全文获取类型
收费全文 | 915篇 |
免费 | 21篇 |
国内免费 | 13篇 |
专业分类
化学 | 514篇 |
力学 | 12篇 |
数学 | 266篇 |
物理学 | 157篇 |
出版年
2023年 | 4篇 |
2021年 | 10篇 |
2020年 | 9篇 |
2019年 | 8篇 |
2018年 | 14篇 |
2017年 | 12篇 |
2016年 | 15篇 |
2015年 | 25篇 |
2014年 | 15篇 |
2013年 | 51篇 |
2012年 | 50篇 |
2011年 | 45篇 |
2010年 | 32篇 |
2009年 | 23篇 |
2008年 | 66篇 |
2007年 | 49篇 |
2006年 | 52篇 |
2005年 | 43篇 |
2004年 | 40篇 |
2003年 | 34篇 |
2002年 | 31篇 |
2001年 | 19篇 |
2000年 | 17篇 |
1999年 | 14篇 |
1998年 | 10篇 |
1997年 | 12篇 |
1996年 | 16篇 |
1995年 | 16篇 |
1994年 | 10篇 |
1993年 | 22篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 17篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 14篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 13篇 |
1983年 | 14篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 4篇 |
排序方式: 共有949条查询结果,搜索用时 15 毫秒
21.
Ma Y Lu Y Zeng H Ron D Mo W Neubert TA 《Rapid communications in mass spectrometry : RCM》2001,15(18):1693-1700
A two-step mass spectrometric method for characterization of phosphopeptides from peptide mixtures is presented. In the first step, phosphopeptide candidates were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) based on their higher relative intensities in negative ion MALDI spectra than in positive ion MALDI spectra. The detection limit for this step was found to be 18 femtomoles or lower in the case of unfractionated in-solution digests of a model phosphoprotein, beta-casein. In the second step, nanoelectrospray tandem mass (nES-MS/MS) spectra of doubly or triply charged precursor ions of these candidate phosphopeptides were obtained using a quadrupole time-of-flight (Q-TOF) mass spectrometer. This step provided information about the phosphorylated residues, and ruled out nonphosphorylated candidates, for these peptides. After [(32)P] labeling and reverse-phase high-performance liquid chromatography (RP-HPLC) to simplify the mixtures and to monitor the efficiency of phosphopeptide identification, we used this method to identify multiple autophosphorylation sites on the PKR-like endoplasmic reticulum kinase (PERK), a recently discovered mammalian stress-response protein. 相似文献
22.
Anthraquinone-2-sulfonate (AQS) photosensitizes pyrimidine dimer splitting. Electron abstraction from the dimer is thought to induce dimer splitting, but direct evidence for the existence and intermediacy of dimer radical cations has been lacking. By employing photochemically induced dynamic nuclear polarization, we have found emission signals in the NMR spectra of dimers upon photolysis of dimers in the presence of anthraquinone-2-sulfonate. The two dimers employed were cis, syn-thymine dimer in which the N(1)-positions were linked by a three-carbon bridge and the N(3), N(3')-dimethyl derivative of that compound. The anthraquinone-2-sulfonate sensitized photochemically induced dynamic nuclear polarization spectrum of the methylated derivative exhibited an emission signal from the dimer-C(6) hydrogens. This result implied the existence of a dimer radical cation (mD+.) formed by electron abstraction by excited anthraquinone-2-sulfonate and nuclear spin sorting within a solvent caged radical ion pair [mD+. AQS-.]. Product pyrimidine photochemically induced dynamic nuclear polarization signals were also seen [enhanced absorption by C(6)-hydrogens and emission by C(5)-methyl groups]. Nuclear spin polarization in the product resulted from spin sorting in one or more of its precursors, including mD+. The results support the conclusion that dimer radical cations not only exist but are intermediates in the photosensitized splitting of pyrimidine dimers by anthraquinonesulfonate. 相似文献
23.
Ron Wright 《International Journal of Theoretical Physics》1977,16(8):567-573
A projection-valued state is defined to be a completely orthoadditive map from the projections on one Hilbert space into the projections on another Hilbert space, which preserves the unit. Any such mapping is shown to have the formP U
1(P 11)U
1
–1
U
2(P 12)U
2
–1
, whereU
1 is unitary andU
2 is antiunitary, generalizing Wigner's theorem on symmetry transformations. A physical interpretation is given and the relation to quantum logic is discussed.The contents of this paper are a portion of the author's dissertation at the University of Massachusetts at Amherst. 相似文献
24.
25.
Kagan Kerman Yasutaka Morita Yuzuru Takamura Mehmet Ozsoz Eiichi Tamiya 《Analytica chimica acta》2004,510(2):169-174
The unique binding event between Escherichia coli single-stranded DNA binding protein (SSB) and single-stranded oligonucleotides conjugated to gold (Au) nanoparticles is utilized for the electrochemical detection of DNA hybridization. SSB was attached onto a self-assembled monolayer (SAM) of single-stranded oligonucleotide modified Au nanoparticle, and the resulting Au-tagged SSB was used as the hybridization label. Changes in the Au oxidation signal was monitored upon binding of Au tagged SSB to probe and hybrid on the electrode surface. The amplified oxidation signal of Au nanoparticles provided a detection limit of 2.17 pM target DNA, which can be applied to genetic diagnosis applications. This work presented here has important implications with regard to combining a biological binding event between a protein and DNA with a solid transducer and metal nanoparticles. 相似文献
26.
