从溶剂笼到分子笼

在物理化学中,对溶液中的反应通常用笼效应来描述溶剂与反应物质间的关系。一般认为,溶质(反应物质)是被溶剂分子所构成的笼包围;溶剂笼是柔性的,其大小、形状取决于溶质分子;反应物分子会在出入溶剂笼的过程中遭遇、碰撞、发生反应。随着合成技术的发展,人们合成出了以共价键、配位键连接的分子笼。相对于由范德华力所构筑的溶剂笼,分子笼表现出更高的刚性,因此其对化学反应可产生更为显著的影响。有别于传统催化剂的化学方式(与反应物、中间体等成键),当化学反应在分子笼中进行时,分子笼可通过富集反应物、改变反应物构型、影响中间产物的结构以及限制产物尺寸等一系列物理方式对反应的速率、选择性造成影响。该领域的研究对于深入揭示化学反应机理、调控所进行的反应均具有重要意义。     

粒度对多相反应动力学参数的影响

以纳米氧化锌与硫酸氢钠溶液为反应体系, 研究反应物粒度对动力学参数的影响规律. 讨论了表观活化能降低的原因. 结果表明：当反应物粒径、反应温度和搅拌速率一定时, 纳米氧化锌与硫酸氢钠溶液的反应速率仅与反应物的浓度有关；反应物粒度对多相反应的反应级数、速率常数、表观活化能和指前因子均有较大的影响；随着反应物粒径的减小, 表观活化能和指前因子减小, 而反应级数和速率常数增大, 并且速率常数和表观活化能与反应物粒径的倒数呈线性关系；反应物粒度是通过摩尔表面积、摩尔表面能和摩尔表面熵三个方面影响多相反应的动力学参数的.     

预测溶剂影响S_N1与S_N2反应速率的Hughes-Ingold规则

理论上，在溶液中进行的均相化学反应的反应速率由活化能及溶剂化作用所决定，但实际上活化能很难测定。ＨｕｇｈｅｓＩｎｇｏｌｄ规则可预测不同荷电类型的反应物按ＳＮ１或ＳＮ２机理反应时溶剂极性对其反应速率的影响。分析了电荷数与溶剂化及活化能的关系，列出了该规则所适用的反应类别，并有实例说明。     

溶液反应机理与动力学-1：经典处理方法及其问题分析

通过具体的实验数据,对溶液反应与气相反应动力学特征的差异进行了对比。对采用"溶剂笼"、"遭遇对"模型并引入扩散过程对溶液反应进行处理的经典过程进行了分析,指出该处理过程存在理论假设和近似处理不合理、结果综合性差、无法全面反映溶剂影响等问题。     

表面张力对PEO稀溶液粘度行为的影响

测定了不同分子量聚氧化乙烯(PEO)在水和苯溶剂中的粘度,发现在低浓度区PEO水溶液的比浓粘度出现负偏离, PEO苯溶液比浓粘度与浓度之间依旧满足线性关系.表面张力测定结果表明, PEO分子显著降低了水的表面张力,而苯的表面张力则不受影响.PEO水溶液和纯溶剂水表面张力的不同干扰了高分子溶液和溶剂在粘度计中流过时间的测量,导致低浓度区PEO水溶液比浓粘度出现负偏离.利用PEO水溶液和水表面张力测定结果,结合乌式粘度计的几何尺寸,定量分析了PEO水溶液和纯溶剂水表面张力的差异对粘度测量结果的影响,计算结果与实验结果基本相符.如果用PEO水溶液流过时间对浓度作图的外推值t0*计算相对粘度,可以完全消除PEO水溶液和水表面张力差异对粘度测量的影响.     

