Formation of PAHs and soot platelets: multiconfiguration theoretical study of the key step in the ring closure–radical breeding polyyne‐based mechanism

Polyynes of general formula H? (C?C? )_nH are known to play a significant role in combustion and pyrolysis, possibly being intermediates in the formation of polycyclic aromatic hydrocarbons (PAHs) and soot. They have also been detected in astrophysical investigations. The key step in the polyyne‐based radical breeding mechanism for PAH growth is a cyclization, put forward by Krestinin, which implies disruption of electron couples, plausibly expected to be energy demanding. We explore the electronic features and energy requirements of such a process by quantum mechanical multiconfiguration methods (CASSCF and CASPT2). The features of the wavefunction are analyzed, and the free energy barriers are estimated over a wide range of temperatures, for three molecular models. The initial radical adduct A, generated by H ^. , HC?C ^. (ethynyl), or HC?C? C ^. H₂ ( propargyl ) addition to butadiyne (BD, HC?C? C?CH), undergoes a cyclization with the generation of two new radical centers. However, in most of the cases, one of these new singly occupied sp² orbitals has some overlap with the unpaired electron lobe already existent in A: some sort of bonding builds up and consequently the triradical character cannot be large. Only one model suggests a possible role of the radical breeding mechanism during combustion. Copyright © 2009 John Wiley & Sons, Ltd.