Thermodynamic restrictions on mechanosynthesis of strontium titanate

Chemical potential phase stability diagrams were calculated from relevant thermodynamic properties and used to predict the thermodynamic driving force under prospective conditions of room temperature mechanosynthesis. One analysed the dependence of chemical potential diagrams on temperature and partial pressure of evolving gases such as oxygen or carbon dioxide, as expected on using strontium peroxide or strontium carbonate as precursor reactants for the alkali earth component. Thermodynamic calculations were also obtained for changes in titania precursor reactants, including thermodynamic predictions for reactivity of strontium carbonate with amorphous titania. Experimental evidence showed that strontium titanate can be obtained by mechanosynthesis of strontium carbonate+anatase mixtures, due to previous amorphization under high energy milling. Ability to perform mechanosynthesis with less energetic milling depends on the suitable choice of alternative precursor reactants, which meet the thermodynamic requirements without previous amorphization; this was demonstrated by mechanosynthesis from anatase+strontium peroxide mixtures.     

Evidence for concerted pathways in ion-pairing coupled electron transfers

Ion-pairing with electro-inactive metal ions may change drastically the thermodynamic and kinetic reactivity of electron transfer in chemical and biochemical processes. Besides the classical stepwise pathways (electron-transfer first, followed by ion-pairing or vice versa), ion-pairing may also occur concertedly with electron transfer. The latter pathway avoids high-energy intermediates but a key issue is that of the kinetic price to pay to benefit from this thermodynamic advantage. A model is proposed leading to activation/driving force relationships characterizing such concerted associative electron transfers for intermolecular and intramolecular homogeneous reactions and for electrochemical reactions. Contrary to previous assertions, the driving force of the reaction (defined as the opposite of the reaction standard free energy), as well as the intrinsic barrier, does not depend on the concentration of the ion-pairing agent, which simply plays the role of one of the reactants. Besides solvent and intramolecular reorganization, the energy of the bond being formed is the main component of the intrinsic barrier. Application of these considerations to reactions reported in recent literature illustrates how concerted ion-pairing electron-transfer reactions can be diagnosed and how competition between stepwise and concerted pathways can be analyzed. It provided the first experimental evidence of the viability of concerted ion-pairing electron-transfer reactions.     

The reaction force. A scalar property to characterize reaction mechanisms

The reaction force is a global property of a chemical reaction that arises when applying the Hellmann–Feynmann theorem to the potential energy surface that links reactants, products and transition states. In the present work, the reaction force is defined rigorously from the cartesian components coming out from all forces exerted over each atom of a molecular system during the chemical reaction; it is demonstrated that the reaction force is a scalar property. An erratum to this article can be found at     

Kinetics of catalyzed esterification of acetic acid with n-butanol using carbonized agro-waste

The objective of this research work was to investigate the kinetics of esterification of acetic acid with n-butanol through the variation of experimental parameters. The reaction mixture was catalyzed heterogeneously by a sulfonated catalyst in batch mode of operation. The catalyst was prepared from abundantly available agro-waste, Cajanus cajan husk by chemical activation process, which produces a carbon-based solid catalyst with high surface area. The catalyst was characterized by a Brunauer-Emmet-Teller surface analyzer and Fourier transform infrared spectroscopy to know the surface morphology. Process parameters such as contact time, reaction temperature, and catalyst loading, which can influence the extent of conversion of reactants, were studied. Furthermore, the kinetic investigation was also carried out to estimate the kinetic parameters for uncatalyzed and catalyzed reaction using the second-order pseudo-homogeneous (P-H), Eley-Rideal (E-R), and Langmuir-Hinshelwood (LH) kinetic models for this research work. The kinetic parameters such as activation energy, preexponential factor, and the thermodynamic parameters such as enthalpy and entropy were estimated for uncatalyzed and catalyzed reactions using these three models. The process conditions were optimized for catalyzed and uncatalyzed reactions to obtain the maximum product yield by minimizing root mean square error of each experimental data using the MS-excel solver tool. Thus, this study reveals the high potential of an agro-waste, Cajanus cajan husk as raw material for the synthesis of catalyst. The results show that the E-R model is more appropriate for predicting the dynamic data of an esterification reaction, as the forward rate of reaction estimated using the E-R model are more modified than P-H and L-H models.     

Concerted proton-electron transfer reactions in water. are the driving force and rate constant depending on pH when water acts as proton donor or acceptor?

The competition between stepwise and concerted (CPET) pathways in proton-coupled electron-transfer reactions in water is discussed on thermodynamic and kinetic bases. In the case where water is the proton acceptor, the CPET pathway may compete favorably with the stepwise pathway. The main parameter of the competition is pK of the oxidized form of the substrate being smaller or larger than 0. The driving force of the forward reaction is however independent of pH, despite the equilibrium redox potential of the proton-electron system being a function of pH. At high pH values, CPET reactions involving OH- as proton acceptor may likewise compete favorably with stepwise pathways. The overall reaction rate constant is an increasing function of pH, not because the driving force depends on pH but because OH- is a reactant. In buffered media, association of the substrate with the basic components of the buffer offers an alternative CPET route; the driving force comes closer to that offered by the pH-dependent equilibrium redox potential.     

