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51.
AlGaN/GaN HEMT外部边缘电容Cofd是由栅极垂直侧壁与二维电子气水平壁之间的电场构成的等效电容.本文基于保角映射法对Cofd进行物理建模,考虑沟道长度调制效应,研究外部偏置、阈值电压漂移和温度变化对Cofd的影响:随着漏源偏压从零开始增加,Cofd先保持不变再开始衰减,其衰减速率随栅源偏压的增加而减缓;AlGaN势垒层中施主杂质浓度的减小和Al组分的减小都可引起阈值电压的正向漂移,正向阈值漂移会加强沟道长度调制效应对Cofd的影响,导致Cofd呈线性衰减.在大漏极偏压工作情况下,Cofd对器件工作温度的变化更加敏感. 相似文献
52.
《Journal of Saudi Chemical Society》2020,24(10):754-764
A kinetic study of the reactions of potentially bioactive 2-amino-4-arylthiazoles with highly reactive 4,6-dinitrobenzofuroxan (DNBF) is reported herein in acetonitrile solution. The complexation reaction was followed by recording the UV–vis spectra with time at λmax = 482 nm. Electronic effects of substituents influencing the rate of reaction have been studied using structure-reactivity relationships. It is shown that the Hammett plot relative to the reaction of DNBF with 2-amino-4-(4-chlorophenyl)thiazole exhibit positive deviation from the log k1 versus σ correlation, while it showed excellent linear correlation in terms of Yukawa–Tsuno equation. It has be noticed that the nonlinear Hammett plot observed for 2-amino-4-(4-chlorophenyl) thiazole is not attributed to a change in rate-determining step but is due to nature of electronic effect of substituent caused by the resonance of stabilization of substrates. The second-order rate constant (k1) relating to the bond C–C and C-N forming step of the complexation processes of DNBF with 4-substituted-aminothiazoles and 2-amino-5-methyl-4-phenylthiazole, respectively, is fit into the linear relationship log k = sN (N + E), thereby permitting the assessment of the nucleophilicity parameter (N) of the 2-amino-4-arylthiazoles of the range (4.90 < N < 6.85). 2-amino-4-arylthiazoles is subsequently ranked by positioning its reactivity on the general nucleophilicity scale developed recently by Mayr and coworkers (2003) leading an interesting and a direct comparison over a large domain of π-, σ -, and n-nucleophiles. The global electrophilicity/nucleophilicity reactivity indexes of the 2-amino-4-arylthiazoles have been investigated by means of a density functional theory (DFT) method. . 相似文献
53.
The friction at the liquid-solid interfaces is widely involved in various phenomena ranging from nanometer to micrometer scales. By the molecular dynamic(MD)simulation, the friction properties of liquid-solid interfaces at the molecular level are calculated via the Green-Kubo relation. It is found that the system size will influence the value of the friction coefficient, especially for the solid surfaces with the larger polar charge. The value of the friction coefficient decreases with the increase in the system size and converges at large system sizes. The large polar charge will lead to a significant friction coefficient. However, the diffusion of water molecules on this surface is almost a constant, indicating that the diffusion coefficient seems to be independent of the system size and polar charge. This work provides insights for the selection of the system size in modeling the frictional properties of hydrophobic/hydrophilic surfaces. 相似文献
54.
Dr. Zhibo Liu Daniel Chao Dr. Ying Li Prof. Dr. Richard Ting Jihyoon Oh Prof. Dr. David M. Perrin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(10):3924-3928
Organotrifluoroborates solvolyze in water at rates that vary over five orders of magnitude. The negative logarithm of the solvolytic rate constant, pk(B?F), correlates exceptionally well with the pKa of the analogous carboxylic acid (R2=0.984). This unforeseen correlation may be of predictive value for several applications including Suzuki–Miyaura cross‐coupling reactions and the design of 18F‐organotrifluoroborate radioprosthetic groups. 相似文献
55.
Palladium‐Catalyzed Direct C2‐Arylation of an N‐Heterocyclic Carbene: An Atom‐Economic Route to Mesoionic Carbene Ligands 下载免费PDF全文
Dr. Rajendra S. Ghadwal Sven O. Reichmann Dr. Regine Herbst‐Irmer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(11):4247-4251
Blocking the C2 position of an imidazole‐derived classical N‐heterocyclic carbene (NHC) with an aryl group is an essential strategy to establish a route to mesoionic carbenes (MICs), which coordinate to the metal via the C4 (or C5) carbon atom. An efficient catalytic route to MIC precursors by direct arylation of an NHC is reported. Treatment of 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) with an aryl iodide (RC6H4I) in the presence of 0.5 mol % of [Pd2(dba)3] (dba=dibenzylideneacetone) precatalyst affords the C2‐arylated imidazolium salts {IPr(C6H4R)}I (R=H, 4‐Me, 2‐Me, 4‐OMe, 4‐COOMe) in excellent (up to 92 %) yields. Treatment of {IPr(C6H5)}I with CuI and KN(SiMe3)2 exclusively affords the MIC–copper complex [(IPrPh)CuI]. 相似文献
56.