纳米氧化铜的粒度对多相反应动力学参数的影响

本文提出了纳米体系多相反应动力学活化能的模型，并以纳米氧化铜与硫酸氢钠溶液的反应为体系，研究了反应物粒度对动力学参数的影响规律，讨论了表观活化能降低的原因。结果表明：反应物粒度对多相反应的反应级数、速率常数、表观活化能和指前因子均有较大的影响；随着反应物粒径的减小，表观活化能和指前因子减小,而反应级数和速率常数增大，并且速率常数和表观活化能与反应物粒径的倒数呈线性关系；这些影响规律与理论模型是一致的。另外，还发现反应物粒度是通过摩尔表面积、摩尔表面能和摩尔表面熵影响多相反应的动力学参数的。     

甲烷与HO2反应的微观动力学研究

应用从头算方法和变分过渡态理论,在B3LYP/6-311+G^**方法下和300～2000K温度范围内研究甲烷与HO₂反应的微观动力学特性,得到由过渡态向反应物方向、向产物方向的能垒分别是11.83和102.90kJ/mol,理论计算正向反应速率常数与实验值之比为1.08～2.85,用此方法还可以预测没有实验数据的温度点反应的速率常数.     

浓度对草酸与酸性高锰酸钾溶液褪色反应速率的影响研究

通过比较同体积、不同浓度的草酸溶液与同体积、同浓度的酸性高锰酸钾溶液反应时溶液褪色时间的不同,使学生体会反应物浓度对反应速率的影响。为适应课堂教学,研究了各种反应物浓度对该反应速率的影响,为一线教师配置适合课堂教学的各种反应物的浓度提供了较为准确的数据。     

甲烷与H02反应的微观动力学研究

应用从头算方法和变分过渡态理论，在B3LYP／6—3ll G^**方法下和300～2000K温度范围内研究甲烷与HO2反应的微观动力学特性，得到由过渡态向反应物方向、向产物方向的能垒分别是11．83和l02．90kJ／mol。理论计算正向反应速率常数与实验值之比为1．08～2．85，用此方法还可以预测没有实验数据的温度点反应的速率常数。     

四氟乙烯齐聚体的反应VII:多氟烷基硫醚的分子内重排及其动力学的研究

本文通过交叉反应、溶剂效应、反应物浓度、截捕实验、动力学参数研究了多氟烷基硫醚的分子内热重排过程.确认并根据Arrhenius方程计算出在四氯化碳溶液中反应的活化能.多氟烷基硫醚分子内重排系经分子内亲核协同过程进行.     

A microscopic theory for solution chemical reactions: Introduction of reactant and medium structures into generalized langevin equation formalism

A microscopic formulation of solution chemical reactions, taking reactants and medium structures into consideration, is presented on the basis of microscopic understandings obtained by recent quantum chemical methods (i.e., ab initio molecular orbital theory, etc.). Assuming thermal equilibrium of the medium bath, an effective internal Hamiltonian is derived, and, further, its derivative with respect to internal normal coordinates is proved explicitly to give the same force field as is provided by the free-energy surface or potential of mean force. The free-energy surface can be expressed in the composite normal coordinate system (CNCS ) consisting of some normal coordinate systems of isolated reactants and surrounding solvent molecules (i.e., medium solvent molecules). In CNCS , in use of diagonal elements obtained in the Hessian matrix of the free-energy surface, effective normal-mode frequencies, which reflect the equilibrium solvent effect, are estimated. Furthermore, on the generalized Langevin equation (GLE ) treatment, a closed expression of the time-dependent frictional coefficient is derived on a microscopic basis, reflecting the reactant and solvent structures. The nonequilibrium effect is estimated by an analytical expression similar to that in the Grote–Hynes theory. The rate constant is evaluated for a typical model system and it is shown that the equilibrium rate constants should be reduced by a factor 0.997. Finally, it is concluded that the present microscopic theory is reasonably applicable to the estimation of chemical reaction rate constants in solution. © 1994 John Wiley & Sons, Inc.     

