Ephemeral collision complexes mediate chemically termolecular transformations that affect system chemistry

Termolecular association reactions involve ephemeral collision complexes—formed from the collision of two molecules—that collide with a third and chemically inert ‘bath gas’ molecule that simply transfers energy to/from the complex. These collision complexes are generally not thought to react chemically on collision with a third molecule in the gas-phase systems of combustion and planetary atmospheres. Such ‘chemically termolecular’ reactions, in which all three molecules are involved in bond making and/or breaking, were hypothesized long ago in studies establishing radical chain branching mechanisms, but were later concluded to be unimportant. Here, with data from ab initio master equation and kinetic-transport simulations, we reveal that reactions of H + O 2 collision complexes with other radicals constitute major kinetic pathways under common combustion situations. These reactions are also found to influence flame propagation speeds, a common measure of global reactivity. Analogous chemically termolecular reactions mediated by ephemeral collision complexes are probably of significance in various combustion and planetary environments. Chemically termolecular reactions — arising from the collision of ephemeral collision complexes with other chemically reactive species — have been neglected in current gas-phase chemical mechanisms of combustion and planetary atmospheres. First-principles calculations reveal that such chemically termolecular reactions constitute major pathways affecting macroscopic observables.