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61.
Vladimír Sychrovský David Šaman Radovan Fiala Otakar Humpa Jan Sýkora Pavel Kessler Vratislav Blechta Petre Dobrev Jan Schraml 《Magnetic resonance in chemistry : MRC》2019,57(12):1084-1096
The connectivities of all atoms in ascorbigen A, an important metabolite, were determined unambiguously for the first time. The connectivity between carbon atoms was established by 2D INADEQUATE, and one-bond 13C–13C coupling constants were determined for all pairs of directly connected carbon atoms except for two strongly coupled carbon pairs. The 13C–13C coupling in one of the pairs was proved by a modification of standard INADEQUATE; however, the signals from the other pair were too weak to be observed. The connectivity within the two strongly coupled C–C pairs was confirmed by a combination of COSY and gHSQC; the latter experiment also identified all C–H bonds. The proton nuclear magnetic resonance (1H NMR) spectra in dry dimethyl sulfoxide allowed identification and assignment of the signals due to NH and OH protons. The derived structure, 3-((1H-indol-3-yl)methyl)-3,3a,6-trihydroxytetrahydrofuro[3,2-b]furan-2(5H)-one, agrees with the structure suggested for ascorbigen A in 1966. The density functional theory (DFT) calculations showed that among 16 possible stereoisomers, only two complied with the almost zero value of the measured 3J(H6–H6a). Of the two stereoisomers, 3S,3aS,6S,6aR and 3R,3aR,6R,6aS, the latter was excluded on synthetic grounds. The nuclear Overhauser effect measurements unveiled close proximity between H2′ proton of the indole and the H6a proton of the tetrahydrofuro[3,2-b]furan part. Detailed structural interpretation of the measured NMR parameters by means of DFT NMR was hampered by rotational flexibility of the indole and tetrahydrofuro[3,2-b]furan parts and inadequacy of Polarizable Continuum Model (PCM) solvent model. 相似文献
62.
Spectral analysis of multichannel MRS data 总被引:2,自引:0,他引:2
Sandgren N Stoica P Frigo FJ Selén Y 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,175(1):79-91
The use of phased-array receive coils is a well-known technique to improve the image quality in magnetic resonance imaging studies of, e.g., the human brain. It is common to incorporate proton (1H) magnetic resonance spectroscopy (MRS) experiments in these studies to quantify key metabolites in a region of interest. Detecting metabolites in vivo is often difficult, requiring extensive scans to achieve signal-to-noise ratios (SNR) that provide suitable diagnostic results. Combining the MR absorption spectra obtained from several receive coils is one possible approach to increase the SNR. Previous literature does not give a clear overview of the wide range of possible approaches that can be used to combine MRS data from multiple detector coils. In this paper, we consider the multicoil MRS approach and introduce several signal processing tools to address the problem from different nonparametric, semiparametric, and parametric perspectives, depending on the amount of available prior knowledge about the data. We present a numerical study of these tools using both simulated 1H MRS data and experimental MRS data acquired from a 3T MR scanner. 相似文献
63.
Petre Birza 《Journal of Molecular Spectroscopy》2005,229(2):276-282
The rotationally resolved electronic spectrum of a vibrational band in the transition of the cumulene carbon chain C3H2 was measured in the 625 nm region in a supersonic discharge using cw cavity ring down spectroscopy. The rotational structure was analysed using a conventional Hamiltonian for an asymmetric top molecule, and the constants A′, B′, and C′ in the upper state were determined. The observed band is assigned to a combination of a1 and b2 vibrations with the frequency around 2000 cm−1. The geometries in the 1A1, 1A2, 1B1 states and the intersection point between the latter two were obtained using ab initio calculations. The effective structure in the measured vibrational level of the 1A2 state was inferred from the determined constants. 相似文献
64.
Anca Paun Irina Zarafu Miron T. Caproiu Constantin Draghici Maria Maganu Ani I. Cotar Mariana C. Chifiriuc Petre Ionita 《Comptes Rendus Chimie》2013,16(7):665-671
Starting from benzocaine, a well-known anaesthetic, ten derivatives were synthesized and characterized by UV–vis, IR, NMR, and elemental analysis. Most of the compounds contain residues with recognized biological activity, like nicotinic acid (vitamin B3 or PP), biotin (vitamin B7 or H), lipoic acid (thioctic acid), adamantine, as well as other residues of crown-ether type, benzofurazane, naphtylurea, di- and tri-nitrobenzene, and a nitroxide radical. The biological evaluation of the obtained compounds included hydrophobicity (lipophobicity) assay, total antioxidant and microbiological activity tests. 相似文献
65.