Time evolution of the activation energy in a batch chemical oscillator

The batch-operated bromate/phosphate/acetone/dual catalyst system was studied at four temperatures between 5 and 35 degrees C. The dynamics was simultaneously followed by potential measurements with platinum and bromide selective electrodes, and spectroscopically at two different wavelengths. By simultaneously recording these four time series it was possible to characterize the dynamics of the sequential oscillations that evolve in time. The existence of three sequential oscillatory patterns at each temperature allowed estimating the activation energies in each case. Along with the activation energy of the induction period, it was possible to trace the time evolution of the overall activation energy at four different stages as the reaction proceeds. The study was carried out for two different sets of initial concentrations and it was observed that the overall activation energy increases as reactants turn into products. This finding was propounded as a result of the decrease in the driving force, or the system's affinity, of the catalytic oxidative bromination of acetone with acidic bromate, as the closed system evolves toward the thermodynamic equilibrium.     

NCS自由基与NO反应动力学的理论研究

用量子化学密度泛函理论B3LYP/6-31+G^*和高级电子相关校正的偶合簇[CCSD(T)/6-311+G^*]方法,对NCS自由基与NO反应的机理和动力学进行了理论研究,得到了体系的势能面信息和可能的反应机理.计算了反应的热力学参数及反应能垒.采用传统过渡态理论计算了各反应通道的速率常数.研究结果表明,NCS自由基与NO反应中存在4个反应通道,产物分别为OCS+N₂,CS+N₂O,ONS+CN和ONCNS.从能量变化和反应速率两方面考虑,NCS+NOOCS+N₂应为主反应通道.     

Phenomenological Kineticsan; an alternative approach

Traditional phenomenological kinetics describes the process rate in terms of the progress degree (mass fraction which has already undergone the change), a, and with a monomial function (or combination of monomial functions for multistep processes) of (1-a), without any connection to the underlying mechanism at the molecular level. The approach proposed in the present work aims at the direct treatment of the experimental data, like DSC records, without suggesting any specific reaction mechanism and excluding any Arrhenius like behaviour. Formal expressions are proposed that include the thermodynamic constraints for any spontaneous process, viz. a negative drop of the Gibbs function throughout the process, and describe the process rate as the result of the effects of a thermodynamic driving force, identified with the drop of the Gibbs function, and of the medium hindrance. This revised version was published online in July 2006 with corrections to the Cover Date.     

The reaction force: Three key points along an intrinsic reaction coordinate

The concept of the reaction force is presented and discussed in detail. For typical processes with energy barriers, it has a universal form which defines three key points along an intrinsic reaction coordinate: the force minimum, zero and maximum. We suggest that the resulting four zones be interpreted as involving preparation of reactants in the first, transition to products in the second and third, and relaxation in the fourth. This general picture is supported by the distinctive patterns of the variations in relevant electronic properties. Two important points that are brought out by the reaction force are that (a) the traditional activation energy comprises two separate contributions, and (b) the transition state corresponds to a balance between the driving and the retarding forces.     

Interacting Quantum Atoms Analysis of the Reaction Force: A Tool to Analyze Driving and Retarding Forces in Chemical Reactions

The analysis of the reaction force and its topology has provided a wide range of fruitful concepts in the theory of chemical reactivity over the years, allowing to identify chemically relevant regions along a reaction profile. The reaction force (RF), a projection of the Hellmann-Feynman forces acting on the nuclei of a molecular system onto a suitable reaction coordinate, is partitioned using the interacting quantum atoms approach (IQA). The exact IQA molecular energy decomposition is now shown to open a unique window to identify and quantify the chemical entities that drive or retard a chemical reaction. The RF/IQA coupling offers an extraordinarily detailed view of the type and number of elementary processes that take reactants into products, as tested on two sets of simple reactions.     

Organic synthesis using chemical tags: the third Leg of parallel synthesis

Increasing emphasis has recently been placed on the development of synthetic methods which effectively couple chemical synthesis and purification. For example, new formats for parallel synthesis are being developed which involve attachment of chemical tags to both reagents, reactants, and substrates to permit their chemoselective removal from reaction mixtures. The driving force for the development of tagged organic reagents is the ability to use standard solution-phase chemistry methods and reaction monitoring techniques (e.g. TLC and HPLC). In this mini-review, we will outline recent developments on the growing class of chemically tagged reagents, reactants, and substrates and highlight examples of their use in multistep synthesis.     

非线性化学反应动力学*

本文从实验和理论的角度综述非线性化学反应动力学。介绍了若干非线性化学反应动力学现象如时钟反应、反馈、混合振荡、化学混沌及反应扩散图案等; 对时空序列、功率谱、Lyapunov 指数、吸引子及其维数、Poincare 截面及分叉理论等常用于分析非线性动力学特征的手段作了介绍; 探讨了对非线性化学反应的实验装置、测试方法、机理和模型研究的方法; 简要回顾了非线性化学反应动力学的发展历史, 并展望了其发展趋势。     

Thermodynamic functions for state-selected chemical reactions

Thermodynamic functions for chemical reactions of state-selected reactants to state resolved products are introduced and discussed. The equilibrium constant, change in free energy and other thermodynamic functions can all be computed using standard thermochemical tables without any kinetic input. It is also shown that the usual type of thermodynamic cycles can be employed for reactions even when the reactants and/or products are state-selected.     

