全文获取类型
收费全文 | 839篇 |
免费 | 30篇 |
国内免费 | 2篇 |
专业分类
化学 | 635篇 |
晶体学 | 2篇 |
力学 | 2篇 |
数学 | 101篇 |
物理学 | 131篇 |
出版年
2023年 | 6篇 |
2022年 | 11篇 |
2021年 | 8篇 |
2020年 | 19篇 |
2019年 | 13篇 |
2018年 | 4篇 |
2017年 | 5篇 |
2016年 | 30篇 |
2015年 | 10篇 |
2014年 | 26篇 |
2013年 | 49篇 |
2012年 | 40篇 |
2011年 | 44篇 |
2010年 | 28篇 |
2009年 | 22篇 |
2008年 | 56篇 |
2007年 | 41篇 |
2006年 | 43篇 |
2005年 | 45篇 |
2004年 | 58篇 |
2003年 | 37篇 |
2002年 | 27篇 |
2001年 | 16篇 |
2000年 | 22篇 |
1999年 | 13篇 |
1998年 | 6篇 |
1997年 | 13篇 |
1996年 | 9篇 |
1995年 | 15篇 |
1994年 | 9篇 |
1993年 | 5篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 9篇 |
1985年 | 6篇 |
1984年 | 15篇 |
1983年 | 11篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 10篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1888年 | 2篇 |
排序方式: 共有871条查询结果,搜索用时 9 毫秒
61.
62.
MacKay A Laule C Vavasour I Bjarnason T Kolind S Mädler B 《Magnetic resonance imaging》2006,24(4):515-525
T2 weighting is particularly sensitive, but notoriously unspecific, to a wide range of brain pathologies. However, careful measurement and analysis of the T2 decay curve from brain tissue promise to provide much improved pathological specificity. In vivo T2 measurement requires accurate 180 pulses and appropriate manipulation of stimulated echoes; the most common approach is to acquire multiple echoes from a single slice. The T2 distribution, a plot of component amplitude as a function of T2, can be estimated using an algorithm capable of fitting a multi-exponential T2 decay with no a priori assumptions about the number of exponential components. T2 distributions from normal brain show peaks from myelin water, intra/extracellular water and cerebral spinal fluid; they can be used to provide estimates of total water content (total area under the T2 distribution) and myelin water fraction (MWF, fractional area under the myelin water peak), a measure believed to be related to myelin content. Experiments on bovine brain suggest that magnetization exchange between water pools plays a minor role in the T2 distribution. Different white matter structures have different MWFs. In normal white matter (NWM), MWF is not correlated with the magnetization transfer ratio (MTR) or the diffusion tensor fractional anisotropy (FA); hence it provides unique information about brain microstructure. Normal-appearing white matter (NAWM) in multiple sclerosis (MS) brain possesses a higher water content and lower MWF than controls, consistent with histopathological findings. Multiple sclerosis lesions demonstrate great heterogeneity in MWF, presumably due to varying myelin contents of these focal regions of pathology. Subjects with schizophrenia were found to have significantly reduced MWF in the minor forceps and genu of the corpus callosum when compared to controls, suggesting that reduced frontal lobe myelination plays a role in schizophrenia. In normal controls, frontal lobe myelination was positively correlated with both age and education; this result was not observed in subjects with schizophrenia. A strong correlation between MWF and the optical density from the luxol fast blue histological stain for myelin was observed in formalin-fixed brain, supporting the use of the MWF as an in vivo myelin marker. 相似文献
63.
Snyder DA Chen Y Denissova NG Acton T Aramini JM Ciano M Karlin R Liu J Manor P Rajan PA Rossi P Swapna GV Xiao R Rost B Hunt J Montelione GT 《Journal of the American Chemical Society》2005,127(47):16505-16511
X-ray crystallography and NMR spectroscopy provide the only sources of experimental data from which protein structures can be analyzed at high or even atomic resolution. The degree to which these methods complement each other as sources of structural knowledge is a matter of debate; it is often proposed that small proteins yielding high quality, readily analyzed NMR spectra are a subset of those that readily yield strongly diffracting crystals. We have examined the correlation between NMR spectral quality and success in structure determination by X-ray crystallography for 159 prokaryotic and eukaryotic proteins, prescreened to avoid proteins providing polydisperse and/or aggregated samples. This study demonstrates that, across this protein sample set, the quality of a protein's [15N-1H]-heteronuclear correlation (HSQC) spectrum recorded under conditions generally suitable for 3D structure determination by NMR, a key predictor of the ability to determine a structure by NMR, is not correlated with successful crystallization and structure determination by X-ray crystallography. These results, together with similar results of an independent study presented in the accompanying paper (Yee, et al., J. Am. Chem. Soc., accompanying paper), demonstrate that X-ray crystallography and NMR often provide complementary sources of structural data and that both methods are required in order to optimize success for as many targets as possible in large-scale structural proteomics efforts. 相似文献
64.
