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排序方式: 共有220条查询结果,搜索用时 31 毫秒
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The reversible reduction protons to dihydrogen: 2H+ + 2e [symbol: see text] H2 is deceptively the simplest of reactions but one that requires multistep catalysis to proceed at practical rates. How the metal-sulfur clusters of the hydrogenases catalyse this interconversion is currently the subject of extensive structural, spectroscopic and mechanistic studies of the enzymes, of synthetic assemblies and of in silico models. This is driven both by curiosity and by the view that an understanding of the underlying chemistry may inform the design of new electrocatalytic systems for hydrogen production or uptake, pertinent to energy transduction technology in an 'Hydrogen Economy'. Can chemists design materials that replace the expensive platinum metal catalysts of fuel cells with metal-sulfur cluster assemblies utilising abundant Ni, Fe and S as in the natural systems? Here we review the state of the art. 相似文献
95.
Chehade KA Kiegiel K Isaacs RJ Pickett JS Bowers KE Fierke CA Andres DA Spielmann HP 《Journal of the American Chemical Society》2002,124(28):8206-8219
Farnesylation is a posttranslational lipid modification in which a 15-carbon farnesyl isoprenoid is linked via a thioether bond to specific cysteine residues of proteins in a reaction catalyzed by protein farnesyltransferase (FTase). We synthesized analogues (3-6) of farnesyl pyrophosphate (FPP) to probe the range of modifications possible to the FPP skeleton which allow for efficient transfer by FTase. Photoaffinity analogues of FPP (5, 6) were prepared by substituting perfluorophenyl azide functional groups for the omega-terminal isoprene of FPP. Substituted anilines replace the omega-terminal isoprene in analogues 3 and 4. Compounds 3-5 were prepared by reductive amination of the appropriate anilines with 8-oxo-geranyl acetate, followed by ester hydrolysis, chlorination, and pyrophosphorylation. Additional substitution of three methylenes for the beta-isoprene of FPP gave photoprobe 6 in nine steps. Preparation of the analogues required TiCl(4)-mediated imine formation prior to NaBH(OAc)(3) reduction for anilines with a pK(a) < 1. The azide moiety was not affected by Ph(3)PCl(2) conversion of allylic alcohols 13-16 into corresponding chlorides 17-20. Analogues 3-6 are efficiently transferred to target N-dansyl-GCVLS peptide substrate by mammalian FTase. Comparison of analogue structures and kinetics of transfer to those of FPP reveals that ring fluorination and para substituents have little effect on the affinity of the analogue pyrophosphate for FTase and its transfer efficiency. These results are also supported with models of the analogue binding modes in the active site of FTase. The transferable azide photoprobe 5 photoinactivates FTase. Transferable analogues 5 and 6 allow the formation of appropriately posttranslationally modified photoreactive peptide probes of isoprene function. 相似文献
96.
The rates of radiative recombination to form HD+ and HeH+ along the ground electronic state potential surface have been calculated from known potentials. These exact quantum calculations are compared with a classical model. For the formation HD+, the classical model predicts the rate in the range of 100–1000 K. Below 100 K, the rate is dominated by the effect of low energy shape resonances. For the formation of HeH+, the classical model predicts the correct temperature dependence. 相似文献
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Jam Farhoomand Herbert M. Pickett 《International Journal of Infrared and Millimeter Waves》1987,8(5):441-447
We report the generation of 1.25 watts of CW laser power at the 119 m (2522.8 GHz) methanol line. The maximum frequency fluctuation of the free running laser is less than ±100 KHz per hour. 相似文献
100.
Stewart AI Clark IP Towrie M Ibrahim SK Parker AW Pickett CJ Hunt NT 《The journal of physical chemistry. B》2008,112(32):10023-10032
Ultrafast two-dimensional infrared (2D) spectroscopy has been applied to study the structure and vibrational dynamics of (mu-S(CH2)3S)Fe2(CO)6, a model compound of the active site of the [FeFe]-hydrogenase enzyme system. Comparison of 2D-IR spectra of (mu-S(CH2)3S)Fe2(CO)6 with density functional theory calculations has determined that the solution-phase structure of this molecule is similar to that observed in the crystalline phase and in good agreement with gas-phase simulations. In addition, vibrational coupling and rapid (<5 ps) solvent-mediated equilibration of energy between vibrationally excited states of the carbonyl ligands of the di-iron-based active site model are observed prior to slower (approximately 100 ps) relaxation to the ground state. These dynamics are shown to be solvent-dependent and form a basis for the future determination of the vibrational interactions between active site and protein. 相似文献