Solution reactions, potential energy surfaces . reaction path

It is the purpose of this review to discuss and illustrate the methods presently employed to obtain potential energy surfaces by approximate, but non-empirical solutions to Schrodinger s electronic equation. In addition to discussing the different levels of approximation employed in these ab initio calculations, we emphasize the type of chemical system (in terms of its electronic structure) to which each level of calculation may be expected to yield usable results, i.e. results with acceptable errors or with predictable bounds on the error. Our interest will be primarily in surfaces which have been determined for the prediction and understanding of chemical reactions. This will include a survey of those calculations which have concentrated on determining the reaction path, and the geometry and properties of the system at points on this path, as well as those in which an essentially complete surface has been determined. The latter type of calculation coupled with either classical or quantal treatments of the nuclear motion on such a surface provides a total theoretical prediction of a chemical reaction. This ultimate objective has been achieved in the case of the H + Ha exchange reaction. 

The methods of geometrical analysis described are only a very rough solution because the intersection of the potential energy surfaces is not identical with the energy profile of the reaction path. Considerations of resonance splitting of potential energy surfaces in the transition complex range lead to further refinement [25-27],... 

A similar relationship is also derived by the absolute reaction rate theory, which is used almost exclusively in considering, and understanding, the kinetics of reactions in solution. The activated complex in the transition state is reached by reactants in the initial state as the highest point of the most favorable reaction path on the potential energy surface. The activated complex Xms in equilibrium with the reactants A and B, and the rate of the reaction V is the product of the equilibrium concentration of X and the specific rate at which it decomposes. The latter can be shown to be equal to kT/h, where k is Boltzmannn s constant and h is Planck s constant ... 

For electrochemists whose principal objective is analysis of constituents in solution, the path is straighter and the hill less steep. The reaction model often used is a redox reaction in which the interfacial reaction is simply electron transfer, and surface chemical reactions among radicals can be neglected. The electrode is regarded as stable during the reaction and is not intended to take any chemical part in it. The function of the surface is not electrocatalytic, it is simply to be a source and sink of electrons, the energy of which may be controlled by variation of the electrode potential. 

A reaction-path based method is described to obtain information from ab initio quantum chemistry calculations about the dynamics of energy disposal in exothermic unimolecular reactions important in the initiation of detonation in energetic materials. Such detailed information at the microscopic level may be used directly or as input for molecular dynamics simulations to gain insight relevant for the macroscopic processes. The semiclassical method, whieh uses potential energy surface information in the broad vicinity of the steepest descent reaction path, treats a reaction coordinate classically and the vibrational motions perpendicular to the reaction path quantum mechanically. Solution of the time-dependent Schroedinger equation leads to detailed predictions about the energy disposal in exothermic chemical reactions. The method is described and applied to the unimolecular decomposition of methylene nitramine. 

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