热动力学的研究(XXlV)——等浓度三级反应的热动力学

本文根据热动力学的基础理论,建立了等浓度三级反应的无量纲参数法,并用此法研究了甲醛与氨反应的热动力学.实验测得的速率常数与文献值吻合.     

热动力学的研究(Ⅳ)——等浓度二级反应的热动力学研究法

本文根据热动力学的基础理论, 讨论了等浓度二级反应的无量纲参数法, 并用以研究了苯甲酸乙酯皂化反应的热动力学。实验测得的速度常数在误差范围内与文献值很好符合。     

特征参量法1: 简单级数反应的热动力学研究法

建立了测定热动力学体系冷却常数的线性拟合法和简单级数反应通用的热动力学研究法---特征参量法。导出了简单级数反应的动力学参数Kn和速率常数kn的通用计算式。应用冷却常数的线性拟合法在不同温度下测定了两个反应体系的冷却常数, 探讨了温度、浓度、介质及装样方式对冷却常数的影响。应用简单级数反应的特征参量法研究了一级、二级、三级和分数级反应的热动力学, 结果文献值吻合。讨论了热动力学体系的冷却常数对速率常数计算结果的影响。     

特征参量法1: 简单级数反应的热动力学研究法

建立了测定热动力学体系冷却常数的线性拟合法和简单级数反应通用的热动力学研究法---特征参量法。导出了简单级数反应的动力学参数Kn和速率常数kn的通用计算式。应用冷却常数的线性拟合法在不同温度下测定了两个反应体系的冷却常数, 探讨了温度、浓度、介质及装样方式对冷却常数的影响。应用简单级数反应的特征参量法研究了一级、二级、三级和分数级反应的热动力学, 结果文献值吻合。讨论了热动力学体系的冷却常数对速率常数计算结果的影响。     

热动力学时间变量法——简单级数化学反应

在热动力学理论基础上提出了一种新的热动力学研究法--时间变量法.该法无须将反应进行到底,只需知道一张热谱图上的3个时间点t,t时刻峰高△和t前峰面积a,就可用计算机数值方法求算反应的速率常数.应用该法研究了一级反应、等浓度二级反应、不等浓度二级反应、三级反应和1.5级反应的热动力学.     

热动力学的研究 Ⅲ. 正脂肪酸乙酯皂化反应的热动力学

根据热动力学的基础理论, 应用一级反应的无量纲参数法, 在20和30 ℃下测定了乙酸乙酯、丙酸乙酯和丁酸乙酯在乙醇-水混合溶剂中皂化反应速度常数, 并计算了它们的活化能, 结果在误差范围内与文献值相符合。还计算了这三个反应体系的活化自由能。本文讨论了反应物初始浓度和反应的速度常数与量热计冷却常数的关系。     

壳聚糖超声可控降解及降解动力学研究

通过正交实验法考察了壳聚糖溶液浓度、反应温度、超声强度以及醋酸溶液浓度对超声降解反应的影响,确定了最佳反应条件,制备了一系列不同分子量的壳聚糖.研究了壳聚糖溶液浓度、反应温度以及壳聚糖原料分子参数与降解速率常数的关系.通过红外光谱、X-射线衍射和凝胶渗透色谱对降解产物进行了表征.结果表明,超声降解壳聚糖的最佳条件为10℃,壳聚糖溶液浓度2.5g/L.降解速率常数随壳聚糖溶液浓度和反应温度的降低而增大.高分子量和低脱乙酰度的壳聚糖原料有较高的降解速率和降解速率常数,壳聚糖原料的分子量对降解速率和降解速率常数的影响大于脱乙酰度对其的影响.超声波导致了壳聚糖分子量的降低和产物晶体结构的破坏,但没有改变产物的脱乙酰度和糖残基结构.     

COMSOL有限元分析方法在循环伏安法教学中的应用

以循环伏安法为例，介绍了如何通过COMSOL Multiphysics有限元软件建立电化学反应模型及其数值求解。通过改变电极尺寸、电子转移速率常数、均相化学反应速率常数等参数，实现了循环伏安图与扩散层内反应物浓度分布的关联和动态演示，帮助初学者形象深刻地理解循环伏安法的基本原理。     

耦联一级随后化学反应的准可逆表面体系循环伏安法数字模拟研究

用数字模拟方法研究了耦联一级不可逆随后化学反应的可逆、准可逆及不可逆表面吸附体系的循环伏安响应.建立了有关数字模型.模拟计算得到不同无量纲动力学参数时电流函数-电位极化曲线.给出了测定电荷传递速率常数k和随后反应速率常数中k₁的方法.并研究了对甲氧基偶氮苯在酸性介质中的伏安响应,结果尚好.     

