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991.
DFT calculations have been performed to determine the isomer shift for a series of iron(II) clusters with nitrogen-containing ligands which serve as models of coordination units in Fe(II) complexes with 1,2,4-triazoles possessing a 1 A 1 ? 5 T 2 spin transition. Good agreement has been found between the theoretical and experimental values of the isomer shift for both low-and high-spin phases. Our calculations confirmed the hypothesis about relationship between the experimentally observed differences in the isomer shift for the low-spin phases of the complexes and variations of the Fe-N mean bond length. 相似文献
992.
对于烷基乙酰胺的初始热解反应机理, 通常认为与酯类的热解反应相类似。Maccoll和Nagra通过对该热解反应的动力学研究, 认为两者存在不同。差异之一, 烷基乙酰胺存在两种可能的热解途径【参见本文(129页)前述反应方程(1),(2)】。而在酯类热解反应中(2)的活化能过高, 且四元环过渡态极不稳定。差异之二, 极性溶剂(比如乙酸)对酰胺热解反应的催化作用不明显, 而对酯类等气相热解反应的催化作用是十分显著的。为此, 我们用MINDO/3分子轨道法对乙基乙酰胺的初始热解反应进行了较全面的研究。用能量梯度法对此反应的反应物、中间体和生成物的平衡几何构型进行了全优化。(如图1所示)用极小能量途径法分别寻找反应(1)和反应(2)的初始过渡态, 继而用Powell法全优化过渡态的几何构型, 计算所得的过渡态TS1、TS2和TS3分别见图2a, 图3a和图4a。为了确证这些过渡态, 进行了振动分析研究, 结果表明这些过渡态的力常数矩阵的诸本征值中均只有一个负值, 且虚振动模式展示了走向各自的反应物和生成物的趋势, (如图2b,图3b和图4b所示)。它们的总能量及反应(1)和反应(2)的活化能列于表1. 对整个热解反应(1)作了内禀反应坐标(IRC)理论分析, 反应历程见图5所示. 与IRC相应的总能偶极矩以及部分关键的键长和原子净电荷变化一并列于表2.本文研究结果表明, 在乙基乙酰胺的初始反应中主反应即反应(1)与酯类反应相类似, ... 相似文献
993.
M. P. Carril M. Soledad Corbillón J. Manuel Madariaga 《Accreditation and quality assurance》1997,2(6):301-308
The development of an analytical method for the determination of some heavy metals (Fe, Cu, Co, Zn and Ni) in fluoride compounds
[Cu(BF4)2, Sn(BF4)2, Pb(BF4)2 and HBF4] by flame atomic absorption spectroscopy is described. This method is to be used as a routine analytical method in an industrial
quality control laboratory. To this end the "performance characteristics" of an instrumental analytical method such as matrix
effects, sensitivity, linearity, detection and quantitation limits, precision and accuracy were evaluated for every system
under study. The results of these investigations showed that non-spectral interferences (due to the presence of large concentrations
of major metals such as Cu, Sn and Pb) were observed. Nevertheless it was possible to define a matrix concentration interval
where matrix effects were not statistically significant, and therefore a direct calibration approach could be used as the
calibration tool whenever the major metal concentration was not higher than 40×10–3 kg l–1. A guide to the developement of an analytical method for trace metal determination is provided. General tools for quality
control have been used in order to show how an analytical method can be tested daily and evaluated in a convenient manner.
Received: 29 January 1997 Accepted: 11 March 1997 相似文献
994.
基于Mn(Ⅱ)对试剂2-(8'-羟基喹啉-5'-磺基-7'-偶氮基)-1-羟基-8-氨基一萘二磺酸(以下简称HQSAH)分解反应的催化作用,提出了锰的荧光催化动力学分析新方法,其λex/em=230/415nm,适宜酸度范围为pH11.0~12.0,Mn(Ⅱ)含量在0~0.08μg/mL呈线性关系,该法用于分析铸造铝合金中的痕量锰,效果良好。还初步探讨了反应机制,确定了反应动力学方程,测定了反应速率和活化能。 相似文献
995.
