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371.
372.
在B3LYP/6-31+G(d)级别水平上对4种Be配合物的几何结构进行了全优化,并探讨了烷基取代基对其分子的几何结构和电子结构等方面的影响。采用TD-B3LYP方法在同样级别水平上研究了各配合物的电子吸收光谱,分析了光谱的变化规律。上述计算结果表明,随烷基取代基团给电子能力的增强,前线分子轨道能级升高、能隙增大、最大吸收波长发生蓝移,且最大吸收波长的跃迁类型为配体内的π→π*跃迁. 相似文献
373.
Theoretical observation of hexaatomic molecules containing pentacoordinate planar carbon 总被引:1,自引:1,他引:0
Qiong Luo 《中国科学B辑(英文版)》2008,51(11):1030-1035
The smallest molecules up to date containing a D5h pentacoordinate planar carbon (PPC) atom, CBe5 and CBe54-, are presented by means of ab initio calculations. To gain a better understanding about which electronic factors contribute to their stabilization, natural bond orbital (NBO) analysis and the nucleus independent chemical shifts (NICS) were calculated. The data reported here suggest that D5h CBe5 is σ aromaticity in nature, while in D5h CBe54- π aromaticity is dominating. The classical octet rule is well satisfied in both molecules, and is one of the fundamental reasons to understand the stability of the pentagon structures. The Be5 ring serves as σ donor in D5h CBe5, and π-acceptor in D5h CBe54-. The D5h CBe54- possessing 18 valence electrons with a closed-shell electron configuration is the most plau-sible candidate for experimental detection. 相似文献
374.
The effects of disorder and incipient magnetism in MgC(Ni1-xTx)3 (T≡Fe, Co or Cu) alloys are studied using coherent-potential approximation and Ginzburg–Landau coefficients. The first-principles, local-density-functional-based calculations for substitutionally disordered Fe and Co impurities in the Ni sub-lattice of MgCNi3, in low concentrations, show that incipient magnetism resides in these materials. The overestimation of the calculated magnetic properties points to the limitations of the local-density approximation. However, using a phenomenological approach based on Ginzburg–Landau coefficients and the fixed-spin moment method, we show that MgC(Ni1-xTx)3 alloys remain paramagnetic. At expanded volumes, we also find the possibility of a ferromagnetic state for MgC(Ni0.95Fe0.05)3 and MgC(Ni0.90Co0.10)3 alloys. 相似文献
375.
Medical endoscopy constitutes a basic and powerful medical tool for the development of minimally invasive procedures for a wide range of medical applications. Fiber endoscopes show a high flexibility, but are severely limited by optical crosstalk. Although they have been manufactured for decades, some of their quality limiting factors have not been exhaustively studied yet, and no design curves have been published so far. In this work, we apply a theoretical model for the study of coupling between multimode optical waveguides to the analysis of crosstalk influence in fiber imaging endoscopes. The fundamental equations and novel design curves will be obtained, enabling the adjustment of the fiber endoscope parameters in order to fulfill the quality requirements for a certain application. 相似文献
376.
采用基于密度泛函理论的平面波赝势方法,对纤锌矿和岩盐矿结构Be1-xMgxO合金的晶格常数、能带特性和形成能进行计算,分析了不同Mg组分下不同结构的Be1-xMgxO合金晶格常数和能带差异.结果表明:随着Mg组分的增大,纤锌矿和岩盐矿Be1-xMgxO合金的晶格常数都线性增加,但它们的能隙都逐渐减小.对于相同Mg组分的Be1-xMgxO合金,岩盐矿结构的能隙要大于纤锌矿结构.当Mg组分为0.89时,Be1-xMgxO合金由纤锌矿相转变为岩盐矿相.为了使理论值与实验值相一致,对Be1-xMgxO合金的能隙计算值进行修正,得到纤锌矿和岩盐矿Be1-xMgxO合金的能隙弯曲系数b值分别为3.451 eV和4.96 eV.对纤锌矿BeO-MgO-ZnO三元合金的能隙和弯曲系数与晶格常数关系做了分析. 相似文献
377.
Xiaojun Zhang 《Solid State Communications》2006,139(8):412-414
Copper nanorods with rectangular cross sections have been successfully prepared in high yield via a hydrothermal reduction process at a low temperature. Ascorbic acid acts as both the complexing agent and reducer, so it plays an important role for the growth of nanorods. The highly crystalline filaments exhibit the face-centered cubic structure, growing mainly along the directions. An interesting red-shift was observed in the optical absorption measurement due to the nanorods with rectangular cross sections. XRD, SEM, TEM, electron diffraction, and UV-visible spectroscopy were used to characterize these copper nanorods. 相似文献
378.
An efficient photoelectrode is fabricated by sequentially assembled CdS and CdSe quantum dots (QDs) onto a ZnO-nanowire film. As revealed by UV-vis absorption spectrum and scanning electron microscopy (SEM), CdS and CdSe QDs can be effectively adsorbed on ZnO-nanowire array. Electrochemical impedance spectroscopy (EIS) measured demonstrates that the electron lifetime for ZnO/CdS/CdSe (13.8 ms) is calculated longer than that of ZnO/CdS device (6.2 ms), which indicates that interface charge recombination rate is reduced by sensitizing CdSe QDs. With broader light absorption range and longer electron lifetime, a power conversion efficiency of 1.42% is achieved for ZnO based CdS/CdSe co-sensitized solar cell under the illumination of one Sun (AM 1.5G, 100 mW cm−2). 相似文献
379.
T. Asshauer G. Delacrétaz E.D. Jansen A.J. Welch M. Frenz 《Applied physics. B, Lasers and optics》1997,65(4-5):647-657
5 Pa served as tissue phantoms to evaluate such effects. Holmium laser pulses (wavelength: 2.12 μm, duration: 180 μs FWHM),
were delivered through 400 and 600 μm diameter optical fibers inserted into cubes of clear gel. Bubble effects were investigated
using simultaneous flash micro-videography and pressure recording for radiant exposures of 20–382 J/cm2.
Bubble formation and bubble collapse induced pressure transients were observed regardless of phantom stiffness. Bubbles of
up to 4.2 mm in length were observed in gels with a Young’s modulus of 2.9×105 Pa at a pulse energy of 650 mJ. An increase of Young’s modulus (reduction in water content) led to a monotonic reduction
of bubble size. In the softest gels, bubble dimensions exceeded those observed in water. Pressure amplitudes at 3 mm decreased
from 100±14 bars to 17±6 bars with increasing Young’s modulus over the studied range. Theoretical analysis suggested a major
influence on bubble dynamics of the mass and energy transfer through the bubble boundary.
Received: 26 August 1996/Revised version: 10 February 1997 相似文献
380.
S. Cebrat M.R. Dudek 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(2):271-276
We have performed several kinds of DNA walks which often are the first steps for further analysis of DNA structure and long
range correlations. The DNA walks analysing frequency of G+C
versus
A+T cannot indicate the coding strand while purine versus pyrimidine DNA walks or two-dimensional (A-T,G-C) DNA walks in some instances can indicate the coding strand but cannot resolve the coding frame. The modified two-dimensional
(A-T,G-C) DNA walks respecting the three-nucleotide codon structure show very high correlation in nucleotide composition of DNA coding
sequences. They can distinguish between coding and non-coding sequences and indicate the strand and the phase in which DNA
is coding.
Received: 8 January 1998 / Received in final form and Accepted: 28 January 1998 相似文献