Hydrogen migration in polyaromatic growth

Abstract

New reaction pathways for polyaromatic growth in combustion environments are explored theoretically. The analysis is based on semiempirical quantum calculations of potential energy barriers and vibrational frequencies, followed by standard transition state theory evaluation of reaction rates. The reaction systems considered include formation of five-and six-member aromatic rings, their interconversion, and migration of cyclopenta rings along zigzag edges of aromatic surfaces. All reactions have a distinctive mechanistic feature: the reaction pathways are induced or assisted by hydrogen atom migration. The present calculations show that such migration is rapid and leads to a faster surface growth than reaction pathways previously considered. The results extend the mechanism for the involvement of five-member aromatic rings in surface growth of soot particles. The migrating cyclopenta rings propagate the growth as a continuous replicating front. This is achieved by extending a six-member-ring step upon encounter or converting to a six-member ring at an open end, thus providing a nucleus for the next aromatic layer. Collisions of the propagating fronts may be responsible for creation of sites that cannot be filled by cyclization and thus cannot support further growth. Formation of such surface defects may be responsible for the loss of reactivity of soot particle surfaces to growth, consistent with a prior suggestion.