Chemical reactions between immiscible polymers in the melt: Transesterification of poly(ethylene-co-methyl acrylate) with mono-hydroxylated polystyrenes

This article presents a unique example dealing with how chemical reactions between immiscible polymers in the melt behave differently than they would do in solution. Specifically, a model reaction was chosen: the transesterification between poly(ethylene-co-methyl acrylate) (EMA) and polystyrene mono-hydroxylated at the chain end (PSOH). It was carried out in the melt in a batch mixer. The overall rate of this reaction has a similar dependence of temperature, composition of reactants, and the nature and concentration of catalyst as in solution. The reactivity of PSOH decreases drastically with increasing molecular weight, and it becomes very weak when the molecular weight exceeds 8000 g/mol. As opposed to a reaction in solution or in a homogeneous melt, mechanical mixing increases the reaction rate since it generates interfacial area and reduces the diffusion length. The EMA-g-PS graft copolymer formed at the interfaces reduces the interfacial tension, and increases the miscibility of the reaction mixture. However, its occupation of the interfaces reduces contact between the reactive moieties, thus decreasing the overall reactivity. More importantly and much to our surprise, adding 1 to 2 wt % of an inert solvent increased greatly the overall reaction rate. While an increased interfacial mixing and diffusion by the presence of minor amounts of solvent are thought to be the major factors contributing to the drastic increase in reactivity, numerous questions still remain. Nevertheless, this study clearly showed that as opposed to a reaction in solution, mechanical mixing and the presence of minor amounts of solvent are two additional and critical means to control chemical reactions between immiscible polymer melts. © 1995 John Wiley & Sons, Inc.     

Chemoselective Synthesis of Stable Phosphorus Ylides from 6-Azauracil and Mechanistic Investigation of the Reaction by UV Spectrophotometry

Stable crystalline phosphorus ylides were obtained in excellent yields from the 1:1:1 addition reaction between triphenylphosphine and dialkyl acetylenedicarboxylates, in the presence of NH-acids such as 6-azauracil. These stable ylides exist in solution as a mixture of two geometrical isomers as a result of restricted rotation around the carbon–carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. To determine the kinetic parameters of the reactions, they were monitored by UV spectrophotometry. The second order fits were automatically drawn, and the values of the second order rate constants (k₂) were automatically calculated using standard equations. At the temperature range studied, the dependence of the second order rate constant (Ln k₂) on reciprocal temperature was in agreement with the Arrhenius equation. This provided the relevant plots to calculate the activation energy of all the reactions. Furthermore, useful information was obtained from studies of the effect of solvent, structure of reactants (different alkyl groups within the dialkyl acetylenedicarboxylates), and also concentration of reactants on the rate of reactions. The proposed mechanism was confirmed according to the obtained results, and a steady-state approximation and first step (k₂) of the reaction was recognized as a rate-determining step on the basis of experimental data.     

Kinetics and Mechanism of the Reactions Between Triphenylphosphine,Dialkyl Acetylenedicarboxilates and a NH-Acid,Pyrazole, by UV Spectrophotometry

Kinetic studies were made of the reactions between triphenylphosphine and dialkyl acetylenedicarboxylates in the presence of a NH-acid such as pyrazole. To determine the kinetic parameters of the reactions, the reaction progress was monitored by UV spectrophotometry. The second-order fits were automatically drawn and the values of the second-order rate constant (k ₂) were automatically calculated using standard equations. In the temperature range studied, the dependence of ln k ₂ on the reciprocal temperature was consistent with the Arrhenius equation. Furthermore, useful information was obtained from studies of the effect of solvent, structure of the reactants (different alkyl groups within the dialkyl acetylenedicarboxylates), and also the concentration of reactants on the rate of reaction. The mechanism was confirmed to involve a steady-state condition with the first step of the reaction being the rate-determining step.     

