Some unusual aspects of unimolecular falloff of importance in combustion modeling

This review is concerned with the appearance and treatment of unimolecular falloff under typical combustion conditions where it represents one of the most difficult problems in modeling. It begins with the proposal of a normal or standard form of RRKM theory, most appropriate for hydrocarbons in rare gases, which can be used for evaluation and prediction of falloff effects. Successful applications of this standard model to large-molecule dissociation are presented. Several important and common deviations from the standard model are then discussed. The first appears in the dissociation of large and unstable radicals that can be formed by either abstraction or dissociation. It is suggested that these may need to be treated as separate species, those formed by abstraction dissociating rapidly without falloff, whereas the others are deep into falloff, often dissociating much more slowly. In the case of very stable large radicals, recombination of the thermalized radical with other pool radicals can produce a species so highly energized that many otherwise inaccessible decomposition channels are now open. The standard model can also be used to uncover anomalies that lead to new and interesting theoretical insights. Several recent examples, HCN, C2H2, and allene/propyne, are considered, wherein anomalously large low-pressure C−H fission rates have been ascribed to a state-density increase resulting from a conversion of bending modes to hindered rotors at high energies.