Tanţa Spătaru Maria Marcu Loredana Preda Petre Osiceanu Jose Maria Calderon Moreno Nicolae Spătaru 《Journal of Solid State Electrochemistry》2011,15(6):1149-1157
Polytyramine (PTy) is shown to be a possible alternative to other conducting polymers as a support material for fuel cell
electrocatalysts such as platinum. In this work, a Pt–PTy composite was prepared via potentiodynamic deposition of polytyramine
on graphite substrate, followed by the electrochemical deposition of Pt nanoparticles. The material obtained by this straightforward
method exhibited, for platinum loadings as low as ca. 0.12 mg cm−2, a specific electrochemically active surface area of the electrocatalyst of ca. 54 m2 g−1, together with a good electrocatalytic activity for methanol oxidation in acidic media, thus ensuring better efficiency of
Pt utilization. The system Pt–PTy appears to be worthy of development for methanol fuel cell applications also because the
results suggested that, when deposited as small particles in a PTy matrix, platinum is less sensitive to fouling during CH3OH oxidation. 相似文献
66.
Simona Somacescu Viorica Parvulescu Jose Maria Calderon-Moreno Soong-Hyuck Suh Petre Osiceanu Bao-Lian Su 《Journal of nanoparticle research》2012,14(6):1-17
The design of cell-based bioreactors for inorganic particles formation requires both a better understanding of the underlying processes and the identification of most suitable organisms. With this purpose, the process of Au3+ incorporation, intracellular reduction, and Au0 nanoparticle release in the culture medium was compared for four photosynthetic microorganisms, Klebsormidium flaccidum and Cosmarium impressulum green algae, Euglena gracilis euglenoid and Anabaena flos-aquae cyanobacteria. At low gold content, the two green algae show maintained photosynthetic activity and recovered particles (ca. 10?nm in size) are similar to internal colloids, indicating a full biological control over the whole process. In similar conditions, the euglenoid exhibits a rapid loss of biological activity, due to the absence of protective extracellular polysaccharide, but could grow again after an adaptation period. This results in a larger particle size dispersity but larger reduction yield. The cyanobacteria undergo rapid cell death, due to their prokaryotic nature, leading to high gold incorporation rate but poor control over released particle size. Similar observations can be made after addition of a larger gold salt concentration when all organisms rapidly die, suggesting that part of the process is not under biological control anymore but also involves extracellular chemical reactions. Overall, fruitful information on the whole biocrystallogenesis process is gained and most suitable species for further bioreactor design can be identified, i.e., green algae with external coating. 相似文献
67.
Petre Makreski Tom
e Run
evski Gligor Jovanovski 《Journal of Raman spectroscopy : JRS》2011,42(1):72-77
The characterization and spectra–structure correlations of grossular and uvarovite garnets by Raman and infrared (IR) spectroscopy were carried out. Attention was focused on the explanation of the spectral differences between both minerals from structural and compositional points of view because they are isomorphous and share the general chemical formula [Ca3Y2(SiO4)3; Y = Al (grossular) and Cr (uvarovite)]. It was established that the prominent spectral differences are mainly due to the presence of various Y3+ cations and the possibility for their interchange. It was found that the substitution of Al3+ with Cr3+ promotes blue shifting of the high‐energy Raman bands (SiO4 stretchings) and red shifting of the corresponding IR bands. This peculiar observation is explained by the introduction of two different effects that promote band shifting in opposite directions—the effect of different cation crystal field stabilization energies versus the effect of different cation masses and radii. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
68.
Take σ to be a continuous semiflow on the locally compact metric space Θ, and let {A(θ)}θ∈Θ be a family of (possibly unbounded) densely defined closed operators on the Banach space X. 相似文献
69.
Petre?Birtea Ioan?Ca?uEmail author Tudor?S.?Ratiu Murat?Turhan 《Journal of Nonlinear Science》2012,22(2):187-212
The stability for all generic equilibria of the Lie–Poisson dynamics of the
\mathfrakso(4)\mathfrak{so}(4) rigid body dynamics is completely determined. It is shown that for the generalized rigid body certain Cartan subalgebras
(called of coordinate type) of
\mathfrakso(n)\mathfrak{so}(n) are equilibrium points for the rigid body dynamics. In the case of
\mathfrakso(4)\mathfrak{so}(4) there are three coordinate type Cartan subalgebras whose intersection with a regular adjoint orbit gives three Weyl group
orbits of equilibria. These coordinate type Cartan subalgebras are the analogues of the three axes of equilibria for the classical
rigid body in
\mathfrakso(3)\mathfrak{so}(3). In addition to these coordinate type Cartan equilibria there are others that come in curves. 相似文献
70.