Gas-phase route to polycyclic aromatic hydrocarbon formation in protoplanetary atmospheres: role of stabilized benzyne anions

The possibility that condensation reactions, leading to polycyclic aromatic hydrocarbon (PAH) formation in protoplanetary atmospheres, could occur with negative aromatic species, such as stabilized anionic ortho-benzyne (o-C6H4-), is analysed through accurate quantum dynamical modelling of the initial steps. Once the primary electron attachment on the neutral o-C6H4- has taken place, producing the corresponding metastable negative ion, the evolution of this resonant anionic intermediate in terms of its energy locations and lifetimes suggests the possibility of a radiationless stabilization by a very rapid intramolecular vibrational redistribution of the excess energy carried by the attached electron. The possible involvement of this stabilized highly reactive molecular anion in an ensuing PAH synthesis is also qualitatively discussed.