氧离子导体La2Mo2O9中α→β相变非等温动力学的研究

以差热分析(differential thermal analysis,DTA)为手段研究了La2Mo2O9中α→β相变的非等温动力学。采用Flynn-Wall-Ozawa法计算了不同转化分数下的相变活化能E,并用Ozawa迭代法对E值进行了修正。应用Criado-Ortega法获得了α-La2Mo2O9→β-La2Mo2O9相变各温度区间的动力学机理函数,并用譒atava-譒esták法进行了验证。结果表明:La2Mo2O9中α→β相变的表观活化能与转化分数有关,说明该相变不属于一步简单反应。不同温度区间α→β相变的动力学机理函数不同,836～840K、842～848 K和850～853 K的动力学机理函数G(x)可依次表示为:-ln(1-x)、[-ln(1-x)]2/3、[-ln(1-x)]3/4。     

Solvent effects on the spectrokinetic studies on the molecular complexes of dextromethorphan and atenolol drugs with DDQ

The kinetics and mechanism of the interaction between 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) and dextromethorphan and atenolol drugs has been investigated spectroscopically. In the presence of large excess of donor drug, the 1:1 charge transfer (CT) complex is transformed into a final product, which has been isolated and characterized by using FT‐IR and GC‐MS techniques. The rate of formation of product has been measured as a function of time in different solvents at three temperatures. The thermodynamic parameters, viz. activation energy, enthalpy, entropy, and free energy of activation were computed from temperature dependence of rate constants. On the basis of the spectrokinetic results, a plausible mechanism for the formation of the CT complex and its transformation into final product is presented and discussed. Cyclic voltammetric study supports the observed solvent effect on the extent of CT complexation and the rate with which it is converted into the product. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 559–568, 2008     

Theoretical study of the reaction of Cr+ with SCO in gas phase

To elucidate the mechanism of reaction M⁺ + SCO, the reaction of Cr⁺ + SCO has been investigated using density functional theory (DFT) with the popular hybrid functional, B3LYP, in conjunction with 6‐311+G* basis set on both the sextet and quartet potential energy surfaces (PESs). To obtain an accurate evaluation of the activation barrier and reaction energy, the coupled cluster single‐point calculations using the B3LYP structures is performed. The crossing points (CPs) of the different PESs have been localized with the approach suggested by Yoshizawa and colleagues. The involving potential energy curve‐crossing dramatically affects reaction mechanism. The present results show that the reaction mechanism is insertion‐elimination mechanism both along the C? S and C? O bond activation branches, but the C? S bond activation is much more favorable than the C? O bond activation in energy. All theoretical results not only support the existing conclusions inferred from early experiment study, but also complement the pathway and mechanism for this reaction. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Trimethylenemethane: Activation energy for ring-closure of the diradical

The energy of activation for ring-closure of ground state triplet trimethylenemethane (I) to methylenecyclopropane has been measured by following the rate of dissappearance of the electron spin resonance spectrum over the temperature range –155° to –140°, in a series of frozen solid matrices. The experiments described make use of 3-methylenecyclobutanone and methylenecyclopropane as precursors to trimethylene-methane.Kinetic results obtained starting from methylenecyclopropane were most satisfactory and lead to an energy of activation for ring-closure of 7 kcal/mole. This value is significantly smaller than the aprpox. 20 kcal/mole barrier estimated on the basis of theoretical models. Truncation of the barrier by a tunnelling mechanism is made unlikely by the finding that trimethylenemethane-d₆(I-d₆) undergoes ring-closure with the same 7 kcal/mole energy of activation as the parent I.     

Chlorine‐Enhanced Surface Mobility of Au(100)

Motivated by experimental studies of two‐dimensional Ostwald ripening on Au(100) electrodes in chlorine‐containing electrolytes, we have studied diffusion processes using density functional theory. We find that chlorine has a propensity to temporary form AuCl complexes, which diffuse significantly faster than gold adatoms. With and without chlorine, the lowest activation energy is found for the exchange mechanism. Chlorine furthermore reduces the activation energy for the detachment from kink sites. Kinetic Monte Carlo simulations were performed on the basis of extensive density functional theory calculations. The island‐decay rate obtained from these Monte Carlo simulations, as well as the decay rate obtained from the theoretical activation energies and frequency factors when inserted into analytical solutions for Ostwald ripening, are in agreement with experimental island‐decay rates in chlorine‐containing electrolytes.     

