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排序方式: 共有148条查询结果,搜索用时 109 毫秒
21.
Skwarczynski M Sohma Y Noguchi M Kimura T Hayashi Y Kiso Y 《The Journal of organic chemistry》2006,71(6):2542-2545
O-N Intramolecular alkoxycarbonyl (carbonate-carbamate) migration was found to occur as a common reaction of hydroxyamino acids under mild basic aqueous conditions with no formation of side products. Carbonate protective groups migrate to produce amino-protected carbamate derivatives of hydroxyamino acids with high efficiency and purity. 相似文献
22.
The holographic principle has revealed that phyical systems in 3-D space, black holes included, are basically two-dimensional as far as their information content is concerned. This conclusion is complemented by one sketched here: as far as entropy or information flow is concerned, a black hole behaves as a one-dimensional channel. We define a channel in flat spacetime in thermodynamic terms, and contrast it with common entropy emitting systems. A black hole is more like the former: its entropy output is related to the emitted power as it would be for a one-dimensional channel, and disposal of an information stream down a black hole is limited by the power invested in the same way as for a one-dimensional channel. 相似文献
23.
The effect of a remote substituent on regioselectivity in the oxymercuration of 2-substituted norbornenes has been investigated experimentally and theoretically using density functional theory (DFT). Regioselectivities of 1:1 to 14:1 were observed with various 2-substituted norbornenes. Exo-2-substituted norbornenes always gave greater regioselectivities compared to the corresponding endo-2-substituted norbornenes. The effects of solvents on the regioselectivity have also been examined, and ethereal solvents were found to be the best choice giving the optimal yield and regioselectivity. The relative rate of oxymercuration was estimated by competition experiments. The least reactive substrate (X = OAc) gave the highest regioselectivity. According to DFT predictions, the increased difference between the reaction barriers that results in the greater regioselectivity is correlated directly with the larger polarity of the C=C double bond, which is attacked by the mercury and oxygen. A number of stable exo and endo conformers were predicted. All exo conformers show the same polarity of the double bond, while some endo conformers have a reversal of this polarity. All the conformers except those with the OAc substituent are very close in energy and thus should react. The existence of a mixture of endo conformers with the C=C double bond of opposite polarity clearly explains a decrease in regioselectivity for the endo species. The origin of the greatest regioselectivity for the OAc-2-norbornenes lies in the fact that the conformer with the largest polarity is notably lower in energy than others due to an internal C-H-O hydrogen bond. 相似文献
24.
Summary: New crystallization procedures have been developed for the analysis of the chemical composition distribution in polyolefins by pumping a small flow of solvent during the crystallization cycle. One of the new techniques, crystallization elution fractionation (CEF) combines the separation power of TREF and CRYSTAF and has been shown to provide very fast analysis of the composition distribution. 相似文献
25.
R. Mayo J. Campos M. Ortiz H. Xu S. Svanberg G. Malcheva K. Blagoev 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2006,40(2):169-173
We report on radiative lifetimes of 4d5p excited states
of Zr III produced in a laser produced plasma. The ions were
populated either in the ground state or in metastable states, and
the number of ions is strongly dependent on the application of an
external magnetic field, which is shown to be very important when
using the time-resolved laser-induced fluorescence technique for
lifetime measurements in highly charged ions. The experimental
lifetime results fall in the region 1–2 ns with statistical
uncertainties less than 7%. The experimental values were compared
with multi-configuration Hartree-Fock calculations showing an
agreement within 12–20%. The experimental values are systematically
higher than the theoretical ones. 相似文献
26.
The biharmonic equation arises in a variety of problems in applied mathematics, most notably in plane elasticity and in viscous incompressible flow. Integral equation methods are natural candidates for the numerical solution of such problems, since they discritize the boundary alone, are easy to apply in the case of free or moving boundaries, and achieve superalgebraic convergence rates on sufficiently smooth domains, regardless of shape. In this paper, we follow the work of Mayo and Greenbaum and make use of the Sherman-Lauricella integral equation which is a Fredholm equation with bounded kernel. We describe a fast algorithm for the evaluation of the integral operators appearing in that equation. When combined with a conjugate gradient like algorithm, we are able to solve the discretized integral equation in an amount of time proportional to N, where N is the number of nodes in the discretization of the boundary. 相似文献
27.
Vladimir A. Daragan Kevin H. Mayo 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,130(2):329-334
A simple approach to deriving motional dynamics information of protein and peptide side chains by using13C NMR relaxation data is presented. By using linear approximation of internal rotational correlation functions, simple equations for relating side-chain conformation, bond rotational amplitudes, and rotational correlation coefficients with different NMR relaxation parameters have been obtained. Auto- and cross-correlation spectral densities are considered, and it is shown that proton-coupled13C NMR relaxation measurements allow detailed motional information to be obtained. 相似文献
28.
The beech leaf-mining weevil, Orchestes fagi (L.), also known as the beech flea weevil, is a common and widespread pest of beech, Fagus sylvatica L., in its native Europe. It now appears to be well established in Nova Scotia, Canada. We have previously reported the synthesis of 9-geranyl-p-cymene and 9-geranyl-α-terpinene, as they are both found in eclosing beech buds, and have researched the synthesis of other diterpenes that are isomers of 9-geranyl-α-terpinene. We now wish to report a synthesis of the diterpene 9-geranylterpinolene as a mixture of isomers, as a novel diterpene, and as a possible kairomone of O. fagi. Also, all trans-α-springene, its 3Z-isomer, and β-springene were synthesized as a mixture, as well as pure β-springene, utilizing known methods. 相似文献
29.
Theodore Mill Harold Richardson Frank R. Mayo 《Journal of polymer science. Part A, Polymer chemistry》1973,11(11):2899-2907
There appear to be no unusual effects of phase change or near-ultraviolet radiation on the reactions of alkoxy radical models for atactic polypropylene. The oxidation of purified atactic polypropylene by singlet oxygen is not an important process at 50°. Photochemical and thermal cleavages of tert-amyloxy radicals, a model for polypropylene oxy radicals, give nearly the same ratio of ka/kd at 50°C. Photochemical cleavage of the model compound 1,1-dinonylethyl hydroperoxide (10-methyl-10-nonadecyl hydropenoxide) gives exclusive cleavage of a nonyl group both in solution and in polypropylene film at 50°C. 相似文献
30.