The metathetic degradation of polyisoprene and polybutadiene in block copolymers using Grubbs second generation catalyst

A degradation study of polystyrene-polybutadiene-polystyrene and polyisoprene-polystyrene-polyisoprene in both dichloromethane and hexane solvents is presented. Alternative solvents for metathetic degradation provide the potential for greener chemistry, better selectivity, and control over the products. The catalyst concentration and solvent selection both determine the products formed. The degradation of polyisoprene and polybutadiene in a particular solvent was controlled by the solubility of polyisoprene/polybutadiene, and by its solubility relative to polystyrene. A large difference in solubility between the polymers in the selected solvent provides an additional driving force for block separation, encouraging reaction close to the interface between different blocks. Furthermore, solubility of the block copolymer speeds the degradation reaction. This tailoring of the reaction mechanism yields a new control over the products of polymer degradation.     

HOSO+NO反应的机理及动力学

在B3LYP/6-311++G(d,p)水平上研究了HOSO+NO的反应机理. 优化得到了反应势能面上各驻点的几何构型, 通过内禀反应坐标(IRC)确认了反应物、中间体、过渡态和产物的相关性. 在CCSD(T)/6-311++G(d,p)水平上对计算得到的构型进行了能量校正. 应用经典过渡态理论(TST)与变分过渡态理论(CVT), 并结合小曲率隧道(SCT)效应模型校正的方法计算了标题反应在200-3000 K温度范围内的速率常数k^TST、k^CVT和k^CVT/SCT. 计算结果表明: HOSO+NO反应在单重态和三重态条件下均可发生, 其中单重态反应为主反应通道, HNO+SO₂为主产物. 并利用电子密度拓扑分析方法研究主反应通道反应过程中的化学键变化.     

Nonoscillation in closed reversible chemical systems

For a closed reversible chemical system obeying mass action kinetics, we prove that structurally stable closed orbits do not exist if there are no more reaction steps than reactants. We derive this using an appropriate Lyapunov function for a special case in which the number of reaction steps equals the number of reactants and a stationary point is shown to be unique and asymptotically stable.     

一元氯化镁化合物热分解的热力学研究

本文利用Gaussian 03程序,采用量子化学理论,在RHF/6-31G(d)水平上,对一元氯化镁化合物热分解反应机理进行了研究。在对现有4种水氯镁石脱水技术的反应物和产物几何构型进行能量梯度法全优化的同时,计算了不同温度下4种方法的主副反应路径的标准热力学参数(298.15~1000 K)。热力学计算结果表明:所有反应均为吸热反应,当压力为1.01×105 Pa、温度低于1000 K时,所有反应都不能自发进行;从热力学的角度分析,热分解更有利于以苯胺为助剂的复盐法的发生。     

密度泛函理论研究香草醛与氨基硫脲的加成-缩合反应

针对氨基硫脲和香草醛亲核加成-缩合反应的反应物和生成物,用量子化学密度泛函方法在B3LYP/6-31G水平上进行几何优化,通过Mulliken电荷分析推测出香草醛具有较高的亲核加成反应活性.对优化后的构型进行振动分析,得到不同温度下反应物和生成物的热力学性质,据此计算出相应温度下的反应焓变、吉布斯自由能变和平衡常数.实验表明,在298K～1000K内,氨基硫脲与香草醛的亲核加成-缩合反应是一个放热反应,吉布斯自由能变成负值,平衡常数很大,说明反应在温和条件下即能自发进行.在此基础上采用含时密度泛函方法(TD-DFT)计算了分子激发态的电子跃迁能,得到对应激发态的吸收波长,所得结果与文献值基本吻合.     

A comprehensive study of the solvent effects on the cycloaddition reaction of diethyl azodicarboxylate and ethyl vinyl ether: Efficient implementation of QM and TD‐DFT study

In this work, quantum chemistry calculations performed to study the kinetics and thermodynamic parameters of [2+2] cycloaddition reaction of diethyl azodicarboxylate and ethyl vinyl ether in eighty‐three solvents and gas phase. The solvent effect on the reaction path and electron density of the C2? N6 critical bond as the reaction coordinate at the TS was investigated. Calculated rate constants in various solvents showed that increase in the activation dipole moment accelerates the reaction. Based on the time‐dependent studies, using a conductor like polarizable continuum model solvation model, the solvent effects on the excitation energies of the reactants and transition states (TSs) and the corresponding chemical shifts were analyzed. Finally, some correlations between the rate constant and quantum reactivity indices such as electrophilicity index, chemical hardness, and electronic chemical potential were investigated. © 2014 Wiley Periodicals, Inc.