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Ab initio SCF calculations have been performed for the methyl internal rotation and for aldehyde hydrogen wagging of thioacetaldehyde in the first triplet excited state, a3A, as well as in the singlet ground state, X1A. The preferred conformations for these states are the anti-eclipsed and the eclipsed ones, respectively. The calculated barrier heights to methyl rotation (118.3 and 455.6 cm–1 for a3A and X1A, respectively) are in good agreement with the available experimental data. The singlet ground state and the triplet excited state exhibit a planar and pyramidal configuration, respectively. The inversion barrier of the pyramidal configuration is found to be very low 67.4 cm–1. Finally, the change of conformation and structure with the transition is explained by a change of hydridization of the aldehyde carbon atom due to an n
* excitation.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday 相似文献
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Barquín Montserrat Garmendia María J. González Bellido Víctor 《Transition Metal Chemistry》2003,28(3):356-360
Mononuclear [M(hfacac)2(H2biim)] complexes, where M = MnII, FeII, CoII, NiII, CuII or ZnII, hfacac = hexafluoroacetylacetonate, H2biim = 2,2-biimidazole; dinuclear K2[M2(acac)4(-biim)] (M = CuII or ZnII) and tetranuclear K2[M4(acac)8(
4-biim)] (M = CoII or NiII) complexes have been prepared and characterized by chemical analysis, conductance measurements, i.r., electronic and e.p.r. spectroscopies and by magnetic susceptibility measurements (in the 2–300 K range). MnII, FeII and CoII are in a high spin state. The e.p.r. spectra of CuII and MnII compounds have been recorded. 相似文献
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Dussault PH Lee IQ Lee HJ Lee RJ Niu QJ Schultz JA Zope UR 《The Journal of organic chemistry》2000,65(25):8407-8414
The Lewis acid-mediated reaction of alkene nucleophiles with peroxyacetals provides an effective route for the synthesis of homologated peroxides and hydroperoxides. In the presence of Lewis acids such as TiCl(4), SnCl(4), and trimethylsilyl triflate, peroxyacetals and peroxyketals undergo reaction with allyltrimethylsilane, silyl enol ethers, and silyl ketene acetals to afford homoallyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, respectively. Reactions of peroxyacetals are Lewis acid dependent; TiCl(4) promotes formation of ethers while SnCl(4) and trimethylsilyl triflate promote formation of peroxides. Lewis acid-promoted reactions of silylated hydroperoxyacetals furnish silylated hydroperoxides, which can be deprotected to homologated hydroperoxides. Hydroperoxyketals undergo Lewis acid-mediated allylation to furnish 1,2-dioxolanes via attack of hydroperoxide on the intermediate carbocation. Lewis acid-mediated cyclization of unsaturated peroxyacetals furnishes 1,2-dioxanes, 1,2-dioxepanes, and 1,2-dioxacanes through 6-endo/exo, 7-endo/endo, and 8-endo/endo pathways. The corresponding reactions involving 6-endo/endo and 5-endo/exo pathways were unsuccessful. 相似文献
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We consider an optimal design problem in wave propagation proposed in Sigmund and Jensen (Roy. Soc. Lond. Philos. Trans. Ser. A 361:1001–1019, 2003) in the one-dimensional situation: Given two materials at our disposal with different elastic Young modulus and different density, the problem consists of finding the best distributions of the two initial materials in a rod in order to minimize the vibration energy in the structure under periodic loading of driving frequency Ω. We comment on relaxation and optimality conditions, and perform numerical simulations of the optimal configurations. We prove also the existence of classical solutions in certain cases. 相似文献
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