Although acetylene loss is well known to constitute the main breakdown pathway of polycyclic aromatic hydrocarbon (PAH) species, the molecular structure of the dissociation products remains only poorly characterized. For instance, the structure of the C8H6 product ion formed upon acetylene loss from the smallest PAH naphthalene (C10H8) has not been experimentally established. Several C8H6(+) isomers are conceivable, including phenylacetylene, benzocyclobutadiene, pentalene as well as a number of a-cyclic products. Here we present infrared (IR) spectroscopic evidence for the formation of the (anti-aromatic) pentalene structure using a combination of tandem mass spectrometry and IR laser spectroscopy. The formation of pentalene is suggestive of facile 6- to 5-membered ring conversion, which possibly has implications for the PAH/fullerene interrelationship in energetic settings such as the interstellar medium and combustion environments.

Energetic processing of gaseous polycyclic aromatic hydrocarbons (PAHs) plays a pivotal role in the chemistries of inter- and circumstellar environments, certain planetary atmospheres, and also in the chemistry of combustion and soot formation. Although the precursor PAH species have been extensively characterized, the products from these gaseous breakdown reactions have received far less attention. It has been particularly challenging to accurately determine their molecular structure in gas-phase experiments, where comparisons against theoretical modeling are best made. Here we report on a combined experimental and theoretical study of the dissociative ionization of two nitrogen containing polycyclic aromatic hydrocarbons of CHN composition, acridine and phenanthridine. The structures of HCN-loss fragments are resolved by infrared multiple-photon dissociation (IRMPD) spectroscopy of the mass-isolated products in an ion trap mass spectrometer. Quantum-chemical computations as well as reference IRMPD spectra are employed to unambiguously identify the molecular structures. Furthermore, computations at the density functional level of theory provide insight into chemical pathways leading to the observed products. Acenaphthylene˙ and benzopentalene˙ - two aromatic species containing pentagons - are identified as the main products, suggesting that such species are easily formed and may be abundant in regions where thermal or photoprocessing of polyaromatics occurs.