全文获取类型
收费全文 | 1409篇 |
免费 | 46篇 |
国内免费 | 7篇 |
专业分类
化学 | 928篇 |
晶体学 | 22篇 |
力学 | 31篇 |
数学 | 212篇 |
物理学 | 269篇 |
出版年
2023年 | 12篇 |
2022年 | 12篇 |
2021年 | 19篇 |
2020年 | 22篇 |
2019年 | 24篇 |
2018年 | 10篇 |
2017年 | 11篇 |
2016年 | 32篇 |
2015年 | 28篇 |
2014年 | 26篇 |
2013年 | 78篇 |
2012年 | 52篇 |
2011年 | 88篇 |
2010年 | 31篇 |
2009年 | 38篇 |
2008年 | 57篇 |
2007年 | 62篇 |
2006年 | 68篇 |
2005年 | 66篇 |
2004年 | 72篇 |
2003年 | 39篇 |
2002年 | 50篇 |
2001年 | 15篇 |
2000年 | 21篇 |
1999年 | 16篇 |
1998年 | 10篇 |
1997年 | 19篇 |
1996年 | 14篇 |
1994年 | 19篇 |
1993年 | 20篇 |
1992年 | 13篇 |
1991年 | 15篇 |
1989年 | 12篇 |
1988年 | 14篇 |
1987年 | 15篇 |
1985年 | 21篇 |
1984年 | 23篇 |
1983年 | 18篇 |
1982年 | 12篇 |
1981年 | 27篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 19篇 |
1977年 | 19篇 |
1976年 | 18篇 |
1975年 | 23篇 |
1974年 | 20篇 |
1973年 | 21篇 |
1972年 | 12篇 |
1966年 | 11篇 |
排序方式: 共有1462条查询结果,搜索用时 15 毫秒
941.
Katarzyna Chruszcz Krzysztof Czarniecki Leonard M Proniewicz 《Journal of Molecular Structure》2003,648(3):215-224
Pyridine-2-phosphono-4-carboxylic acid (MC1) is a compound that possesses potential neuroactivity. In this work the 1H, 13C and 31P NMR spectra of MC1 dissolved in D2O in solution, in the 1.5-9.0 pD range, are presented. Theoretical calculations of the NMR spectra, as well as structural parameters of expected compounds, were performed at the B3PW91/6-311G** and B3PW91/6-31G** level, respectively, for all five possible forms of MC1 (cation, zwitteranion and three anions). Consecutive deprotonation of MC1 and its influence on the structure of the ligand are discussed in detail. 相似文献
942.
The purpose of this paper is to describe a software/hardware systemfor the analysis of digitized video images and a number of applications for which it may be used. The system described includes a Macintosh computer, a frame-grabber board, and Image, a public domain software program available at no cost from the U.S. National Institutes of Health. In our clinic and laboratory, this system is routinely used to make quantitative measurements from videofluoroscopic x-ray images of dynamic swallow studies and studies performed to assess velopharyngeal dysfunction in speech. It can also be used to examine various laryngeal parameters obtained from videotaped endoscopic and stroboscopic examinations. With a videocamera attached to a microscope, the system permits quantitative analysis of tissue characteristics, e.g., thickness of epithelial or connective tissue layers of the vocal folds. The relatively low cost and ease of use of the image analysis system make it a particularly attractive option when quantitative assessment of clinical or research materials in video format is desirable. 相似文献
943.
Tao Zhang Soo‐Young Park B. L. Farmer Leonard V. Interrante 《Journal of polymer science. Part A, Polymer chemistry》2003,41(7):984-997
A series of comb‐type polycarbosilanes of the type [Si(CH3)(OR)CH2]n {where R = (CH2)mR′, R′ = ? O‐p‐biphenyl? X [X = H (m = 3, 6, 8, or 11) or CN (m = 11)], and R′ = (CF2)7CF3 (m = 4)} were prepared from poly(chloromethylsilylenemethylene) by reactions with the respective hydroxy‐terminated side chains in the presence of triethylamine. The product side‐chain polymers were typically greater than 90% substituted and, for R′ = ? O‐p‐biphenyl? X derivatives, they exhibited phase transitions between 27 and 150 °C involving both crystalline and liquid‐crystalline phases. The introduction of the polar p‐CN substituent to the biphenyl mesogen resulted in a substantial increase in both the isotropization temperature and the liquid‐crystalline phase range with respect to the corresponding unsubstituted biphenyl derivative. For R = (CH2)11? O‐biphenyl side chains, an analogous side‐chain liquid‐crystalline (SCLC) polysiloxane derivative of the type [Si(CH3)(O(CH2)11? O‐biphenyl)O]n was prepared by means of a catalytic dehydrogenation reaction. In contrast to the polycarbosilane bearing the same side chain, this polymer did not exhibit any liquid‐crystalline phases but melted directly from a crystalline phase to an isotropic liquid at 94 °C. Similar behavior was observed for the polycarbosilane with a fluorocarbon chain, for which a single transition from a crystalline phase to an isotropic liquid was observed at ?0.7 °C. The molecular structures of these polymers were characterized by means of gel permeation chromatography and high‐resolution NMR studies, and the crystalline and liquid‐crystalline phases of the SCLC polymers were identified by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 984–997, 2003 相似文献
944.
