Activation of C-H, C-C and C-I bonds by Pd and cis-Pd(CO)2I2. Catalyst-substrate adaptation

We have studied the oxidative addition reactions of methane and ethane C-H, ethane C-C and iodomethane C-I bonds to Pd and cis-Pd(CO)₂I₂ at the ZORA-BP86/TZ(2)P level of relativistic density functional theory (DFT). Our purpose, besides exploring these particular model reactions, is to understand how the mechanism of bond activation changes as the catalytically active species changes from a simple, uncoordinated metal atom to a metal-ligand coordination complex. For both Pd and cis-Pd(CO)₂I₂, direct oxidative insertion (OxIn) is the lowest-barrier pathway whereas nucleophilic substitution (S_N2) is highly endothermic, and therefore not competitive. Introducing the ligands, i.e., going from Pd to cis-Pd(CO)₂I₂, causes a significant increase of the activation and reaction enthalpies for oxidative insertion and takes away the intrinsic preference of Pd for C-I over C-H activation. Obviously, cis-Pd(CO)₂I₂ is a poor catalyst in terms of activity as well as selectivity for one of the three bonds studied. However, its exploration sheds light on features in the process of catalytic bond activation associated with the increased structural and mechanistic complexity that arises if one goes from a monoatomic model catalysts to a more realistic transition-metal complex. First, in the transition state (TS) for oxidative insertion, the C-X bond to be activated can have, in principle, various different orientations with respect to the square-planar cis-Pd(CO)₂I₂ complex, e.g., C-X or X-C along an I-Pd-CO axis, or in between two I-Pd-CO axes. Second, at variance to the uncoordinated metal atom, the metal complex may be deformed due to the interaction with the substrate. This leads to a process of mutual adjustment of catalyst and substrate that we designate catalyst-substrate adaptation. The latter can be monitored by the Activation Strain model in which activation energies ΔE^≠ are decomposed into the activation strain of and the stabilizing TS interaction between the reactants in the activated complex: .     

Synthesis,isolation, characterization,and properties of small-size aromatic macrocycles for poly(arylene ether ketone)s

A series of macrocyclic arylene ether ketone oligomers from 4,4′-difluorobenzophenone, 2,4′-difluorobenzophenone and 1,3-bis(4′-fluorobenzoyl)benzene were prepared via aromatic nucleophilic substitution according to the pseudo-high dilution principle. Small-size aromatic macrocycles were isolated by silica gel column chromatography with cyclohexane/ethyl acetate as eluent. The chemical structures of these small-size macrocycles were characterized by matrix-assisted laser desorption ionization–time-of-flight–mass spectrometry (MALDI–TOF–MS), IR, ¹⁹F-,¹H-, and ¹³C-NMR, and GPC techniques. Molecular chain length and steric hindrance of monomers affected the product compositions. The NMR results show that there are different chemical shifts in the different ring-size macrocyclic poly arylene ether ketones in spite of having the same repeating unit. The crystallizability and thermal properties of small-size arylene ether ketone macrocycles were also investigated by DSC, WAXD, TGA, and the results suggest that the crystallization and thermal properties are related to their intrinsic chemical structures. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1957–1967, 1999     

Characterization of stress‐strain behavior for binary blends of isotactic polypropylene with ethylene‐α‐olefin copolymer

The characterization of the mechanical nonlinear behavior of isotactic polypropylene/ethylene‐1‐hexene copolymer blends with various kinds of morphology was carried out using a nonlinear constitutive equation in which the plastic deformation and the anharmonicity of elastic response are taken into account. It was found that the mechanical nonlinearity of the incompatible blends showing phase separation is much greater than that of the compatible blends having rubbery components in the interlamellar regions. Moreover, the mechanical behavior is governed by the plastic deformation for the incompatible blends, whereas the anharmonicity strongly affects the mechanical behavior for the compatible blends. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1513–1521, 1999     

面向血管病变力学特性表征的光学相干弹性成像研究进展

由于血管病主要发病机制是动脉粥样硬化斑块破裂,所以动脉斑块的力学特性表征对血管病病理研究和诊治具有重要意义.光学相干断层扫描(OCT)作为心血管成像的新技术,能识别斑块的细微组织成分,但是不能评估或预测斑块的破裂风险.面向斑块力学特性表征的弹性成像技术(OCE)近年来逐渐成为了血管力学领域的一个研究热点.本综述首先介绍...     

