High-Pressure Kinetics and Highly Viscous Media

High-Pressure Kinetics and Highly Viscous Media 

The transition state theory (TST) may be considered to be established in 1941 by publication of a momunental book The Theory of Rate Processes [1. In Chapter VIII of the book, the authors discuss solution reactions and conclude. . that the ratedetermining step in solution is. .. the formation from the reactants of an activated complex which subsequently decomposes . Though the authors pointed out the importance of diffusion in bimolecular reactions, they did not consider a possible break down of their two key assumptions, that is, thermal equilibrium between the initial and the transition state and neglecting recrossing, in imimolecular rate processes. The remarkable success of TST in the interpretation of kinetic effects of pressure [2] turned the attention of high-pressure kineticists away from a possible failure of TST and efforts were concentrated on the interpretation of the activation volume obtained from pressure dependence of a rate constant fe at a constant temperature (Eq. 3.1). 

In this model, chemical transformations are considered to be in concert with thermal fluctuations of solvent molecules and, therefore, only one reaction coordinate was needed. At high solvent viscosities, the model predicts that the rate constant would be inversely proportional to the medium viscosity tj (Eq. 3.3). 

The difference between the Kramers-Grote-Hynes one-dimensional reaction coordinate (KGH) model and the Agmon-Hopfield two-dimensional reaction coordinate (AH) model can be seen clearly by comparing Figs 3.2a and b. 

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I 3 High-Pressure Kinetics and Highly Viscous Media... 

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