The 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–(CC–)nH, are known to play a significant role in combustion and pyrolysis, possibly being intermediates in the formation of polycyclic aromatic hydro-carbons (PAHs) and soot. They have also been detected in astrophysical investigations. The key step in the polyyne-based radical breeding mechanism for PAH growth, put forward by Krestinin, is a cyclization that implies disruption of electron couples, hence plausibly expected to be energy demanding. We explore by quantum mechanical multiconfiguration methods (CASSCF and CASPT2) the electronic features and energy requirements of such a process in itself. The wavefunction features are analyzed, and free energy barriers estimated over a wide range of temperatures, for three molecular models. The initial radical adduct A, generated by H•, HCC• (ethynyl), or HCC–C•H2 (propargyl) addition to butadiyne (HCC–CCH) undergoes a cyclization with generation of two new radical centers. However, in most 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 the triradical character cannot consequently be large. Only one model suggests a possible role of the radical breeding mechanism during combustion.