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排序方式: 共有52条查询结果,搜索用时 93 毫秒
1.
The 3',5'-bis-O-TBDMS derivative of 2'-deoxyguanosine can be converted to its O6-alkyl and O6-aryl ethers as well as to N6-substituted diaminopurine nucleosides in two simple steps. Also described is a novel, nonaqueous, one-step O6-desulfonylation method that leads to deprotection of the carbonyl moiety of 2'-deoxyguanosine. 相似文献
2.
Isolation of teratogenic alkaloids by reversed-phase high-performance liquid chromatography 总被引:1,自引:0,他引:1
Reversed-phase high-performance liquid chromatography was used for both analytical and preparative separations of several steroidal alkaloids which occur in extracts of Veratrum californicum. The inclusion of 0.1% trifluoroacetic acid in the mobile phase improved the efficiency of the chromatography and the solubility of the compounds in aqueous acetonitrile. Nuclear magnetic resonance was used to assist the identification of the isolated steroidal alkaloids. The effect of the interaction of trifluoroacetic acid with the alkaloids could be clearly seen by changes in the chemical shifts in the nuclear magnetic resonance spectra. 相似文献
3.
4.
Salman Z Wang D Chow KH Hossain MD Kreitzman SR Keeler TA Levy CD MacFarlane WA Miller RI Morris GD Parolin TJ Saadaoui H Smadella M Kiefl RF 《Physical review letters》2007,98(16):167001
A low energy radioactive beam of polarized 8Li has been used to observe the vortex lattice near the surface of superconducting NbSe2. The inhomogeneous magnetic-field distribution associated with the vortex lattice was measured using depth-resolved beta-detected NMR. Below Tc, one observes the characteristic line shape for a triangular vortex lattice which depends on the magnetic penetration depth and vortex core radius. The size of the vortex core varies strongly with the magnetic field. In particular, in a low field of 10.8 mT, the core radius is much larger than the coherence length. The possible origin of these giant vortices is discussed. 相似文献
5.
Suppression of strong coupling artefacts in J-spectra 总被引:3,自引:0,他引:3
Thrippleton MJ Edden RA Keeler J 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,174(1):97-109
A major problem in J-resolved spectroscopy and many other experiments is the appearance of "artefacts" due to strong coupling. The signals giving rise to these artefacts follow the same coherence transfer pathways as the required signals, making it impossible to suppress them using phase cycling or pulsed field gradients. We present new methods for suppressing these artefacts, applicable to any experiment containing a J-resolved pulse sequence element, as well as to constant-time experiments. 相似文献
6.
We describe new techniques involving trapped atoms as targets for measurements of electron-impact ionization cross sections out of an excited level. By directly detecting the ions formed by electron collisions, we have obtained the first experimental ionization cross sections out of the 5(2)P excited level in rubidium at energies from 40 to 300 eV. 相似文献
7.
Kimberly L. Colson Lloyd M. Jackman Tikam Jain Gerald Simolike James Keeler 《Tetrahedron letters》1985,26(38):4579-4582
The epoxyenedione 1a has been isolated from a strain and its structure has been established by nmr spectroscopy and by its conversion to 2,5-diphenyl-3,4-dihydroxyphenol. Selective hetero J-resolved nmr spectroscopy played a pivotal role in the structure elucidation. 相似文献
8.
Baltisberger JH Walder BJ Keeler EG Kaseman DC Sanders KJ Grandinetti PJ 《The Journal of chemical physics》2012,136(21):211104
We present an improved and general approach for implementing echo train acquisition (ETA) in magnetic resonance spectroscopy, particularly where the conventional approach of Carr-Purcell-Meiboom-Gill (CPMG) acquisition would produce numerous artifacts. Generally, adding ETA to any N-dimensional experiment creates an N + 1 dimensional experiment, with an additional dimension associated with the echo count, n, or an evolution time that is an integer multiple of the spacing between echo maxima. Here we present a modified approach, called phase incremented echo train acquisition (PIETA), where the phase of the mixing pulse and every other refocusing pulse, φ(P), is incremented as a single variable, creating an additional phase dimension in what becomes an N + 2 dimensional experiment. A Fourier transform with respect to the PIETA phase, φ(P), converts the φ(P) dimension into a Δp dimension where desired signals can be easily separated from undesired coherence transfer pathway signals, thereby avoiding cumbersome or intractable phase cycling schemes where the receiver phase must follow a master equation. This simple modification eliminates numerous artifacts present in NMR experiments employing CPMG acquisition and allows "single-scan" measurements of transverse relaxation and J-couplings. Additionally, unlike CPMG, we show how PIETA can be appended to experiments with phase modulated signals after the mixing pulse. 相似文献
9.
B. A. Demidov M. V. Ivkin I. A. Ivonin V. A. Petrov V. P. Efremov V. E. Fortov N. Keeler 《Technical Physics》1997,42(11):1264-1269
Optical methods are used to investigate the dynamics of the interaction of a high-current electron beam with an aerogel (a
highly porous transparent dielectric with a low density ρ=0.36 g/cm3). The measured profile of the glow of the aerogel and the pattern of its expansion are compared with the results of a numerical
simulation. The influence of the space charge on the profile of the energy absorption from the high-current relativistic electron
beam is discussed.
Zh. Tekh. Fiz. 67, 26–32 (November 1997) 相似文献
10.
This paper describes a number of improvements to a method, developed in this laboratory and described in J. Magn. Reson. 85 (1989) 111-113, which makes it possible to determine values of long-range 13C-1H coupling constants from heteronuclear multiple bond correlation (HMBC) spectra. First, it is shown how pulsed-field gradients can be introduced into the HMBC experiment without perturbing the form of the cross-peak multiplets; a one-dimensional version of the experiment is also described which permits the rapid measurement of a small number of couplings. Second, the experiment is modified so that one-bond and long-range cross-peaks can be separated, and so that the one-bond cross-peaks have more reliable intensities. Finally, it is shown how these one-bond cross-peaks can be used to advantage in the fitting procedure. 相似文献