Kinetics of non-isothermal crystallization of some glass-ceramics based on basalt

The crystallization kinetics of some glass-ceramics obtained from Romanian (Şanoviţa) basalt has been studied in non-isothermal conditions using DTA technique. The activation energies of the crystallization processes were calculated using the isoconversional methods Kissinger-Akahira-Sunose and Ozawa-Flynn-Wall. The results obtained show a dependence of the activation energy (E _α) on the crystallized fraction (α) that proves the complex mechanism of the glass-ceramics crystallization process. It has been proved that the Johnson-Mehl-Avrami model cannot be applied for the studied glass-ceramics crystallization process. The effect of 2% TiO₂ as nucleating agent upon the crystallization kinetics and upon the microstructure of the studied glass-ceramics was analyzed.     

Rates and mechanism of fluoride and water exchange in UO(2)F(5)(3-) and [UO(2)F(4)(H(2)O)](2-) studied by NMR spectroscopy and wave function based methods

The reaction mechanism for the exchange of fluoride in UO(2)F(5)(3-) and UO(2)F(4)(H(2)O)(2-) has been investigated experimentally using (19)F NMR spectroscopy at -5 degrees C, by studying the line broadening of the free fluoride, UO(2)F(4)(2-)(aq) and UO(2)F(5)(3-), and theoretically using quantum chemical methods to calculate the activation energy for different pathways. The new experimental data allowed us to make a more detailed study of chemical equilibria and exchange mechanisms than in previous studies. From the integrals of the different individual peaks in the new NMR spectra, we obtained the stepwise stability constant K(5) = 0.60 +/- 0.05 M(-1) for UO(2)F(5)(3-). The theoretical results indicate that the fluoride exchange pathway of lowest activation energy, 71 kJ/mol, in UO(2)F(5)(3-) is water assisted. The pure dissociative pathway has an activation energy of 75 kJ/mol, while the associative mechanism can be excluded as there is no stable UO(2)F(6)(4-) intermediate. The quantum chemical calculations have been made at the SCF/MP2 levels, using a conductor-like polarizable continuum model (CPCM) to describe the solvent. The effects of different model assumptions on the activation energy have been studied. The activation energy is not strongly dependent on the cavity size or on interactions between the complex and Na(+) counterions. However, the solvation of the complex and the leaving fluoride results in substantial changes in the activation energy. The mechanism for water exchange in UO(2)F(4)(H(2)O)(2-) has also been studied. We could eliminate the associative mechanism, the dissociative mechanism had the lowest activation energy, 39 kJ/mol, while the interchange mechanism has an activation energy that is approximately 50 kJ/mol higher.     

The mechanism of the low temperature polymerization reaction in diacetylene crystals

The mechanism of the low temperature polymerization reaction in TSHD diacetylene crystals has been investigated by the transient ESR of the paramagnetic triplet (S = 1) and quintet (S ≈ 2) reaction intermediates. The reaction schemes of the thermal and optical addition and transformation reactions after UV photoinitiation are obtained. The activation energies of the chain propagation reaction and the singlet—triplet and singlet—quintet energy separations of the different dicarbene and diradical intermediates are deduced from the experiments.     

Crystallization kinetics of Cu3Ti2 forming from Cu60Ti40 glass

Amorphous to crystalline transformation in Cu₆₀Ti₄₀ alloy has been studied under conditions of constant heating rate experiments, using XRD and DSC. In the high temperature XRD experiment, the transformation has been monitored continuously as the integrated X-ray intensity corresponding to a chosen reflection from one of the two crystalline products forming at close by temperatures. Differentiation of the curve thus obtained gives the transformation rate curve which passes through a maximum. From the peak shift with heating rate, the activation energy for the formation of Cu₃Ti₂ crystalline phase has been obtained. The results have been compared with those obtained by DSC.     

2,6-二巯基嘌呤质子转移异构化的密度泛函理论研究

采用密度泛函B3LYP方法, 在6-311＋G(d,p)基组水平上对2,6-二巯基嘌呤质子转移引起的硫醇式与硫酮式互变异构反应进行了计算研究, 获得了互变异构过程的反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等性质. 计算结果表明, 2,6-二巯基嘌呤无论是孤立分子还是一水合物, 其二硫酮式R是最稳定异构体. 由二硫酮式通过分子内质子转移向二硫醇式异构化共有6条反应通道, 其主通道(1)速控步骤的活化能为139.1 kJ•mol^－1, 速率常数为2.16×10^－12 s^－1; 当水分子参与反应以双质子转移机理异构化时, 活化能显著降低, 有利于硫酮式向硫醇式转变, 其主通道(7)速控步骤的活化能为61.3 kJ•mol^－1, 速率常数为1.33×10 s^－1. 计算结果还表明, 氢键作用在增大2,6-二巯基嘌呤氢键一水合物稳定性、降低质子转移异构化反应活化能等方面起着重要的作用.     