The mechanism of activation and nonequilibrium distribution functions in unimolecular reactions

Nonequilibrium distribution functions for polyatomic molecules undergoing unimolecular reaction are considered. Two limiting cases of activation by collision are investigated: the mechanism of strong impacts, in which each collision with an activated molecule may be regarded as bringing about deactivation; and the diffusion mechanism in which the transition between states is described by the Fokker-Planck equation. An accurate solution of the diffusion equation for the thermal decomposition of a nonlinear triatomic molecule makes it possible to elucidate the kinetics of various reactions in relation to the activation mechansim.     

Rate kernel theory for pseudo-first-order kinetics of diffusion-influenced reactions and application to fluorescence quenching kinetics

Theoretical foundation of rate kernel equation approaches for diffusion-influenced chemical reactions is presented and applied to explain the kinetics of fluorescence quenching reactions. A many-body master equation is constructed by introducing stochastic terms, which characterize the rates of chemical reactions, into the many-body Smoluchowski equation. A Langevin-type of memory equation for the density fields of reactants evolving under the influence of time-independent perturbation is derived. This equation should be useful in predicting the time evolution of reactant concentrations approaching the steady state attained by the perturbation as well as the steady-state concentrations. The dynamics of fluctuation occurring in equilibrium state can be predicted by the memory equation by turning the perturbation off and consequently may be useful in obtaining the linear response to a time-dependent perturbation. It is found that unimolecular decay processes including the time-independent perturbation can be incorporated into bimolecular reaction kinetics as a Laplace transform variable. As a result, a theory for bimolecular reactions along with the unimolecular process turned off is sufficient to predict overall reaction kinetics including the effects of unimolecular reactions and perturbation. As the present formulation is applied to steady-state kinetics of fluorescence quenching reactions, the exact relation between fluorophore concentrations and the intensity of excitation light is derived.     

Collision theory treatment of monomolecular and bimolecular gas reactions

An exact collision theory of unimolecular and bimolecular gas phase reactions is derived from a general quantum-mechanical formulation of reactions rates based on the assumption that the reactants are in thermal equilibrium. In this way the quantum corrections to the classical collision theory expressions are rigorously defined. Approximate formulas for these corrections make it possible to determine well the temperature ranges within which the classical and the semiclassical approximations are valid. A comparison is made between the collision and the transition state theory with emphasis on some conceptual difficulties of the latter in treating the simple decomposition and recombination reactions. It is shown that in the classical (high temperature) limit these theories are incompatible except when the reaction coordinate is entirely separable (i.e., when the transition state theory is no longer useful).     

A kinetic model for steric hindrance effects on quaternization of poly(vinylpyridines)

Quaternization reactions of poly(vinylpyridines) with alkyl halides show retardation in excess of that predicted by the classical second-order kinetics. Based on the classical collision and transition state theories, a kinetic model has been developed to quantify such retardation, in which the overall reaction rate is characterized by a rate constant k₀ of the intrinsic reactivity between a pyridyl group and an alkyl halide group, and by a steric hindrance effect parameter α. The latter accounts for the degree to which the rate of collisions of reactants is reduced, or to which the freedom of movement of the reactants in the transition state is restricted as the reaction proceeds. The resulting kinetic expression has been validated using experimental results reported in the literature and those of our own. The functional dependence of k₀ and α values on the nature of poly(vinylpyridines) and that of alkyl halides is explained. Other factors affecting k₀ and α, including changes in macromolecular dimensions and/or in the distribution of residue environment, quality of solvent, and reaction temperature, are discussed. © 1993 John Wiley & Sons, Inc.     

The kinetics of nucleophilic substitution processes in the alkyl halides. Part A—theory

Activation energies for substitution reactions of the type AC + B → A + CB, occurring in polar media and characterized by an abrupt change of the term along two coordinates have been calculated within the framework of the quantum-mechanical theory of chemical reactions. In the case of nonadiabatic processes, the transmission coefficient and activation energy for these reactions are expressed in terms of characteristic parameters of the medium (reorganization energy, effective frequency of solvent fluctuation polarization) and the potential energy curves for intermolecular interactions between the reactants (AC and B) and between the products (A and BC).