The chemistry of overall reactions in bulk gases can involve essentially isolated reactions of radicals that may not be thermalized. The energy requirements, which we refer to as chemical-activation energies (CEa), of these reactions are dependent on the physical conditions, progress of the overall chemistry, and are not currently measureable. Recently, we presented a generalization of Tolman's concept of activation energy for thermal reactions using Boyd’s local equilibrium theory of chemical kinetics. We will discuss extension of the generalized Tolman activation energy (GTEa) method to non-thermal reactions in molecular dynamics (MD) simulations of reactions in bulk gases. The equipartition theorem is generalized to compute average energies of small isolated systems with internal degrees of freedom, which allows application of the GTEa method to non-thermal reactions. We will present CEa results for H2O2 + OH → H2O + HO2 occurring in microcanonical MD simulations with the ReaxFF force field; where non-thermal OH radicals are the products of the self-dissociation of H2O2 and subsequent reactions. We define reaction clusters in bulk simulations that include all species that are involved in chemical changes as well as other species within interaction range; thus, we can identify individual reactions that are occur in “isolation” and are thus conservative systems. We define the reaction cluster local energy (RCLE), which is a conserved quantity during the course of an individual reaction, as the sum of the relative translational plus internal energies of the species involved in the reaction. We define the chemical-activation energy for a back reaction (BCEa) as the difference between the energy of the products and the average energy of the system. We show that the BCEa is linearly correlated to the CEa, which is in accord with the Bell–Evans–Polanyi principle. We determine transition configurations (TCs), which are unique points that define a seam separating reactants and products at which the local energies of the reactants and products are equal.

Generalized Tolman activation energy method applied to non-thermal reactions

256th National Meeting (2018)

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