Thomas Bernert 《Journal of solid state chemistry》2006,179(3):849-854
Cu2MnGeS4 crystallizes orthorhombic in a wurtzite superstructure type while Cu2MnSnS4 crystallizes in a tetragonal sphalerite superstructure type. Lattice constants and thermal analyses of the solid solution series Cu2MnGexSn1−xS4 are presented. A two-phase region is found from Cu2MnGe0.3Sn0.7S4 to Cu2MnGe0.5Sn0.5S4. The cell volume of the mixed crystals increases with increasing Sn content. The melting points increase smoothly with increasing Ge content to x=0.5 and then steeply for higher Ge contents. The single crystal X-ray structure analysis of Cu2MnGe0.55Sn0.45S4 is presented. The refinement converges to R=0.0270 and wR2=0.0586, Z is 2. The volumes of the tetrahedra [MS4] (M=Cu, Mn, Ge, Sn) are calculated. From these volumes the differences in size of the tetrahedra are derived and compared with the corresponding differences in the end members of the solid solution series. It turns out that the resulting structure type in these materials depends on the volume differences of the constituting tetrahedra [MS4]. 相似文献
996.
Villanueva J Villegas V Querol E Avilés FX Serrano L 《Journal of mass spectrometry : JMS》2002,37(9):974-984
In the post-genomic era, several projects focused on the massive experimental resolution of the three-dimensional structures of all the proteins of different organisms have been initiated. Simultaneously, significant progress has been made in the ab initio prediction of protein three-dimensional structure. One of the keys to the success of such a prediction is the use of local information (i.e. secondary structure). Here we describe a new limited proteolysis methodology, based on the use of unspecific exoproteases coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), to map quickly secondary structure elements of a protein from both ends, the N- and C-termini. We show that the proteolytic patterns (mass spectra series) obtained can be interpreted in the light of the conformation and local stability of the analyzed proteins, a direct correlation being observed between the predicted and the experimentally derived protein secondary structure. Further, this methodology can be easily applied to check rapidly the folding state of a protein and characterize mutational effects on protein conformation and stability. Moreover, given global stability information, this methodology allows one to locate the protein regions of increased or decreased conformational stability. All of this can be done with a small fraction of the amount of protein required by most of the other methods for conformational analysis. Thus limited exoproteolysis, together with MALDI-TOF MS, can be a useful tool to achieve quickly the elucidation of protein structure and stability. 相似文献
997.
The time lag permeation technique has proven to bean effective method for characterisation. Because of the simple nature of the permeation experiment, transport parameters can be directly obtained from experimental data hence avoiding the intensive mathematical treatment required by other techniques. The method has historically been applied to diffusion and adsorption in porous membranes and diffusion in polymer membranes. Since its origins in 1920, interest in the time lag method has expanded because of its value in characterising simple permeation processes and also complex systems of diffusion with simultaneous adsorption and surface diffusion. This review focuses on presenting the asymptotic solution of the mass balance diffusion equations and includes applications of time lag analysis, in order to give a critical and broad perspective of this method as a tool for characterisation. It includes much of the previously published literature in order to show that for most cases the asymptotic solution of the transport equations is simple, and for more complex cases that an analytical solution is possible hence avoiding cumbersome numerical techniques. 相似文献
998.
以Mg(NO3)2,Ca(NO3)2,Eu(NO3)3,Bi(NO3)3和Si(OC2H5)4为反应物,采用溶胶-凝胶法,在比较低的温度,首次合成0.701molMgO-0.175molCaO-1.25molSiO2:0.06molEu^3+,0.002molBi^3+(加入Li^+作为电荷补偿剂)发光体,得到了最佳合成条件,研究了由溶胶向凝胶转变和凝胶向发光晶体的转变过程,探讨了发光体在不同激光 相似文献
999.
T. Saraidarov R. Reisfeld A. Sashchiuk E. Lifshitz 《Journal of Sol-Gel Science and Technology》2003,26(1-3):533-540
PbS nanocrystals (NCs) ranging between 4–8 nm were incorporated into Zirconium-Silica-Urethane (ZSUR) matrix obtained by the sol-gel method. The sizes of the particles were controlled by temperature treatment and by concentration of PbS in ZSUR matrix. The sizes of PbS NCs were determined by TEM measurements. The quantum size effect could also be extracted from optical absorption and photoluminescence spectra. The new matrix allows incorporation of up to 40% PbS forming a characteristic structure of dendrite by reacting lead acetate with ammonium thiocyanate in sol-gel matrix. The sol precursors of the matrix for Zirconium-Silica-Urethane contained zirconium oxide (ZrO2) matrix solution, tetramethoxysilane (TMOS), 3-glycid oxypropyl trimethoxysilane (GLYMO) and polyethylene urethane silane (PEUS) synthesized separately. The ZrO2 matrix solution was obtained from zirconium n-tetrapropoxide in propanol and acetic acid was used as a chelating agent to stabilize the zirconium oxide precursor. 相似文献
1000.