Matthias Schüler Thomas Kovar Hans Lischka Ron Shepard Robert J. Harrison 《Theoretical chemistry accounts》1993,84(6):489-509
Summary In this work a parallel implementation of the COLUMBUS MRSDCI program system is presented. A coarse grain parallelization approach using message passing via the portable toolkit TCGMSG is used. The program is very well portable and runs on shared memory machines like the Cray Y-MP, Alliant FX/2800 or Convex C2 and on distributed memory machines like the iPSC/860. Further implementations on a network of workstations and on the Intel Touchstone Delta are in progress. Overall, results are quite satisfactory considering the complexity and the prodigious requirements, especially the I/O bandwidth, of MRCI programs in general. For our largest test case we obtain a speedup of a factor of 7.2 on an eight processor Cray Y-MP for that section of the program (hamiltonian matrix times trial vector product) which has been parallelized. The speedup for one complete diagonalization iteration amounts to 5.9. An absolute speed close to 1 GFLOPS is found. Results for the iPSC/860 show that ordinary disk I/O is certainly not sufficient in order to guarantee a satisfactory performance. As a solution for that problem, the implementation of a fully asynchronous distributed-memory model for certain data files is in preparation.
On leave from: Bereich Informatik, Universität Leipzig, Augustusplatz 10/11, O-7010 Leipzig, Germany 相似文献
27.
Ron S. Dickson Rhonda J. Nesbit Helen Pateras Jennifer M. Patrick Allan H. White 《Journal of organometallic chemistry》1984,265(2):c25-c28
Complexes of formula (η-C5H52Rh2{CF3C2CF3 · RNCO} have been prepared by three methods, from reactions between organic isocyanates and (η-C5H5)2Rh2(CO)(CF3C2CF3) or (η-C5H5)2Rh2(CO)2(CF3C2CF3); by treatment of (η-C5H5)2Rh2(CO)(CF3C2CF3) with organic azides; and by oxidation with Me3NO of the organic isocyanide in (η-C5H5)2Rh2(CO)(CNR)(CF3C2CF3). The crystal and molecular structure of the complex (η-C5H5)2Rh2{CF3C2CF3 · RNCO} with R = Ph has been determined from single crystal X-ray diffraction data. This reveals that the isocyanate has condensed with the hexafluorobut-2-yne to form an amide ligand of the form C(CF3)C(CF3)C(=O)N(R); this bridges the two rhodium atoms in a μ2η3-manner. 相似文献
28.
Ron Orlando Catherine Fenselau Robert J. Cotter 《Journal of the American Society for Mass Spectrometry》1991,2(3):189-197
Endothermic ion-molecule reactions in a tandem mass spectrometer have been used for a number of years for determining thermodynamic quantities, such as heats of formation and proton affinities, for gaseous ions. Recently, the reactive, endothermic collision has been exploited as an analytical technique for the structural analysis of peptides and other biomolecules. The technique is based upon the endothermic transfer of protons associated with amide bonds to ammonia. This reaction proceeds via a long-lived collision complex. When additional beam energy is supplied, other dissociation channels are opened up, leading to the production of sequence ions for the mass-selected, protonated analyte that are normally observed in high energy collision-induced dissociation spectra. The advantage, however, is that such spectra can be produced at very low beam energies. In this article, the rationale for developing this scheme, and its roots in previous ion-molecule studies, are explored. 相似文献
29.
The approximation order provided by a directed set {S
h
}
h>0 of spaces, each spanned by thehZ
d
-translates of one function, is analyzed. The nearoptimal approximants of [R2] from eachs
h
to the exponential functions are used to establish upper bounds on the approximation order. These approximants are also used on the Fourier transform domain to yield approximations for other smooth functions, and thereby provide lower bounds on the approximation order. As a special case, the classical Strang-Fix conditions are extended to bounded summable generating functions.The second part of the paper consists of a detailed account of various applications of these general results to spline and radial function theory. Emphasis is given to the case when the scale {s
h
} is obtained froms
1 by means other than dilation. This includes the derivation of spectral approximation orders associated with smooth positive definite generating functions. 相似文献
30.
A synthesis of a 1,3-dithiane corresponding to the C1-side chain of zaragozic acid D is described. An aldol reaction using an Evans oxazolidinone is the key step in controlling stereochemistry. Metallation of the derived dithiane monosulfoxide and coupling to an aldehyde effected construction of the C1-C7 bond. Subsequent steps are also reported, including acid-mediated ketalization resulting in formation of an advanced synthetic intermediate containing the bicyclic ketal core of the natural product. 相似文献