Novel simple methods for the synthesis of single-phase valentinite Sb2O3

Several methods with solid and dissolved reactants were investigated as possible routes for synthesis of single-phase valentinite Sb₂O₃. The methods are based on simple chemical reaction between SbCl₃ and NaOH. The method with solid state reactants was established on self-propagating room temperature reaction (SPRT), while wet syntheses were based on the same chemical reaction, and performed in either distilled water or absolute ethanol. The prepared powders were characterized by X-ray powder diffraction, scanning electron microscopy and field emission scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction (SAED) and UV/vis diffuse reflectance spectroscopy. SPRT and aqueous solution syntheses resulted in single-phase valentinite Sb₂O₃, but with significantly different morphologies. In the case of SPRT method the obtained powder contains well crystallized prismatic shaped submicronic particles, with hexagonal or lozenge basis typical for valentinite crystal structure, while aqueous solution synthesis resulted in powder containing micronic agglomerates. The ethanolic solution synthesis product was Sb₂O₃ with cubic senarmontite as predominant phase and traces of orthorhombic valentinite. It was confirmed that not only the aggregate state, but also the choice of solvent has a great influence on the structural and optical characteristics of synthesized Sb₂O₃ powders.     

Calorimetric study of cross-linking polymerization of methyl methacrylate in the presence of a multimonomer

The kinetics of free-radical cross-linking polymerization of methyl methacrylate (MM) in the presence of poly[2-(10-undecenoyloxy)ethyl methacrylate] (PUDEM) as a macromolecular cross-linker has been isothermally examined within the temperature range from 85–100°C using the differential scanning calorimetry (DSC). The activation energy found for this reaction, E _a=89.3 kJ mol^–1, exceeds slightly the literature values of activation energy obtained for the mass polymerization of MM without any cross-linking agent. The activation energy has been also determined by the isoconversion method. It has been found that E _a decreases with the increase in the conversion, which may indicate a change in the reaction mechanism.This work was partly supported by the Committee for Research (KBN) in the framework of project No. 7 T08E 026 20     

Sintering kinetic study of the nano-crystalline 3-mol% yttria-samaria codoped tetragonal zirconia polycrystal ceramics

Dilatometric shrinkage data was utilized to study the sintering kinetics of the in-house synthesized nano-crystalline 3-mol% yttria-samaria codoped tetragonal zirconia polycrystal (TZP) ceramics. The objective was to determine activation energy (Q) of sintering and the sintering mechanism (n) relevant to the initial stage of sintering. The product of activation energy and sintering parameter, i.e., “nQ” was calculated from the shrinkage data acquired from the constant rate of heating experiment. The apparent activation energy of sintering (Q) was calculated using modified-“Dorn” method. Modified Johnson’s equation was used to determine value of “n” using the activation energy obtained from the Dorn method. Stepwise isothermal dilatometry technique was utilized as an independent method to determine the “n” value. The activation energy of sintering was in the range of 400–525 kJ mol^?1 and found to be dependent on the dopant concentration. The value of “n” was found to be ~0.33 for both 3 mol% yttria-doped (3Y-TZP) and yttria-samaria codoped (3(Y,S)-TZP) TZP, whereas for 3 mol% samaria-doped tetragonal zirconia (3S-TZP), the value of “n” was ~0.40. From the obtained “n” values, it may be concluded that grain boundary diffusion (GBD) was the dominant sintering mechanism in 3Y-TZP and 3(Y,S)-TZP, whereas an intermediate of GBD and volume diffusion influences the initial sintering stage in the 3S-TZP.     

光卤石在芒硝溶液中的溶解和转化

研究了光卤石在芒硝溶液中溶解及转化的机理，用离子浓度随时间的变化拟合得到动力学方程，并求得溶解反应的表观活化能。     

A Silica Bilayer Supported on Ru(0001): Following the Crystalline-to Vitreous Transformation in Real Time with Spectro-microscopy

The crystalline-to-vitreous phase transformation of a SiO₂ bilayer supported on Ru(0001) was studied by time-dependent LEED, local XPS, and DFT calculations. The silica bilayer system has parallels to 3D silica glass and can be used to understand the mechanism of the disorder transition. DFT simulations show that the formation of a Stone–Wales-type of defect follows a complex mechanism, where the two layers show decoupled behavior in terms of chemical bond rearrangements. The calculated activation energy of the rate-determining step for the formation of a Stone—Wales-type of defect (4.3 eV) agrees with the experimental value. Charge transfer between SiO₂ bilayer and Ru(0001) support lowers the activation energy for breaking the Si−O bond compared to the unsupported film. Pre-exponential factors obtained in UHV and in O₂ atmospheres differ significantly, suggesting that the interfacial ORu underneath the SiO₂ bilayer plays a role on how the disordering propagates within the film.     