两个4,4'-二甲基-2,2'-联吡啶锰(Ⅱ)配合物切割DNA的动力学研究

对2个4,4'-二甲基-2,2'-联吡啶锰(Ⅱ)配合物在生理条件及H2O2的存在下对DNA切割的动力学进行了研究.结果表明,这2个配合物分别存在下的DNA切割反应具有相似的动力学反应特征.其中对超螺旋DNA切割成缺口DNA步骤,均表现为三级反应,即反应速率分别与底物DNA的浓度、配合物的浓度和H2O2的浓度的一次方成正比;同时得到了2个反应的速率常数、活化能(Ea)、活化焓(△H≠)和活化熵(△S≠)等动力学参数,并根据这些结果提出了一个可能的氧化切割反应机理.     

KINETIC MODEL FOR DIFFUSION-CONTROLLED INTERMOLECULAR REACTION OF HOMOGENOUS POLYMER UNDER STEADY SHEAR

The kinetic model for diffusion-controlled intermolecular reaction of homogenous polymer under steady shear was theoretically studied. The classic formalism and the concept of conformation ellipsoids were integrated to get a new equation, which directly correlates the rate constant with shear rate. It was found that the rate constant is not monotonic with shear rate. The scale of rate constant is N^-1.5 （N is the length of chains）, which is in consistent with de Gennes＇s result.     

Effects of nonequilibrium on velocity and plate height in reactive capillary electrophoresis

Models for velocity and plate height for reactive CE are developed under the formalism of generalized nonequilibrium theory, as described by Giddings. The resultant equations are consistent with chromatographic theory and validated with an independent stochastic simulation. Moreover, unlike prior methods for CE, this model allows calculation of thermodynamic equilibrium constants and kinetic rate constants from a single, undistorted peak. The theoretical development shows that velocity is directly dependent on the equilibrium constant and is independent of the rate constant. On the other hand, plate height varies little with equilibrium constant and is inversely proportional to rate constant. The ability to evaluate equilibrium constants from velocity and rate constants from plate height is most greatly influenced by electric field strength and mobility difference. The accuracy in calculated equilibrium constants is limited by mobility difference; however, the accuracy in rate constants is limited by plate height and equilibrium constant.     

A new method for evaluation of the characteristic constant for primary radical termination in free radical polymerization

A new model has been proposed for estimation of the characteristic rate constant for primary radical termination, using the radical life time, rate of polymerization, and rate of initiation. The model can be used to estimate the characteristic rate constant for primary radical termination under most conditions of free radical polymerization. By applying this model to high conversion polymerization data, it is possible to compare the conversion dependence of the characteristic rate constant for primary radical termination and initiation rate as well as the conversion dependence of the termination rate constant.The model has applied to various published experimental data and the results compared with literature values.     

Rate constants for the combination of trichloromethyl radicals and the electron-transfer reaction between hydroxymethyl radicals and carbon tetrachloride in solution

The rate constant for the bimolecular combination of trichloromethyl radicals in methanol is determined as a function of temperature by steady-state kinetics and electron-spin resonance with modulated radical initiation. The rate constant is in accord with the Smoluchowski equation and indicates a diffusion-controlled radical termination. The temperature dependence of the rate constant for the electron-transfer reaction between hydroxymethyl radicals and carbon tetrachloride in methanol is determined, and is found to disagree with predictions of the Marcus theory. This disagreement is tentatively ascribed to the structure of the transition state.     

Promoting-mode free formalism for excited state radiationless decay process with Duschinsky rotation effect

In the present work, through the path integral of Gaussian type correlation function, a new formalism based on Fermi-Golden Rule for calculating the rate constant of nonradiative decay process with Duschinsky rotation effect in polyatomic molecules is developed. The advantage of the present path-integral formalism is promoting-mode free. In order to get the rate constant, a "transition rate matrix" needs to be calculated. The rate constant calculated previously is only an approximation of diagonal elements of our "transition rate matrix " . The total rate should be the summation over all the matrix elements.     

Derivation of rate constant ratios for consecutive second‐order irreversible reactions

The determination of rate constants for consecutive irreversible reactions is a difficult and time‐consuming problem, especially when the research extends up to many subsequent products. Thus, the derivation of proper mathematical expressions would greatly facilitate the determination of these rate constants when only the rate constant of the first consecutive reaction is known. Many authors have dealt with this problem in the past but the issue is still of interest to the scientific community judging from recent publications. This paper aims at extending our knowledge of mathematical expressions for rate constant ratios of consecutive reactions to more than three reactions, as is the situation now, and offering a simple graphical estimation of the rate constant ratios exploiting the maxima of each intermediate product. Furthermore, the method extends to the derivation of a generic formula for the estimation of the rate constant ratios based on this graphical approach. This approach for the estimation of rate constant ratios based on mathematical expressions and graphical estimations was validated against experimental data found in the literature.     