Stereoselective Preparation of Polyfunctional Alkenylindium(III) Halides and Their Cross‐Coupling with Unsaturated Halides 下载免费PDF全文
Zhi‐Liang Shen Prof. Dr. Paul Knochel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(19):7061-7065
The direct insertion of indium powder to cycloalkenyl iodides in the presence of LiCl in THF allows the preparation of new highly functionalized cycloalkenylindium(III) derivatives. In addition, we discovered that, in contrast to many metal insertions to alkenyl iodides which proceed with a loss of stereochemistry, the insertion of In/LiCl to stereodefined (Z)‐ and (E)‐styryl iodides in THF proceeded with high retention of stereochemistry. After a palladium‐catalyzed cross‐coupling, various polyfunctionalized (Z)‐ and (E)‐stilbenes were obtained with high stereoselectivity. 相似文献
57.
Investigation of Suzuki–Miyaura catalyst‐transfer polycondensation of AB‐type fluorene monomer using coordination‐saturated aryl Pd(II) halide complexes as initiators 下载免费PDF全文
Zilong Zhang Pan Hu Xiao Li Hongmei Zhan Yanxiang Cheng 《Journal of polymer science. Part A, Polymer chemistry》2015,53(12):1457-1463
Novel triarylamine‐based coordination‐saturated aryl Pd(II) halide complexes ligated by PEt3, PCy3, and P(o‐tol)3 were successfully synthesized by direct oxidative addition of aryl halide to the corresponding Pd(0) precursors. Suzuki–Miyaura coupling polymerization of 2‐(7‐halide‐9,9‐dioctylfluoren‐2‐yl)?1,3,2‐dioxaborinane with these Pd(II) complexes as initiators was investigated for the synthesis of poly(fluorene)s with triarylamine end group. Pd(II) complexes with PCy3 or P(o‐tol)3 exhibited catalytic activity and realized the catalyst‐transfer polycondensation at 75 °C and room temperature, respectively, while the polymerization using Pd(II) catalyst ligated by PEt3 did not proceed, which indicated that the bulky phosphine ligands could facilitate the reductive elimination and further promote the polymerization. In addition, the dimeric Pd(II) complex with P(o‐tol)3 can convert into monomeric Pd(II) intermediate with an open coordination site, which had a higher activity. The end groups of the afforded polyfluorene were analyzed by matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectrometry, in which the Ar/H end groups are indicative of the catalyst‐transfer polymerization. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1457–1463 相似文献
58.
Novel hyperbranched polymers synthesized via A3 + B(B′) approach by radical addition‐coupling polymerization 下载免费PDF全文
In this study, a novel application of radical addition‐coupling polymerization (RACP) for synthesis of hyperbranched polymers is reported. By Cu/PMDETA‐mediated RACP of 2‐methyl‐2‐nitrosopropane with trimethylolpropane tris(2‐bromopropionate) or a bromo‐ended 3‐arm PS macromonomer, two types of hyperbranched polymers with high degree of polymerization are synthesized under mild conditions, respectively. The chemical structures of the hyperbranched polymers are carefully characterized. By selective degradations of the ester groups and weak bonds of NO? C in the polymers, high degree of alternative connection of the two monomers in the synthesized polymers have been identified. Based on the experimental results, mechanism of formation of the hyperbranched polymer is proposed, which includes formation of carbon radicals from the tribromo monomer through single electron transfer, its capture by 2‐methyl‐2‐nitrosopropane that results in nitroxide radical, and cross‐coupling reaction of the nitroxide radical with other carbon radicals. Hyperbranched polymer can be formed in a step‐growth mode after multiple steps of such reactions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 904–913 相似文献
59.
“(Diphosphine)Nickel”‐Catalyzed Negishi Cross‐Coupling: An Experimental and Theoretical Study 下载免费PDF全文
Dr. Emmanuel Nicolas Alexia Ohleier Florian D'Accriscio Dr. Anne‐Frédérique Pécharman Dr. Matthieu Demange Dr. Philippe Ribagnac Dr. Jorge Ballester Dr. Corinne Gosmini Dr. Nicolas Mézailles 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(21):7690-7694
The use of a strongly donating “(bis‐dialkylphosphine)Ni” fragment promotes the catalytic coupling of a large range of ArCl and ArZnCl derivatives under mild conditions. Stoichiometric mechanistic investigations and DFT calculations prove that a Ni0/NiII cycle is operative in this system. 相似文献
60.