全文获取类型
收费全文 | 3671篇 |
免费 | 157篇 |
国内免费 | 24篇 |
专业分类
化学 | 2647篇 |
晶体学 | 13篇 |
力学 | 90篇 |
数学 | 596篇 |
物理学 | 506篇 |
出版年
2023年 | 29篇 |
2022年 | 66篇 |
2021年 | 72篇 |
2020年 | 83篇 |
2019年 | 67篇 |
2018年 | 83篇 |
2017年 | 76篇 |
2016年 | 134篇 |
2015年 | 112篇 |
2014年 | 139篇 |
2013年 | 257篇 |
2012年 | 274篇 |
2011年 | 277篇 |
2010年 | 187篇 |
2009年 | 149篇 |
2008年 | 251篇 |
2007年 | 214篇 |
2006年 | 199篇 |
2005年 | 175篇 |
2004年 | 174篇 |
2003年 | 124篇 |
2002年 | 104篇 |
2001年 | 47篇 |
2000年 | 43篇 |
1999年 | 31篇 |
1998年 | 30篇 |
1997年 | 16篇 |
1996年 | 33篇 |
1995年 | 20篇 |
1994年 | 28篇 |
1993年 | 24篇 |
1992年 | 23篇 |
1991年 | 17篇 |
1990年 | 25篇 |
1989年 | 21篇 |
1988年 | 12篇 |
1987年 | 25篇 |
1985年 | 25篇 |
1984年 | 26篇 |
1983年 | 10篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 17篇 |
1979年 | 8篇 |
1978年 | 12篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1975年 | 14篇 |
1973年 | 9篇 |
1971年 | 9篇 |
排序方式: 共有3852条查询结果,搜索用时 78 毫秒
51.
Laura Marcu Javier A. Jo Pramod V. Butte William H. Yong Brian K. Pikul Keith L. Black Reid C. Thompson 《Photochemistry and photobiology》2004,80(1):98-103
Fluorescence spectroscopy of the endogenous emission of brain tumors has been researched as a potentially important method for the intraoperative localization of brain tumor margins. We investigated the use of time‐resolved, laser‐induced fluorescence spectroscopy for demarcation of primary brain tumors by studying the time‐resolved spectra of gliomas. The fluorescence of human brain samples (glioblastoma multiforme, cortex and white matter: six patients, 23 sites) was induced ex vivo with a pulsed nitrogen laser (337 nm, 3 ns). The time‐resolved spectra were detected in a 360–550 nm wavelength range using a fast digitizer and gated detection. Parameters derived from both the spectral‐ (intensities from narrow spectral bands) and the time domain (average lifetime) measured at 390 and 460 nm were used for tissue characterization. We determined that high‐grade gliomas are characterized by fluorescence lifetimes that varied with the emission wavelength (>3 ns at 390 nm, <1 ns at 460 nm) and their emission is overall longer than that of normal brain tissue. Our study demonstrates that the use of fluorescence lifetime not only improves the specificity of fluorescence measurements but also allows a more robust evaluation of data collected from brain tissue. Combined information from both the spectraland the time domain can enhance the ability of fluorescencebased techniques to diagnose and detect brain tumor margins intraoperatively. 相似文献
52.
Adelfinskaya O Nashine VC Bergstrom DE Davisson VJ 《Journal of the American Chemical Society》2005,127(46):16000-16001
Two oxadiazole carboxamide deoxyribonucleoside analogues are described that can be incorporated and efficiently extended by Taq DNA polymerase. The primer strand extension beyond oxadiazole nucleoside analogues occurs at rates similar to the values observed for the canonical Watson-Crick base pairs irrespective of the template nucleobase. These distinctive chemical effects in DNA polymerase extensions are attributed to the smaller size and unique electronic properties of the oxadiazole nucleobase. 相似文献
53.
