011,16(1):738-761
In this work we have characterized the action of the naringin, a flavonoid found in grapefruit and known for its various pharmacological effects, which include antioxidant blood lipid lowering and anticancer activity, on the structure and biochemical activities of a secretory phospholipase A (sPLA2) from Crotalus durissus cascavella, an important protein involved in the releasinge of arachidonic acid in phospholipid membranes. sPLA2 was incubated with naringin (mol:mol) at 37 °C and a discrete reduction in the UV scanning signal and a modification of the circular dichroism spectra were observed after treatment with naringin, suggesting modifications of the secondary structure of the protein. This flavonoid was able to decrease enzymatic activity and some pharmacological effects, such as myonecrosis, platelet aggregation, and neurotoxic activity caused by sPLA2, however, the inflammatory effect was not affected by naringin. In addition, small angle X-ray scattering (SAXS) data were collected for sPLA2 and naringin-treated sPLA2 to evaluate possible modifications of the protein structure. These structural investigations have shown that sPLA2 is an elongated dimer in solution and after treatment with naringin a conformational change in the dimeric configuration was observed. Our results suggest that structural modification may be correlated with the loss of enzymatic activity and alterations in pharmacological properties. 相似文献