Activated-complex theory: current status, extensions, and applications

Abstract

The activated-complex theory of chemical reactions has proved to be very useful in interpreting rate data on a wide variety of chemical reactions. The topics so treated include preexponential factors of bimolecular gas-phase reactions, preexponential factors at high pressures of unimolecular reactions, salt and polar solvent effects on bimolecular-reaction rates, and recombination rates of radicals under certain conditions. In conjunction with additional concepts, the theory has also found extensive use in such subjects as pressure effects on unimolecular reactions (so-called RRKM theory}, chemical activation, and electron-transfer reactions both in solution and at electrodes. In the present chapter the assumptions and several derivations of the theory are considered. Some of the recent tests of activated-complex theory that are based on the numerical solution of the dynamical equations of motion of the reactants are summarized. Topics such as the "free-energy-maximization criterion" for the position of the activated complex are also considered. Several of the applications of activated-complex theory are treated, and some dynamical extensions of it are noted. Some of the work described in the present chapter is drawn from the writer's published studies, but the material in Sections 3 and 5 on an ensemble derivation for reactions in solution and on a basis for the usual free-energy-maximization criterion for location of the activated complex is drawn from unpublished work.