Title: Understanding and Representing the Kinetics of Gas-Phase Reaction Systems in Mixtures
Description: Gas-phase reaction kinetics is at the center of the evolution of reacting systems. Many important reactions in the gas phase proceed through rovibrationally excited complexes AB* that are formed either from the association of two particles A + B or from the activation of thermalized species AB by collisions between AB and inert colliders. The fate of these rovibrationally excited complexes is governed by the competition among energy-transferring collisions, unimolecular decomposition, and bimolecular reactions -- yielding a strong dependence of all emergent rate constants on temperature, pressure, and composition of the surrounding mixture. In nearly all realistic environments critical to combustion and planetary atmospheres, multiple species are present in significant quantities and thus contribute to the evolution of the rovibrationally excited complexes. A substantial fraction of these species is inert in the sense that they merely participate in energy-transferring collisions, and a portion of them are instead reactive, which participate in reactive collisions and induce reactions with the rovibrationally excited complexes rather than merely transferring energy.

Most of the inert species have distinct collisional energy transfer characteristics and thus contribute differently to the energy-transferring collisions -- making the multi-component pressure dependence in mixtures different from the pressure dependence in the constituent components when pure. Accounting for such “mixture effects,” in practice, one employs a “mixture rule” to interpolate kinetic data in mixtures from individual pure bath gas components. While mixture effects for reactions proceeding through a single potential well and a single reaction channel have been extensively investigated, mixture effects and mixture rules for multi-well and/or multi-channel reactions are significantly less characterized despite their ubiquitousness in gas-phase reaction systems. This work presents an investigation of and seeks reliable representations of bath gas mixture effects on multi-channel (both single-well and multi-well) reactions and their impacts on combustion predictions. The performance of different mixture rules for representing multi-component pressure dependence of rate constants for various systems is evaluated through comparisons against ab initio master equation calculations for the mixture.

The comparisons revealed that the classic linear mixture rule, the most commonly applied mixture rule, yields substantial deviations (exceeding a factor of 50) for typical combustion mixtures. The comparisons, together with results from combustion simulations, suggest that recently proposed mixture rules based on the reduced pressure provide a considerably more accurate representation of mixture effects for various systems. These new mixture rules are therefore recommended for use in fundamental and applied chemical kinetics investigations. The importance of reactive collisions between the rovibrationally excited complexes AB*, formed from the association of A + B, and reactive colliders C was largely ignored historically. Recent studies have demonstrated that reactive collisions of AB* with C often occur on the same timescale as energy-transferring collisions. And these reactive collisions can induce non-Boltzmann kinetic sequences that proceed through AB* and propagate across multiple coupled potential energy surfaces.

The non-Boltzmann kinetic sequences can be represented by the chemically termolecular reactions A + B + C → products in phenomenological kinetic models. While these non-Boltzmann kinetic sequences consume the same set of species as their equivalent thermal sequential pathways, they are kinetically and dynamically distinct and can have substantial impacts on the global reactivity in combustion and atmospheric systems beyond those imposed by thermal sequential pathways. Evaluating the kinetics of non-Boltzmann kinetic sequences requires that rovibrational excitation of reacting complexes from one potential energy surface be carried over to the following and appropriate treatments for the augmentation and dissipation of the energy distributions due to reactions. This work presents a theoretical and computational methodology that couples multiple master equations and derives rate constants for all emergent phenomenological reactions for non-Boltzmann kinetic sequences spanning across the coupled master equations.

Results from implementing the methodology for a couple of systems demonstrate that reactive collisions can both increase the overall rate of conversion of reactants to products and alter the branching ratios among final products. Combustion simulations indicate that reactive collisions can have significant impacts on the overall system reactivity. Therefore, suitable rate laws and appropriate treatment are needed for the distinct effects of reactive collisions to be represented in phenomenological kinetic models.