Temperature dependences for the reactions of O- and O2- with O2(a1Deltag) from 200 to 700 K

Rate constants and product ion distributions for the O- and O2- reactions with O2(a 1Deltag) were measured as a function of temperature from 200 to 700 K. The measurements were made in a selected ion flow tube (SIFT) using a newly calibrated O2(a 1Deltag) emission detection scheme with a chemical singlet oxygen generator. The rate constant for the O2- reaction is approximately 7 x 10(-10) cm3 s-1 at all temperatures, approaching the Langevin collision rate constant. Electron detachment was the only product observed with O2-. The O- reaction shows a positive temperature dependence in the rate constant from 200 to 700 K. The product branching ratios show that almost all of the products at 200 K are electron detachment, with an increasing contribution from the slightly endothermic charge-transfer channel up to 700 K, accounting for 75% of the products at that temperature. The increase in the overall rate constant can be attributed to this increase in the contribution the endothermic channel. The charge-transfer product channel rate constant follows the Arrhenius form, and the detachment product channel rate constant is essentially independent of temperature with a value of approximately 6.1 x 10(-11) cm3 s-1.     

A computational study of thiolate and selenolate oxidation by hydrogen peroxide.

Ab initio molecular orbital calculations have been used to study the effects of the molecular environment on the oxidation of thiolate and selenolate by hydrogen peroxide. The reaction was first examined in vacuo at the QCISD(T)/6-311+G(2df,2pd)//MP2/6-311+G(d,p) level of theory. It was found for both thiolate and selenolate that a reactant aggregate is formed, which has a dissociation rate constant comparable to the activation rate constant (about 10(-3) s(-1) for thiolate and 10(-1) s(-1) for selenolate). Using the polarizable continuum model (PCM) it was then found that the dissociation barrier energy decreases dramatically in water giving a dissociation rate constant of the order of 10(9) s(-1). In this case, the predicted overall rate constant of the thiolate reaction was about 10.2 mol(-1) dm3 s(-1), which is in good agreement with the experimental rate constant of cysteine oxidation in aqueous solution. The calculated rate constant for the selenolate reaction was somewhat higher (about 35.4 mol(-1) dm3 s(-1)). However, this value is several orders of magnitude smaller than the experimental value reported for the oxidation of selenocysteine in glutathione peroxidase. By considering the effect of the PCM dielectric constant on the reaction rate constant it was concluded that the high reactivity of the selenocysteine in glutathione peroxidase, as compared with cysteine, could be mainly due to the molecular environment of the selenocysteine residue.     

Controlled-Rate Thermal Desorptton

A new way of conducting thermal desorption experiments is described. The method consists in maintaining the desorption rate constant by a rigorous control of temperature. The equations for two models of constant rate desorption are described: from a surface with only one type of adsorption site, and with two different types. The technique is applied to two real systems and the kinetic parameters are determined. The results are compared with those obtained by using TPD. The main advantages are discussed and the methodology and modifications required for a TPD set-up to work at a constant desorption rate are described.     

Rate constant measurements for the overall reaction of OH + 1-butanol → products from 900 to 1200 K

The rate constant for the overall reaction OH + 1-butanol → products was determined in the temperature range 900 to 1200 K from measurements of OH concentration time histories in reflected shock wave experiments of tert-butyl hydroperoxide (TBHP) as a fast source of OH radicals with 1-butanol in excess. Narrow-linewidth laser absorption was employed for the quantitative OH concentration measurement. A detailed kinetic mechanism was constructed that includes updated rate constants for 1-butanol and TBHP kinetics that influence the near-first-order OH concentration decay under the present experimental conditions, and this mechanism was used to facilitate the rate constant determination. The current work improves upon previous experimental studies of the title rate constant by utilizing a rigorously generated kinetic model to describe secondary reactions. Additionally, the current work extends the temperature range of experimental data in the literature for the title reaction under combustion-relevant conditions, presenting the first measurements from 900 to 1000 K. Over the entire temperature range studied, the overall rate constant can be expressed in Arrhenius form as 3.24 × 10(-10) exp(-2505/T [K]) cm(3) molecule(-1) s(-1). The influence of secondary reactions on the overall OH decay rate is discussed, and a detailed uncertainty analysis is performed yielding an overall uncertainty in the measured rate constant of ±20% at 1197 K and ±23% at 925 K. The results are compared with previous experimental and theoretical studies on the rate constant for the title reaction and reasonable agreement is found when the earlier experimental data were reinterpreted.