Chemical reaction dynamics in solution

J. T. Hynes, Chemical reaction dynamics in solution, Annu. Rev. Phys. Chem. 36, 573 (1985). 

Transition State Theory and Chemical Reaction Dynamics in Solution... 

Simulation of chemical reaction dynamics in solution has a long history as has been demonstrated by the work of Rabinowitch and Wood who... 

The hybrid methods which combine quantum-mechanical (QM) and classical descriptions are surely one of the mostly well-suited strategies in this context. Two main families of hybrid methods can be defined according to the model used to describe the classical part of the system. Either continuum or atomistic formulations can be introduced where, in the first case, the classical subsystem is described as a dielectric medium while, in the second case, a Molecular Mechanics (MM) formulation is generally adopted. While QM/continuum methods have been largely and successfully applied to molecular solutes in liquid solutions [2-5], QM/MM formulations have been more often used in the field of structured (biological) environments [6-10] even if the study of chemical reaction dynamics in solution represents another important field of applications of the method [11, 12]. 

Hynes, J.T., 1985, Chemical Reaction Dynamics in Solution, Aim. Rev. Phys. Chem., 36,573. 

Kuznetsov, A. M., Stochastic and Dynamic Views of Chemical Reaction Kinetics in Solutions, Presses Polytechniques et Universitaires Romandes, Lausanne, Switzerland, 1999. Kuznetsov, A. M., and J. Ulstrup, Electron Transfer in Chemistry and Biology, Wiley, Chichester, West Sussex, England, 1999. 

The understanding of chemical reaction mechanisms in solution is often based on the nature of the interactions between reactants and solvent, which are governed by the physical properties of molecules, such as polarity, or by the possibility of bonds formation (e.g., hydrogen-bonding) and their dynamical evolution. The goal of the majority of works on molecular clusters is to try to fill the gap between the gas phase reaction and the condensed phase reaction by a step-by-step solvation of the reactive system. This approach will give useful... 

We have mentioned salts in solution (i.e., dissolved in water), reversible reactions and equilibrium in solution, and ions in solution. Most chemical reactions occur in solution. It was apparent to Van t Hoff at an early stage that to understand the dynamics and thermodynamics of chemical reactions, he needed to understand the nature of solutions in general. And it was equally clear when he began his work that very little was known about the nature of solutions. Solutions always involve specific chemicals, the solvent (often water) and the dissolved substance or solute (often a salt, e.g., sodium chloride). But although they are always chemical systems, they can also be considered as physical systems, to which the principles of thermodynamics can be applied. 

Refs. [i] Landau LD, Lifshitz EM (1970) Statistical physics, 2nd edn. Pergamon, Oxford [ii] Kuznetsov AM (1997) Stochastic and dynamic views of chemical reaction kinetics in solutions. Presses polytechniques et universitaire romandes, Lausanne [iii] Kornyshev AA, Leikin S, Sut-mann G (1997) Electrochim Acta 42 849... 

Rotational diffusion of molecules in liquids should be quite important to establish the microscopic description of chemical reaction dynamics in the solution phase. Sometimes, geminate process was suggested to be governed by rotational diffusion rather than a translational diffusion. It may play an important role in the radical dimer formation process on the geminate encounter of p-aminophenylthiyl radical pair. ... 

The straightforward approach to this kind of difficulty is to follow the time evolution of the system, for instance by molecular dynamics simulation, and wait until a sufficient number of events have been observed. However, the computational requirements of such a procedure are excessive for most interesting systems. In practice, it is frequently impossible to observe a single transition of interest, let alone collect enough statistics for a microscopic resolution of the mechanism. For instance, reaction times of chemical reactions occurring in solution often exceed the second time scale. Since the simulation of molecular systems typically proceeds in steps of roughly one femtosecond, of the order of 10 steps are required to observe just one transition. Such calculations are far beyond the reach of the fastest computers even in the foreseeable future. 

Kuznetsov, A.M., Stochastic and Dynamic Views of Chemical reaction Kinetics in Solutions, Press Polytechniques et Universitaires Romandes Lausanne, 1999. 

In this chapter, we will recall some basic aspects of chemical reaction kinetics in solution, starting from an oversimplified point of view and gradually bringing in more complications. We will not discuss theory aimed at explaining reaction rates on a molecular level (molecular reaction dynamics). Other rate processes will be discussed in Chapters 5 and 13. 

S. A. Adelman and J. D. Doll, Brownian motion and chemical dynamics on solid surfaces, Acc. Chem. Res. 10 378 (1977) S. A. Adelman, Chemical reaction dynamics in liquid solution, Adv. Chem. Phys. 53 61 (1983). 

S. A. Adelman, Adv. Chem. Phys., 53, 61 (1983). Chemical Reaction Dynamics in Liquid Solution. 

Since acid-base reactions are among the most conunon and elementary chemical reactions, the molecular mechanism of proton-transfer to solvent is of great importance for the understanding of reaction dynamics in solution. 

Hynes J T 1985 The theory of reactions in solution Theory of Chemical Reaction Dynamics ed M Baer (Boca Raton, FL CRC Press) pp 171-234... 

Chemical reaction dynamics is an attempt to understand chemical reactions at tire level of individual quantum states. Much work has been done on isolated molecules in molecular beams, but it is unlikely tliat tliis infonnation can be used to understand condensed phase chemistry at tire same level [8]. In a batli, tire reacting solute s potential energy surface is altered by botli dynamic and static effects. The static effect is characterized by a potential of mean force. The dynamical effects are characterized by tire force-correlation fimction or tire frequency-dependent friction [8]. 

Finally, there is a large body of experimental and theoretical contributions from investigators who are mainly interested in the dynamic and conformational properties of chain molecules. The basic idea is that the cyclisation probability of a chain is related to the mean separation of the chain ends (Morawetz, 1975). Up to date comprehensive review articles are available on the subject (Semiyen, 1976 Winnik, 1977, 1981a Imanishi, 1979). Rates and equilibria of the chemical reactions occurring between functional groups attached to the ends or to the interior of a flexible chain molecule are believed to provide a convenient testing ground for theories of chain conformations and chain dynamics in solution. 

The simplest generalization of free-energy-of-solvation concepts to dynamics in solution is provided by transition state theory. In conventional transition state theory, the rate constant of a chemical reaction at temperature T is given by... 

Most reactions of interest to chemists take place in either solution or at the gas-solid interface. At the atomic level, much less is known about the reaction dynamics in such systems than about the dynamics of gas-phase reactions. In the gas phase one may follow the detailed evolution from reactants to products without disturbing collisions with other molecules, at least in the low pressure limit. Contrary to that, in solution, where reactants and products are continually perturbed by collisions with solvent molecules, it is much more complicated to follow a chemical reaction. 

Computational fluid dynamics involves the analysis of fluid flow and related phenomena such as heat and/or mass transfer, mixing, and chemical reaction using numerical solution methods. Usually the domain of interest is divided into a large number of control volumes (or computational cells or elements) which have a relatively small size in comparison with the macroscopic volume of the domain of interest. For each control volume a discrete representation of the relevant conservation equations is made after which an iterative solution procedure is invoked to obtain the solution of the nonlinear equations. Due to the advent of high-speed digital computers and the availability of powerful numerical algorithms the CFD approach has become feasible. CFD can be seen as a hybrid branch of mechanics and mathematics. CFD is based on the conservation laws for mass, momentum, and (thermal) energy, which can be expressed as follows ... 

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