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1.
Steven R. Woodcock 《Tetrahedron letters》2005,46(42):7213-7215
Ether tethers allow Heck cyclizations to proceed in high yields. Ester tethers lead to low yields. Styrene trapping experiments indicate that ester reactions form viable organopalladium intermediates that cannot cyclize efficiently. 相似文献
2.
The geometry and energy of 1,3-butadiene have been calculated using the 6-311G** basis set as a function of the CCCC dihedral angle-0 ° (trans), 30 °, 60 °, 75 °, 90 °, 120 °, 135 °, 150 °, 165 ° and 180 ° (cis)-assuming that the vinyl groups remain planar. Potential minima are located at 0 ° and 141.4 °, with the trans structure more stable than the gauche by 13.2 kJ mol–1. Potential maxima are located at 76.7 °, giving a barrier height of 25.4 kJ mol–1 relative to the trans structure, and at 180 ° giving a barrier height of 3.0 kJ mol–1 relative to the 141.4 °-gauche structure. Using the 6-31G* basis set the inclusion of electron correlation, accounting for about 52% of the correlation energy, was found to produce no significant change in the shape of the potential energy curve. The magnitude of the expectation energy differences is such that both barriers with respect to the 14l.4 °-gauche maximum structure can be categorized unequivocally as attractive-dominant, whereas the values for the energy barrier with respect to the trans structure, although characteristic of a repulsive-dominant barrier at the 6–311G** level, are sufficiently small that higher level calculations might give the opposite result. Analysis of V
nn
for the conversion reactions cis 150 °-gauche, trans 60 °-gauche, and trans 90 °-gauche in terms of the individual contributions from the various internuclear interactions shows that nonbonded interactions are important, not only in initiating the destabilization of the crowded cis structure, but also through-out the entire range of CCCC dihedral angles, 0 ° to 180 °. 相似文献
3.
DNA纤维的X射线衍射分析与双螺旋结构的发现 总被引:3,自引:0,他引:3
概述了DNA化学结构式确立的过程,介绍了x射线衍射分析的概念、原理和方法,阐述了物理学家威尔金斯和物理化学家富兰克林在用实验事实证明DNA双螺旋结构方面所作出的巨大贡献。 相似文献
4.
《Current Applied Physics》2014,14(3):491-495
Wet chemical passivation of n-GaN surface was carried out by dipping GaN samples in ammonium sulphide diluted in aqueous and alcoholic solvent base solutions. Photoluminescence (PL) investigations indicated that sulphide solution effectively led to the reduction of GaN surface states. Increased band edge PL peak showed that S2− ions are more active in alcohol based solvents. X-ray photoelectron spectroscopy revealed reduction in surface oxides by introduction of sulphide species. Ni/n-GaN Schottky barrier diodes were fabricated on passivated surfaces. Remarkable improvement in the Schottky barrier height (0.98 eV for passivated diodes as compared to 0.75 eV for untreated diodes) has been observed. 相似文献
5.
A review of commonly used analytical and empirical formulae for predicting sound diffracted by a thin screen 总被引:2,自引:0,他引:2
This paper reviews some useful analytical and empirical formulae that are used for the calculation of sound diffracted by a barrier. A brief historic overview of the study of spherical waves diffracted by a rigid half plane is presented also. The physical principles of sound diffraction by a thin plane are explained. The mathematical expressions are cast in a convenient form for ease of numerical implementation. Accurate analytical solutions are expressed in terms of standard mathematical functions that can be computed readily. Among the approximate solutions quoted, quite a few empirical formulations are adequate for most engineering purposes. The information presented in this paper should also be helpful for those who are interested in the study of sound diffracted by a barrier in a room or in a long industrial space. 相似文献
6.
半导体PN结具有电容的性质.在正向直流偏压下,理论计算表明,PN结扩散电容的对数与正向偏压成正比.实验发现,当正向偏压小于30mV时,这种线性关系是成立的;当正向偏压大于30mV时,会偏离这种线性关系.由于PN结还具有电阻特性,对交流信号的相位有影响.随着正向偏压增大,交流信号的相位变化出现一极值.如果将PN结等效为一个电容和一个电阻并联,就可以定性解释这种变化关系. 相似文献
7.
The present experimental status in the search for neutrinoless double beta decay is reviewed, with emphasis on the first indication for neutrinoless double beta decay found in the HEIDELBERG-MOSCOW experiment, giving first evidence for lepton number violation and a Majorana nature of the neutrinos. Future perspectives of the field are briefly outlined. 相似文献
8.
This article reviews the current status of high-density capacitor for volatile memory devices. The dielectric properties for both the Ta2O5 film and the (Ba, Sr)TiO3 (BST) dielectric materials using either the metal organic chemical vapor deposition (MOCVD) or the atomic layer deposition (ALD) are reviewed briefly. New challenges of dielectric material for the next generation, and serious problems emerged during integration to date using Ta2O5 and BST. The material characteristics of many electrode materials for the high dielectric materials are introduced. We present the basic properties and integration issued for MOCVD-ruthenium (Ru). The second part of this review summarized the failure mechanisms from barrier properties of previously reported diffusion barriers and emphasizes new design concepts of diffusion barrier for high-density memory devices. Finally, the future direction for a diffusion barrier to advance high-density memory capacitors is suggested. 相似文献
9.
Zhang Yugeng 《光谱学快报》2013,46(9):1673-1679
The photoacoustic spectra of Co(H2EDTA)·3H20, Ca[Co(EDTA)H20]·4H20, Sr[Co(HEDTA)H20]2·4H20, Ba[Co(HEDTA)H20]2·4H20 complexes crystal are determined in the region of 300–800nm. In these spectra, the separations of absorption bands observed at 510nm and 650nm, the peak widths of 510nm band are variable in this series complexes. It is explained by the electron mutual effect, strength of ligand field and the symmetry of coordination group. The structure characters of these series complexes are also discussed. 相似文献
10.
The electron-phonon interaction in the periodic Anderson model (PAM) is considered. The PAM incorporates the effect of onsite
Coulomb interaction (U) between f-electrons. The influence of Coulomb correlation U on the phonon response of the system is studied by evaluating the phonon spectral function for various parameters of the
model. The numerical evaluation of the spectral function is carried out in the long wavelength limit at finite temperatures
keeping only linear terms in U. The observed behaviour is found to agree well with the general features obtained experimentally for some heavy fermion (HF)
systems. 相似文献