The matrix model formulation of M-theory can be generalized by compactification to ten-dimensional type II string theory, formulated in the infinite momentum frame. Both the type IIA and IIB string theories can be formulated in this way. In the M-theory and type IIA cases, the transverse rotational invariance is manifest, but in the IIB case, one of the transverse dimensions materializes in a completely different way from the other seven. The full O(8) rotational symmetry then follows in a surprising way from the electric-magnetic duality of supersymmetric Yang-Mills field theory. 相似文献
945.
946.
Combinatorial mutasynthesis of flavonoid analogues from acrylic acids in microorganisms 总被引:5,自引:0,他引:5
Flavonoids are plant secondary metabolites often used as nutraceutical supplements, but a growing number of unnatural flavonoids are being investigated as therapeutic agents. Cultures of Saccharomyces cerevisiae expressing recombinant flavonoid enzymes, including 4-coumaroyl:CoA ligase (4CL), chalcone synthase (CHS), chalcone isomerase (CHI), and flavanone 3beta-hydroxylase (FHT), produced novel flavanones and dihydroflavonols when fed with a number of aromatic acrylic acids. The flavonoid network also exhibited broad substrate specificity by converting muconic acid into a unique polypropanoid. 相似文献
947.
Gasparini G Martin M Prins LJ Scrimin P 《Chemical communications (Cambridge, England)》2007,(13):1340-1342
The success of applying the tethering strategy in a synthetic molecular system strongly depends on the experimental conditions and is related to the strength of the noncovalent interaction and the competition between the 'captured' and unbound ligand for the recognition site. 相似文献
948.
949.
Csenki L Alm E Torgrip RJ Aberg KM Nord LI Schuppe-Koistinen I Lindberg J 《Analytical and bioanalytical chemistry》2007,389(3):875-885
In metabolic profiling, multivariate data analysis techniques are used to interpret one-dimensional (1D) 1H NMR data. Multivariate data analysis techniques require that peaks are characterised by the same variables in every spectrum.
This location constraint is essential for correct comparison of the intensities of several NMR spectra. However, variations
in physicochemical factors can cause the locations of the peaks to shift. The location prerequisite may thus not be met, and
so, to solve this problem, alignment methods have been developed. However, current state-of-the-art algorithms for data alignment
cannot resolve the inherent problems encountered when analysing NMR data of biological origin, because they are unable to
align peaks when the spatial order of the peaks changes—a commonly occurring phenomenon. In this paper a new algorithm is
proposed, based on the Hough transform operating on an image representation of the NMR dataset that is capable of correctly
aligning peaks when existing methods fail. The proposed algorithm was compared with current state-of-the-art algorithms operating
on a selected plasma dataset to demonstrate its potential. A urine dataset was also processed using the algorithm as a further
demonstration. The method is capable of successfully aligning the plasma data but further development is needed to address
more challenging applications, for example urine data.
Figure Traces of NMR peaks visualizing the Generalized Fuzzy Hough Transform (GFHT) method for elucidating peak correspondence between
samples. The spectra are sorted according to one shift sensitive peak and reveals that other peaks exhibit a similar shift
pattern. This pattern(s) can now be searched for using the GFHT. The red and black spectra in the figure are the most shifting
spectra (top and bottom), by following the GFHT traces peak correspondence is easily established although peaks change spatial
location
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
950.
Wang X Lu D Austin R Agarwal A Mueller LJ Liu Z Wu J Feng P 《Langmuir : the ACS journal of surfaces and colloids》2007,23(10):5735-5739
Herein we report a new strategy for protein refolding by taking advantage of the unique surface and pore characteristics of ethylene-bridged periodic mesoporous organosilica (PMO), which can effectively entrap unfolded proteins and assist refolding by controlled release into the refolding buffer. Hen egg white lysozyme was used as a model protein to demonstrate the new method of protein refolding. Through loading of denatured proteins inside uniform mesoporous channels tailored to accommodate individual protein, protein aggregation was minimized, and the folding rate was increased. Poly(ethyleneglycol) (PEG)-triggered continuous release of entrapped denatured lysozyme allowed high-yield refolding with high cumulative protein concentrations. The new method enhances the oxidative refolding of lysozyme (e.g., over 80% refolding yield at about 0.6 mg/mL). 相似文献