We prove that a finite solvable group G has at least (49p+1)/60 conjugacy classes whenever p is a prime such that p2 divides the order of G. We also construct an infinite family of finite solvable groups, where this bound is attained. 相似文献
65.
66.
Lee MR Raguse TL Schinnerl M Pomerantz WC Wang X Wipf P Gellman SH 《Organic letters》2007,9(9):1801-1804
[structure: see text] beta-Peptides containing residues derived from trans-2-aminocyclohexanecarboxylic acid (ACHC) display high population of 14-helical secondary structure in aqueous solution. We show that hydrophobic interactions between cyclohexyl rings are not responsible for this conformation-promoting effect, and that polar groups may be attached to the cyclohexyl ring without diminishing the effect. 相似文献
67.
Dr. Leyre Marzo Dr. Santosh K. Pagire Prof. Dr. Oliver Reiser Prof. Dr. Burkhard König 《Angewandte Chemie (International ed. in English)》2018,57(32):10034-10072
Visible‐light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross‐coupling reactions, α‐amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible‐light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible‐light‐absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo‐ or metal catalysis. 相似文献
68.
Thomas M. Klapötke Burkhard Krumm Dieter Naumann 《Journal of fluorine chemistry》2004,125(6):997-1005
The perfluoroaryl tellurolates C6F5TeLi (1) and 4-CF3C6F4TeLi (2) were prepared. These intermediates were identified by NMR spectroscopy and may form, depending on the reaction conditions, either the corresponding ditellanes C6F5TeTeC6F5 (3) and CF3C6F4TeTeC6F4CF3 (4) by subsequent oxidation, or in the case of 1, a telluranthrene (C6F4Te)2 (5) by reaction with itself. The halogenation products of 5, ( C6F4Te)2F4 (6), (C6F4Te)2Cl4 (7), (C6F4Te)2Br4 (8), as well as the azidation product (C6F4Te)2(N3)4 (9) were synthesized. Furthermore, in pursuit of our recent work on tellurium azides, the syntheses and properties of R2Te(N3)2 (R=CF3 (10), C6F2H3 (11)) and RTe(N3)3 (R=CF3 (12) and C6F5 (13)) are reported. The crystal structures of CF3C6F4TeTeC6F4CF3 (4), (C6F4Te)2Br4 (8), and (C6F2H3)2Te(N3)2 (11) were determined. 相似文献
69.
Andreas Baumann Anna Erbacher Camilla Evangelisti Prof. Thomas M. Klapötke Dr. Burkhard Krumm Sebastian F. Rest Martin Reynders Véronique Sproll 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(46):15627-15638
Various energetic polynitro esters, carbamates, and nitrocarbamates that were derived from the amino acid glycine were fully characterized by single‐crystal X‐ray diffraction, vibrational spectroscopy (IR and Raman), multinuclear NMR spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). Owing to their positive oxygen balance, the suitability of these compounds as potential oxidizers in energetic formulations was investigated and discussed. In addition, the heats of formation of the products were calculated by using the Gaussian 09 program package at the CBS‐4M level of theory. From these values and the calculated densities (from the X‐ray data), several detonation parameters, such as detonation pressure, velocity, energy, and temperature, were computed by using the EXPLO5 code. Furthermore, their sensitivities towards impact, friction, and electrostatic discharge were tested by using a drop hammer, a friction tester (both BAM certified), and a small‐scale electrical‐discharge device, respectively. 相似文献
70.
Burkhard Haastert 《manuscripta mathematica》1987,58(4):385-415
This paper is about differential operators andD-modules on a smooth variety over a field of positive characteristic. Beside some generalities the main results are theD-affinity of the projective space, theD-quasi-affinity of the ordinary flag manifolds (G/B) and theD-affinity of the ordinary flag manifold of Sl3. In contrast to characteristic 0 generally there exists some non-vanishing higher cohomology group of the associated graded algebra gr(D) on an ordinary flag manifold. 相似文献