Jo Sakurada 《Tetrahedron》2007,63(18):3806-3817
Treatment of magnesium alkylidene carbenoids, which were generated from 1-chlorovinyl p-tolyl sulfoxides with isopropylmagnesium chloride at −78 °C in toluene, with N-lithio nitrogen-containing heterocycles gave N-alkenylated products in moderate to good yields. Also, the reaction of C-lithio indoles, which were generated from N-protected indoles, with magnesium alkylidene carbenoids gave C-2 or C-3 alkenylated products, corresponding to the protective group. The intermediate of these reactions were found to be the alkenyl anion, which could be trapped with electrophiles to give the heterocycles having fully substituted alkenes. 相似文献
54.
We provide a direct computational proof of the known inclusion
where
is the product Hardy space defined for example by R. Fefferman and
is the classical Hardy space used, for example, by E.M. Stein. We
introduce a third space
of Hardy type and analyze the interrelations among these spaces. We give simple sufficient conditions for a given function
of two variables to be the double Fourier transform of a function in
and
respectively. In particular, we obtain a broad class of multipliers on
and
respectively. We also present analogous sufficient conditions in the case of double trigonometric series and, as a by-product,
obtain new multipliers on
and
respectively. 相似文献
55.
Ferenc Mó ricz Ká roly Tandori 《Proceedings of the American Mathematical Society》1996,124(3):877-885
We study the a.e. convergence of orthogonal series defined over a general measure space. We give sufficient conditions which contain the Menshov-Rademacher theorem as an endpoint case. These conditions turn out to be necessary in the particular case where the measure space is the unit interval and the moduli of the coefficients form a nonincreasing sequence. We also prove a new version of the Menshov-Rademacher inequality.
56.
Ferenc F. Gaál Vukadin M. Leovac Branislava Dj. Avramovi 《Fresenius' Journal of Analytical Chemistry》1973,266(5):355-357
Summary Tertiary amines and salts of organic acids in acetic anhydride with 5% of acetic acid were titrated applying a biamperometric indicating system consisting of a polarised or unpolarised pair of tin electrodes by means of 0.1 N perchloric acid in glacial acetic acid. Amounts of 15.4–29.8 mg were analysed with average deviations of <0.6%, the results obtained being in a good agreement with those of potentiometry. The biamperometric end-point detection using an indicating system of polarised pair of tin electrodes can be applied for coulometric determinations of these bases with satisfying accuracy. By means of anodic generation of hydrogen ions in the presence of hydroquinone amounts of 0.5–1.0 mg were titrated with average deviations of < 1.3%. The results obtained were compared with those obtained of catalytic thermometric and photometric titrations.
Biamperometrische Neutralisationstitrationen in nichtwärigen Lösungsmitteln unter Anwendung eines Zinn-Elektrodenpaares als Indicatorsystem
Zusammenfassung Tertiäre Amine und Salze organischer Säuren werden in Acetanhydrid mit 5% Essigsäure durch Titration mit einer 0,1 N Lösung von Perchlorsäure in Essigsäure bestimmt. Das verwendete biamperometrische Indicatorsystem besteht aus einem Paar polarisierter oder nichtpolarisierter Zinnelektroden. Mengen von 15,4–29,8 mg wurden mit einer mittleren Abweichung von <0,6% analysiert. Die Ergebnisse stimmen gut mit denen von potentiometrischen Titrationen überein. Die biamperometrische Endpunkttechnik mit polarisierten Zinnelektroden kann für coulometrische Bestimmungen dieser Basen mit zufriedenstellender Genauigkeit benutzt werden. Durch anodische Erzeugung von Wasserstoffionen in Gegenwart von Hydrochinon wurden Mengen von 0,5–1,0 mg mit einer mittleren Abweichung von < 1,3% titriert. Die Ergebnisse wurden mit denen von katalytisch-thermometrischen und photometrischen Titrationen verglichen.相似文献
57.