On the mechanisms of initiation and propagation of plastic instability in polyethylene under tensile drawing

The development of a plastic instability in a high-density ethylene-butene copolymer under tensile drawing was monitored by means of a video-controlled optical extensometer in order to study the mechanism of initiation and propagation of necking. True-stress true-strain curves measured along the neck at various times of the drawing are compared with the curve recorded as a function of time at the locus where the neck started. A strain rate gradient is shown to build up during the stage of neck initiation. Evidence is given of tensile normal stress in the concave part of the neck shoulder. X-ray diffraction reveals an oblique preferred orientation of the crystal c-axis that is governed by the lower energy-consuming pathway for the deformation. This is responsible for the plastic flow localization owing to an improved shear ability. The gradual c-axis orientation towards the draw direction as the neck grows involves a strain-hardening effect that leads to neck stabilization. The conclusion is put forward that neck propagation lies in the gradation of the preferred orientation along the neck shoulder rather than in stress triaxiality. Comparison with a parent low-density copolymer shows a better trend for oblique preferred orientation of the c-axis and a reduced propensity for localized plastic flow thanks to a more homogeneous distribution of the stress over the crystallites. © 1996 John Wiley & Sons, Inc.     

力学性能对Bi-2223/Ag超导带材临界电流影响实验研究

本文主要研究了在77K温度下,拉伸应变和弯曲应变对Bi-2223/Ag超导带材临界电流的影响,得到了超导带材的临界电流随拉伸强度、曲率半径变化规律.实验结果显明,拉伸作用在超导带材上产生的形变对其临界电流(Ic)的影响存在一个临界值εirr=0.3%.形变小于此临界值,Ic变化较小,超过此临界值,临界电流急剧下降.弯曲实验同样存在类似关系.同步辐射光源对超导带材检测表明,在形变情况下,超导氧化物陶瓷芯的微裂纹迅速增加和交织是Ic降低的主要原因.     

PREDICTION FOR FORMING LIMIT OF AL2024T3 SHEET BASED ON DAMAGE THEORY USING FINITE ELEMENT METHOD

This paper presents the application of anisotropic damage theory to the study of forming limit diagram of A12024T3 aluminum alloy sheet. In the prediction of limiting strains of the aluminum sheet structure, a finite element cell model has been constructed. The cell model consists of two phases, the aluminum alloy matrix and the intermetallic cluster. The material behavior of the aluminum alloy matrix is described with a fully coupled elasto-plastic damage constitutive equation. The intermetallic cluster is assumed to be elastic and brittle. By varying the stretching ratio, the limiting strains of the sheet under biaxial stretching have been predicted by using the necking criterion proposed. The prediction is in good agreement with the experimental findings. Moreover, the finite element cell model can provide information for understanding the microscopic damage mechanism of the aluminum alloy. Over-estimation of the limit strains may result if the effect of material damage is ignored in the sheet metal forming study.     

Reissner板切口尖端应力应变场

本文利用双重幂级数展开法推导出Reissner板切口特征方程,进而应用Muller迭代法求得不同切口张角下特征值序列解答,最后获得Reissner板切口尖端应力应变场.     

表面承受常静水载荷下弹性半无限体的表面稳定性分析

本文应用小变形叠加到大变形上的分析方法,考察了表面受常静水载荷作用下可压缩和不可压缩弹性半无限体的表面平面应变失稳问题,得到了各自的临界屈曲条件的一般形式.在此基础上.分别对由Blatz-Ko材料和谐和材料所组成的可压缩弹性半无限体以及由Mooney材料所组成的不可压缩弹性半无限体,详细讨论了表面所受的静水压力对临界屈曲条件的影响.     

轴向冲击下理想塑性直杆动态屈曲的过应力简化分析模型*

本文在理想塑性直杆的动态屈曲分析中引入应变率效应,得到相应的动力学微分方程,求出了屈曲半波长,临界载荷和屈曲时间的表达式.讨论了应变率效应对杆的塑性动态屈曲的影响.并与文[4]的理论和试验结果作了比较.