全文获取类型
收费全文 | 4694篇 |
免费 | 139篇 |
国内免费 | 19篇 |
专业分类
化学 | 3298篇 |
晶体学 | 14篇 |
力学 | 86篇 |
数学 | 837篇 |
物理学 | 617篇 |
出版年
2021年 | 36篇 |
2020年 | 44篇 |
2019年 | 50篇 |
2018年 | 35篇 |
2016年 | 82篇 |
2015年 | 106篇 |
2014年 | 78篇 |
2013年 | 175篇 |
2012年 | 184篇 |
2011年 | 230篇 |
2010年 | 166篇 |
2009年 | 116篇 |
2008年 | 225篇 |
2007年 | 207篇 |
2006年 | 208篇 |
2005年 | 200篇 |
2004年 | 153篇 |
2003年 | 125篇 |
2002年 | 133篇 |
2001年 | 63篇 |
2000年 | 72篇 |
1999年 | 68篇 |
1998年 | 64篇 |
1997年 | 78篇 |
1996年 | 69篇 |
1995年 | 71篇 |
1994年 | 67篇 |
1993年 | 95篇 |
1992年 | 73篇 |
1991年 | 37篇 |
1990年 | 44篇 |
1988年 | 39篇 |
1987年 | 56篇 |
1986年 | 73篇 |
1985年 | 96篇 |
1984年 | 60篇 |
1983年 | 59篇 |
1982年 | 85篇 |
1981年 | 56篇 |
1980年 | 75篇 |
1979年 | 55篇 |
1978年 | 70篇 |
1977年 | 73篇 |
1976年 | 45篇 |
1975年 | 38篇 |
1974年 | 46篇 |
1973年 | 43篇 |
1972年 | 35篇 |
1971年 | 34篇 |
1959年 | 33篇 |
排序方式: 共有4852条查询结果,搜索用时 78 毫秒
931.
Saeid Zamiri Bernhard Reitinger Engelbert Portenkirchner Thomas Berer Enrique Font-Sanchis Peter Burgholzer Niyazi Serdar Sariciftci Siegfried Bauer Fernando Fernández-Lázaro 《Applied physics. B, Lasers and optics》2014,114(4):509-515
We present two laser ultrasonic receivers based on organic photorefractive polymer composites with 2-[4-bis(2-methoxyethyl)aminobenzylidene]malononitrile (AODCST) or 2-dicyanomethylen-3-cyano-5,5-dimethyl-4-(4′-dihexylaminophenyl)-2,5-dihydrofuran nonlinear optical chromophores. Experimental results show sensitivities of the ultrasonic receivers of ~9.5 × 10?8 nm (W/Hz)0.5 for both composites, and a faster response time (~60 ms) for the AODCST-based laser ultrasonic receiver. We show that such LUS detectors are highly suitable for contactless thickness measurements of aluminum, steel sheets and defect detection with an accuracy of 100 μm. 相似文献
932.
Georg Ramer Jürgen Kasberger Markus Brandstetter Ahmed Saeed Bernhard Jakoby Bernhard Lendl 《Applied physics. B, Lasers and optics》2014,116(2):325-332
We present the design and fabrication of a single-mode slab waveguide structure for mid-infrared spectroscopy optimized for broadband coupling. The sensor uses grating couplers for robust off-axis coupling and a silicon nitride guiding layer for mechanical robustness. An external cavity quantum cascade laser-based transmission method is introduced for characterizing the structure’s broadband coupling behavior. Light from an external cavity quantum cascade laser with a spectral range of 0.5 μm around 6 μm was coupled into the waveguide without the need for moving parts. First spectra taken with this sensor are presented. 相似文献
933.
Genomic effects of polyamide/DNA interactions on mRNA expression 总被引:1,自引:0,他引:1
Supekova L Pezacki JP Su AI Loweth CJ Riedl R Geierstanger B Schultz PG Wemmer DE 《Chemistry & biology》2002,9(7):821-827
934.
