全文获取类型
收费全文 | 848篇 |
免费 | 7篇 |
国内免费 | 2篇 |
专业分类
化学 | 665篇 |
晶体学 | 4篇 |
力学 | 17篇 |
数学 | 95篇 |
物理学 | 76篇 |
出版年
2023年 | 3篇 |
2022年 | 12篇 |
2021年 | 5篇 |
2020年 | 5篇 |
2019年 | 6篇 |
2018年 | 4篇 |
2017年 | 7篇 |
2016年 | 8篇 |
2015年 | 8篇 |
2014年 | 12篇 |
2013年 | 34篇 |
2012年 | 55篇 |
2011年 | 70篇 |
2010年 | 39篇 |
2009年 | 28篇 |
2008年 | 70篇 |
2007年 | 61篇 |
2006年 | 60篇 |
2005年 | 59篇 |
2004年 | 41篇 |
2003年 | 43篇 |
2002年 | 22篇 |
2001年 | 9篇 |
2000年 | 4篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1997年 | 15篇 |
1996年 | 11篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 6篇 |
1992年 | 11篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1975年 | 7篇 |
1974年 | 3篇 |
排序方式: 共有857条查询结果,搜索用时 203 毫秒
51.
Perlbarg V Bellec P Anton JL Pélégrini-Issac M Doyon J Benali H 《Magnetic resonance imaging》2007,25(1):35-46
When applied to functional magnetic resonance imaging (fMRI) data, spatial independent component analysis (sICA), a data-driven technique that addresses the blind source separation problem, seems able to extract components specifically related to physiological noise and brain movements. These components should be removed from the data to achieve structured noise reduction and improve any subsequent detection and analysis of signal fluctuations related to neural activity. We propose a new automatic method called CORSICA (CORrection of Structured noise using spatial Independent Component Analysis) to identify the components related to physiological noise, using prior information on the spatial localization of the main physiological fluctuations in fMRI data. As opposed to existing spectral priors, which may be subject to aliasing effects for long-TR data sets (typically acquired with TR >1 s), such spatial priors can be applied to fMRI data, regardless of the TR of the acquisitions. By comparing the proposed automatic selection to a manual selection performed visually by a human operator, we first show that CORSICA is able to identify the noise-related components for long-TR data with a high sensitivity and a specificity of 1. On short-TR data sets, we validate that the proposed method of noise reduction allows a substantial improvement of the signal-to-noise ratio evaluated at the cardiac and respiratory frequencies, even in the gray matter, while preserving the main fluctuations related to neural activity. 相似文献
52.
Michel Prudent Giona Sonego Mélanie Abonnenc Jean-Daniel Tissot Niels Lion 《Journal of the American Society for Mass Spectrometry》2014,25(4):651-661
Pathogen reduction technologies (PRT) are photochemical processes that use a combination of photosensitizers and UV-light to inactivate pathogens in platelet concentrates (PCs), a blood-derived product used to prevent hemorrhage. However, different studies have questioned the impact of PRT on platelet function and transfusion efficacy, and several proteomic analyses revealed possible oxidative damages to proteins. The present work focused on the oxidative damages produced by the two main PRT on peptides. Model peptides containing residues prone to oxidation (tyrosine, histidine, tryptophane, and cysteine) were irradiated with a combination of amotosalen/UVA (Intercept process) or riboflavin/UVB (Mirasol-like process). Modifications were identified and quantified by liquid chromatography coupled to tandem mass spectrometry. Cysteine-containing peptides formed disulfide bridges (R-SS-R, ?2 Da; favored following amotosalen/UVA), sulfenic and sulfonic acids (R-SOH, +16 Da, R-SO3H, +48 Da, favored following riboflavin/UVB) upon treatment and the other amino acids exhibited different oxidations revealed by mass shifts from +4 to +34 Da involving different mechanisms; no photoadducts were detected. These amino acids were not equally affected by the PRT and the combination riboflavin/UVB generated more oxidation than amotosalen/UVA. This work identifies the different types and sites of peptide oxidations under the photochemical treatments and demonstrates that the two PRT may behave differently. The potential impact on proteins and platelet functions may thus be PRT-dependent. Fig. a
? 相似文献
53.
Pan Chen Yoshiharu Nishiyama Jean-Luc Putaux Karim Mazeau 《Cellulose (London, England)》2014,21(2):897-908
We have performed molecular dynamics calculations using a revised version of the Gromos56Acarbo force field to understand the consequences of the different potential hydrogen bonding patterns on the structural stability and thermal behavior of the Iα and Iβ forms of native cellulose. For each allomorph, we considered three patterns of hydrogen bonds: two patterns obtained from neutron diffraction data refinement and a regular mixture of the two. Upon annealing, the hydrogen bonding schemes of cellulose Iβ, irrespective of the starting structure, re-arranged into the main hydrogen bond pattern experimentally observed (pattern A). On the other hand, the Iα structures, irrespective of the starting hydrogen bonding pattern, converged to a non-experimental structure where the adjacent chains are shifted along the chain direction by 0.12 nm in the hydrogen-bonded plane, and the hydroxymethyl group conformation alternates between gt and tg along the chain. The exotic structure in Iα might be a consequence of a deficiency in force field parameters and/or potential molecular arrangement in less crystalline cellulose. 相似文献
54.
