The effect of reagent energy on chemical reaction rates: An information theoretic analysis

The effect of changing reagent vibrational and rotational energy on the reaction rate has been analyzed for over 20 chemical reactions. In most cases the selectivity in energy requirements could be characterized by a single ("consumption potential") parameter, even when the reactivity varied by many orders of magnitude. The reactions analyzed covered atom-diatom and diatom-diatom collisions and included both simple rearrangement ("exchange") reactions as well as collision induced dissociation (CID) and quenching of electronically excited states. The results were derived both from experiments and classical trajectory computations and include the variation in reactivity at both a given total collision energy and at a given translational (and rotational) temperature. In all cases the analysis was based on evaluating the surprisal of the energy consumption, i.e., the observed (or computed) reaction rate constant was compared to the rate expected on prior grounds when all states (at a given total energy) react with the same rate. The excess internal energy of the reactants is not necessarily available for reaction. Hence the consumption potential is not invariably of a definite sign. For highly endoergic processes, both experiments and trajectory computations show that often the increase in reaction rate due to reagent vibrational energy is over and above that expected on purely prior grounds (i.e., that expected due to the increase in the total available energy). The enhancement of an endoergic reaction rate by reagent vibrational energy is particularly significant for the lower vibrational states. As the excitation energy of the reagent approaches the endoergicity of the reaction, the enhancement of the rate is considerably lower. For exoergic, thermoneutral, and mildly endoergic reactions the increase in rate due to reactant vibrational energy is usually less than expected on prior (i.e., statistical) grounds. The effect of reagent rotational energy appears to be strongly correlated with the "steric requirements" of the reaction. For reactions with a preferential direction of attack (a "cone of acceptance") the enhancement in the reaction rate is often less than expected on prior grounds. The surprisal analysis demonstrates different rotational energy consumption for even and odd J states in the F+H₂ (υ, J) reaction. Several examples of reactions with two (or more) different types of products have been analyzed. It is found that often the different reaction paths have qualitatively different energy requirements. In particular, reagent vibrational energy tends to strongly favor the most endoergic path, over and above the energetic effect expected on prior grounds. Selective excitation of reagents does provide a discriminatory (i.e., nonstatistical) method for changing the branching ratio.