Kräutler B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(29):9032-9045
The synthesis of cobyrinic acid derivatives by reduction of dehydrocobyrinates is largely unexplored. It is, however, a rational path to B(12) analogues that lack specific substituents of the corrin moiety of natural B(12) derivatives. The partial syntheses of four epimeric 7-decarboxymethyl-cobyrinates is described, which is achieved by reduction of Δ7-dehydro-7-de[carboxymethyl]-cobyrinate with zinc or with the 'prebiotic' reducing agent formic acid. A direct and remarkably efficient route was found to 7-decarboxymethyl-cobyrinates, which are cobyrinic acid derivatives in which the c-side chain at ring B of vitamin B(12) is missing. The structures of the hexamethyl-7-decarboxymethyl-cobyrinates were characterized and the stereochemical and conformational properties at their newly saturated ring B were analyzed. The stereochemical outcome of the reduction was found to depend strongly on the reaction conditions. In 7-decarboxymethyl-cobyrinates, both peripheral carbon centres of ring B carry a hydrogen atom, and the characteristic quaternary carbon centre at C7 of the cobyrinic acid moiety of vitamin B(12) is lacking. The still highly substituted 7-decarboxymethyl-cobyrinates are readily dehydrogenated in the presence of dioxygen, furnishing 7-de[carboxymethyl]-Δ(7)-dehydro-cobyrinate as the common, unsaturated oxidation product. The noted stability of vitamin B(12) and of other Co(III)-cobyrinates in the presence of air is a consequence of their highly substituted corrin macrocycle, a finding of interest in the context of chemical rationalizations of the B(12) structure. 相似文献
935.
Chiu M Jaun B Beels MT Biaggio I Gisselbrecht JP Boudon C Schweizer WB Kivala M Diederich F 《Organic letters》2012,14(1):54-57
A formal [2 + 2] cycloaddition-cycloreversion (CA-CR) between N,N'-dicyanoquinone diimides (DCNQIs) and electron-rich alkynes was explored, providing a new class of π-conjugated donor-acceptor chromophores. These DCNQI adducts exist in the solid state as single diastereoisomers, but as two interconverting diastereoisomers in solution. Solid- and solution-state evidence for intramolecular charge transfer (CT) was obtained; additionally, the DCNQI adducts exhibit positive solvatochromism and significant solution-state third-order polarizabilities. 相似文献
936.
Weiqi Fu Manuela Magnúsdóttir Sigureur Brynjólfson Bernhard ?. Palsson Giuseppe Paglia 《Analytical and bioanalytical chemistry》2012,404(10):3145-3154
A fast method for quantification and identification of carotenoid and chlorophyll species utilizing liquid chromatography coupled with UV detection and mass spectrometry has been demonstrated and validated for the analysis of algae samples. This method allows quantification of targeted pigments and identification of unexpected compounds, providing isomers separation, UV detection, accurate mass measurements, and study of fragment ions for structural elucidation in a single run. This is possible using parallel alternating low- and high-energy collision spectral acquisition modes, which provide accurate mass full scan chromatograms and accurate mass high-energy chromatograms. Here, it is shown how this approach can be used to confirm carotenoid and chlorophyll species by identification of key diagnostic fragmentations during high-energy mode. The developed method was successfully applied for the analysis of Dunaliella salina samples during defined red LED lighting growth conditions, identifying 37 pigments including 19 carotenoid species and 18 chlorophyll species, and providing quantification of 7 targeted compounds. Limit of detections for targeted pigments ranged from 0.01?ng/mL for lutein to 0.24?ng/mL for chlorophyll a. Inter-run precision ranged for of 3 to 24 (RSD%) while inter-run inaccuracy ranged from ?17 to 11. Figure
Identification of carotenoids and chlorophylls combining UPLC separation, UV detection, accurate mass measurements and study of fragment ions in a single run 相似文献
937.