Silva Silvio S. Ribeiro JoÃo D. Felipe Maria G. A. Vitolo Michelle 《Applied biochemistry and biotechnology》1997,(1):557-564
Batch fermentations of sugar cane bagasse hemicellulosic hydrolysate treated for removing the inhibitors of the fermentation
were performed byCandida guilliermondii FTI20037 for xylitol production. The fermentative parameters agitation and aeration rate were studied aiming the maximization
of xylitol production from this agroindustrial residue. The maximal xylitol volumetric productivity (0.87 g/L h) and yield
(0.67 g/g) were attained at 400/min and 0.45 v.v.m. (KLa 27/h). According to the results, a suitable control of the oxygen input permitting the xylitol formation from sugar cane
bagasse hydrolysate is required for the development of an efficient fermentation process for large-scale applications. 相似文献
58.
Pedro A. Fernandes Elsa S. Henriques Vineet Pande Maria. J. Ramos Ana R. R. Maia André A. S. Almeida Bruno F. B. Silva Carla M. S. Ribeiro César F. B. Ribeiro David S. M. Ribeiro Diana A. P. Fonseca Eva M. S. Cunha Filipe R. N. C. Maia Joana A. A. Pereira João P. G. Pacheco Joaquim A. A. D. Ferreira Liliana R. C. Matos Manuel A. B. P. Pinto Maria C. S. Borges Paulo J. C. R. Magalhães Pedro F. R. D. Teixeira Pedro N. B. C. Veloso Ricarte J. F. Ferreira Sandro S. Gomes Tiago F. Barros Tiago S. J. T. Selão Virgínia M. M. C. Fernandes 《Theoretical chemistry accounts》2005,113(4):197-204
We present a series of new inhibitors of the association between nuclear factor kappa B (NF-B) and the corresponding B site in DNA. They were designed using the lead compound 15-deoxy-12,14 -prostaglandin J2 (PGJ2), which is a natural product with demonstrated inhibitory efficiency for this system. First, the binding mode of PGJ2 to NF-B was unraveled by GOLD docking calculation. Subsequently, substitutions were made to PGJ2 to optimize its association with NF-B. Care was taken not to strongly increase the reactivity of the new compounds, and to keep the overall shape, size and hydrophilicity of the lead compound, which should render them a similar bioavailability. Molecular mechanics calculations were performed to decide on the suitability of the substitutions, and to evaluate the energies of association with NF-B. Density functional theory calculations were performed also to study the overall reactivity of the substituted drugs towards NF-B. Important general conclusions were obtained, concerning the improvement of these natural inhibitors; namely, a set of rational methodologies were deduced to improve the association between the PGJ2 derivatives and NF-B, and their efficiency demonstrated by generating a set of substituted complexes, some of them with a very much increased affinity for NF-B, opening new doors to enlarge the therapeutic capabilities of this class of drugs. 相似文献
59.
A new functional group, the hydroxy group, was inserted into a Betti base by reaction with salicylaldehyde, and the naphthoxazine derivatives thus obtained were converted by ring-closure reactions with formaldehyde, acetaldehyde, propionaldehyde or phosgene to the corresponding naphth[1′2′:5,6][1,3]oxazino[3,2-c][1,3]benzoxazine derivatives. Further, the conformational analysis of these polycyclic compounds by NMR spectroscopy and an accompanying molecular modelling are reported; especially, both quantitative anisotropic ring current effects of the aromatic moieties in these compounds and steric substituent effects were employed to determine the stereochemistry of the naphthoxazinobenzoxazine derivatives. 相似文献
60.
Ferenc Fülp Enik Forr Gbor Bernth Istvn Miskolczi Pirjo Vainiotalo Aarne Martinsen 《Journal of heterocyclic chemistry》1997,34(1):289-293
The Hammett-Brown electrophilic substituent constants σ+ for five heteroaryl-substituted phenyl groups were estimated via the ring-chain tautomeric equilibria of three sets of oxazolidines and 1,3-oxazines in deuteriochloroform solution. For oxazolidines derived from norephedrine and norpseudoephedrine, the σ+ values were also determined via the gas-phase ring-chain equilibria, by means of mass spectrometry. The results show that the heteroaromatic substituents studied have a slight electron-withdrawing character, with σ+ in the range -0.03 to 0.59. The gas-phase σ+ values are significantly higher than the solution values. 相似文献