4 Dispersive kinetics

Chemical reactions proceeding on time-scales comparable to or shorter than those of internal rearrangements in the reaction system renewing the environment of reactants (mixing) are, in general, dispersive. For dispersive kinetics, as for dispersive transport and dispersive relaxation, many time-scales coexist, and the rate coefficients, termed the specific reaction rates, depend on time. In classical kinetics, the concept of an energy profile along a reaction path helps one to visualize the main features of a chemical reaction, including its mechanism. Along the reaction path, the reactants are separated from the products by a potential energy barrier. Specific reaction rates are related to thermally activated over-barrier transitions or to quantum-mechanical tunneling through the barrier. For a constant specific reaction rate a single potential energy barrier is envisaged for the whole reaction course. For a time-dependent specific reaction rate the potential energy barrier separating the reactants from products has to evolve during the reaction course. Since the first half of the last century there have been many studies aimed toward a mathematical description of barrier crossing processes in condensed-phase systems. When the time-scale of the environmental fluctuations is short compared to the time of the overall rate processes, the effect of the environments on the barrier crossing is merely to ‘renormalize’ the rate coefficient. However, there are many cases, such as electron transfer in viscous solvents, reactions of biomolecules, reaction in glasses and femtosecond events in fluid environments, where the fluctuations of the environment are slower than or comparable in time-scale to the overall barrier crossing. In this situation the traditional theory of chemical kinetics breaks down and we enter the vast areas of dispersive kinetics. In Section 2 some recent discussions on the basic concepts of kinetics and the common features of dispersive rate processes in condensed media are presented. Approximation of classical kinetics is discussed in Section 3. The extent to which the models of dispersive kinetics and the approximation of classical kinetics conform to reality is determined by testing them against experimental data. This is done in Section 4. Conclusions comprising Section 5 reflect to some extent the author’s personal point of view and interest, which undoubtedly may be seen also in the choice of the subject matter from this rapidly developing interdisciplinary field of research.