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Kinetics and dynamics of reactions in liquids

Abstract

We discuss the control of the kinetics and dynamics of chemical reactions by the solvent, from a molecular point of view. The kinetics are discussed using a transition state theory (TST) approach, applied to the reactants and their surrounding solvent as one supramolecule. The topics discussed include a molecular interpretation for the changes that take place when one solvent is being replaced by another; the use of local against normal vibrational modes and/or joint description, i.e., local modes for part of the system and normal modes for the other part; and the effect of pressure on the rate in solution. The notion of free volume and volume of activation is extended to a more general phase space in which geometrical volumes may overlap, the approximations that are inherent to cell theory are examined and a molecular interpretation for internal and chemical pressures is suggested. The link to the dynamics is provided by an analysis of the breakdown of TST due to diffusion/cage control of the rate of the reaction. A unified description which interpolates from activation control to diffusion control is presented with a special emphasis on the motion within the solvation cage. Results of molecular dynamics simulations for both activated and activationless reactions are presented. The very detailed computer experiment is interpreted using a reduced mechanical description and the separation of time-scales is discussed using an adiabatic separation of variables. Spectroscopic methods for probing the different time epochs are suggested. The rather short duration typical of the motion within the solvent cage is emphasized, and the possibilities that this affords for studying the short-time dynamical role of the solvent via experiments in clusters or in glasses are noted.

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