Glass transition activation energy of CdS/PMMA nano-composite and its dependence on composition of CdS nano-particles

CdS/PMMA nano-composites at different weight percent of CdS (0, 2, 4, 6, and 8) have been prepared using solution casting method. The obtained nano-composites are characterized through the transmission electron microscope (TEM). The differential scanning calorimetry (DSC) measurements have been done on the nano-composites at different heating rates. The peak glass transition temperature is determined using the DSC thermograms. It is found that the glass transition temperature increases with the increase of CdS content up to 6 wt% and then decreases for higher weight percent (8). It is explained on the basis of molecular motion of PMMA, which is restricted when CdS is added into PMMA. An effort is also made to study the activation energy of glass transition in the case of nano-composites of different weight percent of CdS. Variation of activation energy with CdS nano-particle concentration has also been theoretically predicted by using an empirical relation. Thermal stability of these nano-composites has been explained with the help of activation energy in the glassy region.     

Conductivity relaxation behavior of interpenetrating polymer network composites of polypyrrole and poly(styrene‐co‐butyl acrylate)

Conductivity relaxation spectra of interpenetrating network conducting composites of polypyrrole (PPy) and poly(styrene‐co‐butyl acrylate) (SBA) have been analysed on the basis of coupling model developed by Ngai. The macroscopic activation energy obtained from coupling model using the stretch exponent β compares favourably with the tunnel energy estimated from the overlapping large polaron tunnelling (OLPT) model. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1193–1200, 2000     

Study of non-isothermal crystallization of amorphous Cu<Subscript>50</Subscript>Ti<Subscript>50</Subscript> alloy

The crystallization kinetics of amorphous Cu₅₀Ti₅₀ has been studied using differential scanning calorimetry (DSC) under non-isothermal conditions. The curves at different linear heating rates (2, 4, 8 and 16 K min^–1) show sharp crystallization peaks. The crystallization peak shifts to higher temperatures with increasing heating rate. The Kissingers method of analysis of the shift in the transformation peak is applied to evaluate the activation energy (E _c). The KJMA formalism, which is basically developed for isothermal experiments, is also used to obtain E _c and the Avrami parameter (n).The DSC data have been analysed in terms of kinetic parameters, viz. activation energy (E _c), Avrami exponent (n) and frequency factor K ₀ using three different theoretical models. It is observed that the activation energy values derived from KJMA approach and modified Kissinger equation agree fairly well with each other. The activation energy values obtained from normal Kissinger method, and Gao and Wang expression underestimate the activation energy.The financial support provided by All India Council for Technical Education (AICTE), New Delhi (Govt. of India) is gratefully acknowledged.     

Investigation of the Crystallization Kinetics of Zn(Met)(AcO)_2·H_2O in Mixed Solution of Water and Acetone by Microcalorimetry

Introduction The complexes of zinc salts with -amino acid as additive have a wide application in medicine, foodstuff and cosmetics.1-3 The synthesis methods of the com-plexes of zinc salts with -amino acid have been re-viewed.4 The solubility of Zn(AcO)2-Met-H2O system at 298.15 K has ever been investigated by the semi-micro-phase equilibrium method.5 The phase diagram is a simple one, in which the solid complex of Zn(AcO)2 and Met can not be obtained at 298.15 K. Zn(Met)2+ in solution w…     

A computer method to determine the kinetic law of solid-state reactions from DSC curves

A computer program has been worked out to evaluate the activation energy, the Arrhenius pre-exponential factor and the mechanism of solid-state reactions from non-isothermal measurements. A univocal determination of the reaction mechanism is obtained by the simultaneous application of two selection criteria: (a) activation energy value by the Ozawa method; (b) degree of linearity in the <Satava method.A modification of the Rogers and Smith method was then used to obtain empirically complete agreement between kinetic expression and experimental results when the kinetic law has to be improved to account for a whole experimental curve.The accuracy of the computer method has been checked through a calibration of the program by means of 16 theoretical functions proposed by <Sesták.