Meier SM Tsybin YO Dyson PJ Keppler BK Hartinger CG 《Analytical and bioanalytical chemistry》2012,402(8):2655-2662
The interaction between oxaliplatin and the model protein ubiquitin (Ub) was investigated in a top–down approach by means
of high-resolution electrospray ionization mass spectrometry (ESI-MS) using diverse tandem mass spectrometric (MS/MS) techniques,
including collision-induced dissociation (CID), higher-energy C-trap dissociation (HCD), and electron transfer dissociation
(ETD). To the best of our knowledge, this is the first time that metallodrug–protein adducts were analyzed for the metal-binding
site by ETD-MS/MS, which outperformed both CID and HCD in terms of number of identified metallated peptide fragments in the
mass spectra and the localization of the binding sites. Only ETD allowed the simultaneous and exact determination of Met1
and His68 residues as binding partners for oxaliplatin. CID-MS/MS experiments were carried out on orbitrap and ion cyclotron
resonance (ICR)-FT mass spectrometers and both instruments yielded similar results with respect to number of metallated fragments
and the localization of the binding sites. A comparison of the protein secondary structure with the intensities of peptide
fragments generated by collisional activation of the [Ub + Pt-(chxn)] adduct [chxn = (1R,2R)-cyclohexanediamine] revealed a correlation with cleavages in solution phase random coil areas, indicating that the N-terminal β-hairpin and α-helix structures are retained in the gas phase. 相似文献
938.
The synthesis and characterization of four new Ferrocene (Fc) bioconjugates, bearing a podant (Lys)-Leu-Val-Phe-Phe motif, namely the hydrophobic sequence of amyloid-β-peptides (Aβ), is reported. The Fc-peptide conjugates are characterized by a reversible redox activity and the ability to undergo hydrophobic and hydrogen bonding interactions. Biomolecular interactions between Fc-bioconjugates with Aβ(12-28) fragments were studied by circular dichroism (CD), transmission electron microscopy (TEM), and electrochemistry. All four Fc-peptides interacted favourable with Aβ(12-28) and prevented fibril formation, the extent of which depended on the length of the peptide and the nature of the C-terminal group. The aggregates obtained for the Aβ(12-28)/Fc-peptide mixtures range from short fibrils to spherical aggregates. We demonstrated that in solution the peptide sequence and peptide charge affect the biomolecular interactions. Fc-peptide interactions with immobilized Aβ(12-28)-Cys films on Au surfaces were detected by measuring the current response of the Fc redox process. The formal redox potential, E(0), at ~440 (10) mV and i(pc)/i(pa) at 0.9 were observed characteristic for the monosubstituted Fc-derivative undergoing a one-electron redox process. On the surface, methyl ester-protected Fc-peptides (1 and 3) interacted only weakly with Aβ(12-28)-Cys films, giving rise to minimal redox activity. In contrast, charged Fc-peptides (2 and 4) gave a significant electrochemical readout following the interaction with Aβ(12-28)-Cys films. Interestingly, the Fc-peptide charge dictates the surface-assisted interactions, while hydrophobic and ionic effects contribute to the overall solution behaviour of the Fc-bioconjugates with Aβ(12-28). 相似文献
939.
Aaron C. West Joseph D. Lynch Bernhard Sellner Hans Lischka William L. Hase Theresa L. Windus 《Theoretical chemistry accounts》2012,131(10):1-14
Extensive density functional theory (DFT) calculations have been performed to develop a force field for the classical molecular dynamics (MD) simulations of various azobenzene derivatives. Besides azobenzene, we focused on a thiolated azobenzene’s molecular rod (4′-{[(1,1′-biphenyl)-4-yl]diazenyl}-(1,1′-biphenyl)-4-thiol) that has been previously demonstrated to photoisomerize from trans to cis with high yields on surfaces. The developed force field is an extension of OPLS All Atoms, and key bonding parameters are parameterized to reproduce the potential energy profiles calculated by DFT. For each of the parameterized molecule, we propose three sets of parameters: one best suited for the trans configuration, one for the cis configuration, and finally, a set able to describe both at a satisfactory degree. The quality of the derived parameters is evaluated by comparing with structural and vibrational experimental data. The developed force field opens the way to the classical MD simulations of self-assembled monolayers (SAMs) of azobenzene’s molecular rods, as well as to the quantum mechanics/molecular mechanics study of photoisomerization in SAMs. 相似文献
940.