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41.
Gliseida Z. Melgar Edison P. Wendler Alcindo A. dos Santos André L.M. Porto 《Tetrahedron: Asymmetry》2010,21(18):2271-2274
Hydroxypropargylpiperidones rac-1–3 were efficiently obtained by a one-pot three-component coupling reaction; enantioenriched propargylpiperidones were then obtained by a kinetic resolution process using the lipase from Candida antarctica. Lipase CALB has been shown to efficiently catalyse the stereocontrolled acetylation of hydroxypropargylpiperidones rac-3 by promoting stereodiscrimination at the carbinolic centre. The enzymatic catalytic processes allow the separation of the (S,R)- and (S,S)-3 diastereoisomers into the corresponding acetates produced as a (R,S)- and (R,R)-6 diastereoisomeric pair. The CALB was able to discriminate the stereogenic centre of the secondary (R)-enantiomer of rac-3 according to the Kaslauzkas rule. The remote stereogenic centre was not discriminated by the lipase. The functionalised enantioenriched diastereoisomers obtained are important building blocks in organic synthesis. 相似文献
42.
Osorio E Olson JK Tiznado W Boldyrev AI 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(31):9677-9681
We performed global minimum searches for the B(n) H(n+2) (n=2-5) series and found that classical structures composed of 2c-2e B-H and B-B bonds become progressively less stable along the series. Relative energies increase from 2.9?kcal?mol(-1) in B(2) H(4) to 62.3?kcal?mol(-1) in B(5) H(7). We believe this occurs because boron atoms in the studied molecules are trying to avoid sp(2) hybridization and trigonal structure at the boron atoms, as in that case one 2p-AO is empty, which is highly unfavorable. This affinity of boron to have some electron density on all 2p-AOs and avoiding having one 2p-AO empty is a main reason why classical structures are not the most stable configurations and why multicenter bonding is so important for the studied boron-hydride clusters as well as for pure boron clusters and boron compounds in general. 相似文献
43.
We present an extension of Karmarkar's linear programming algorithm for solving a more general group of optimization problems: convex quadratic programs. This extension is based on the iterated application of the objective augmentation and the projective transformation, followed by optimization over an inscribing ellipsoid centered at the current solution. It creates a sequence of interior feasible points that converge to the optimal feasible solution in O(Ln) iterations; each iteration can be computed in O(Ln
3) arithmetic operations, wheren is the number of variables andL is the number of bits in the input. In this paper, we emphasize its convergence property, practical efficiency, and relation to the ellipsoid method. 相似文献
44.
45.
Apart from long-known and applied nanostructures like carbon black for tyres or pigments for coatings nanotechnology has created highly sophisticated structures used for nano/molecular electronics,diagnostics,drug delivery, UV-absorbers etc.Often the main question to be solved analytically is the local determination of tiny amounts of chemicals resulting in an ever increasing need for highly sensitive as well as locally resolved techniques.Applications of techniques like mass spectroscopy,transmission el... 相似文献
46.
Alexander C. Edison Stephen G. Naculich 《Nuclear Physics B》2012,858(3):488-501
Color-ordered amplitudes for the scattering of n particles in the adjoint representation of SU(N) gauge theory satisfy constraints arising solely from group theory. We derive these constraints for n=5 at all loop orders using an iterative approach. These constraints generalize well-known tree-level and one-loop group theory relations. 相似文献
47.
Fabio Simonelli F.A. Marques Alberto Wisniewski Jr. Edison P. Wendler 《Tetrahedron letters》2004,45(43):8099-8101
The oxazoline cyanocuprates addition to conjugated cyanoalkenes, was shown to be an efficient method for the synthesis of a variety of γ-cyanooxazolines, interesting intermediates with different, but convertible, functions in a symmetrical position. 相似文献
48.
We present an application of a recently developed dynamic mean field theory to the study of relaxation dynamics in adsorption
and desorption from pore networks. The theory predicts the evolution of density distribution in the system, based on an underlying
free energy functional from static mean field theory and the system evolves to equilibrium or metastable equilibrium states
consistent with the static theory. The theory makes it possible to follow the evolution of the density distribution with time
in response to a change in the bulk pressure or chemical potential. We compare uptake dynamics for a 2D slit pore network
with that in a single slit pore. We see more rapid uptake dynamics in the pore network in some cases, due to the greater access
of the pore space to the bulk. We also observe that the formation of liquid bridges can slow down the mass transfer in the
pore network in certain situations. 相似文献
49.
A design of polarization-maintaining retro-reflectors(PMRRs) for folded-path applications is proposed and analyzed.The prism-based scheme enables the output light,which is parallel to the input,to have an identical state of polarization.The principle of the design is theoretically verified,and the related error is analyzed due to possible manufacturing imperfection.The maximum spatial angle error is ±2.75°.The effect on the extinction ratio and insertion loss is also discussed,which further proves the design’s feasibility in practical applications. 相似文献
50.
Omar. D. Gutierrez Edison Osorio Carlos. G. Paucar C.Z. Hadad 《Journal of luminescence》2009,129(8):836-1528
Vitroceramic powders of Li2O-Al2O3-SiO2 systems (LAS), doped with 1% (LAS:1Ce) and 10% (LAS:10Ce) molar of cerianite (CeO2) were synthesized by means of the gelification technique of metal formates of aluminum and lithium, in the presence of tetraethoxy silane and CeO2. The gels obtained were dried (120 °C, 2.5 h), calcined (480 °C, 5 h) and sinterized (1250 °C, 30 min). The sinterized samples were characterized by X-ray difraction (XRD), scanning electron microscopy (SEM) and microchemical analysis (EDS). There is evidence for a mixture of two phases of 64% β-spodumene (Li2O-Al2O3-4SiO2) and 36% β-eucryptite (Li2O-Al2O3-2SiO2). The LAS:1Ce system was enriched in aluminum, the LAS:10Ce system showed areas of heterogeneous composition; some regions with a shortage of CeO2, while others zones with cerium cumulus. From the microscopy images it was found that CeO2 acts as a densificant agent in LAS system, favoring the sintering in the host. The chemical route and the sintering processes utilized allow the production of samples exhibiting an acceptable linear correlation between total thermoluminescent emission intensity and the irradiation dose when the CeO2 concentration is low (less than 1%), opening the possibility of using this kind of glass-ceramic in dosimetry. 相似文献