Gram‐Scale Enantioselective Formal Synthesis of Morphine through an ortho–para Oxidative Phenolic Coupling Strategy
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Matthieu Tissot Dr. Robert J. Phipps Dr. Catherine Lucas Dr. Rafael M. Leon Dr. Robert D. M. Pace Dr. Tifelle Ngouansavanh Prof. Matthew J. Gaunt 《Angewandte Chemie (International ed. in English)》2014,53(49):13498-13501
A gram‐scale catalytic enantioselective formal synthesis of morphine is described. The key steps of the synthesis involve an ortho–para oxidative phenolic coupling and a highly diastereoselective “desymmetrization” of the resulting cyclohexadienone that generates three of the four morphinan ring junction stereocenters in one step. The stereochemistry is controlled from a single carbinol center installed through catalytic enantioselective hydrogenation. These transformations enabled the preparation of large quantities of key intermediates and could support a practical and scalable synthesis of morphine and related derivatives. 相似文献
55.
This paper studies the focusing of high-frequency solutions of semilinear hyperbolic equations. In previous papers, we studied two opposite phenomena. First, the focusing of nonlinear waves can force the solutions to blow up, even before reaching the caustics. Second, for strongly dissipative equations, nonlinear oscillations can be completely absorbed when they reach the caustic set. In this paper, we study the intermediate case of equations with globally Lipschitz nonlinearities. The nonlinear oscillations persist after crossing the caustic set. The solutions are described using oscillatory integrals, which are associated with Lagrangian manifolds in the cotangent bundle. The equations of nonlinear geometric optics lift to these manifolds. In contrast to the linear case, the transport equations for amplitudes living above the same points of spacetime are coupled. © 1996 John Wiley & Sons, Inc. 相似文献
56.
57.
Apperloo JJ Groenendaal LB Verheyen H Jayakannan M Janssen RA Dkhissi A Beljonne D Lazzaroni R Brédas JL 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(10):2384-2396
The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n=1-4) and their beta,beta'-unsubstituted analogues (Tn, n=1-4) are described. Both series are end capped with phenyl groups to prevent irreversible alpha-coupling reactions during oxidative doping. Absorption and fluorescence spectra of both series reveal a significantly higher degree of intrachain conformational order in the EDOTn oligomers. Oxidation potentials (E(PA1) and E(PA2)) determined by cyclic voltammetry reveal that those of EDOTn are significantly lower than the corresponding Tn oligomers as a consequence of the electron-donating 3,4-ethylenedioxy substitution. Linear fits of E(PA1) and E(PA2) versus the reciprocal number of double bonds reveal significantly steeper slopes for the EDOTn than for the Tn oligomers. This could indicate a more effective conjugation for the EDOTn series, confirmed by the fact that coalescence of E(PA1) and E(PA2) is reached already at relatively short chain lengths ( approximately 5 EDOT units) in contrast to the Tn series (>10 thiophene units). The stepwise chemical oxidation of the EDOTn and Tn oligomers in solution was carried out to obtain radical cations and dications. The energies of the optical transitions of the radical cations and dications as determined by UV/Vis/NIR spectroscopy were similar for the two series. These spectroscopic observations are consistent with quantum-chemical calculations performed on the singly charged molecules. Cooling solutions containing T2.+, T3.+, EDOT2.+, and EDOT3.+ revealed the reversible formation of dimers, albeit with a somewhat different tendency, expressed in the values for the dimerization enthalpy. 相似文献
58.
Routaboul C Dumas L Gautier-Luneau I Vergne J Maurel MC Décout JL 《Chemical communications (Cambridge, England)》2002,(10):1114-1115
A remarkable stereoselective reaction of methylglyoxal with 2-aminopyridine, the nucleic base adenine and adenine nucleosides leads in good yield to heterocycles of a new family in water under mild conditions and should be of interest in the understanding of the biological effects of methylglyoxal which is toxic, mutagenic and involved in diabetic complications. 相似文献
59.
60.
Five pentapeptides with the amino acid sequence L -tyrosyl-D -alanyl-glycyl-L -p-nitrophenylalanyl-X, wherein X = L -leucine, L -leucine amide, L -methionine amide, L -1-adamantylalanine amide, and L -neopentylglycine amide were prepared by chemical synthesis in solution and characterized chemically and physically. The effect of the incorporation of the unnatural amino acids adamantylalanine and neopentylglycine, ‘fat’-analogues of leucine and methionine, on the inhibition of electrically evoked contractions of the myenteric plexus-longitudinal muscle preparation from guinea pig ileum was studied. Their activities relative to the peptides with X = Leu · OH, Leu-NH2, and Met-NH2 (X = Neo-NH2 is particularly potent) are discussed in terms of hydrophobic and steric factors, and resistance towards enzymic degradation. 相似文献