Why Do Large Ionized Polycyclic Aromatic Hydrocarbons Not Lose C2H2?

The reaction mechanisms for the loss of C 2 H 2 from the ions of anthracene, phenanthrene, tetracene, and pyrene were calculated at the B3-LYP/6-311++G(2d,p) level of theory and compared to that previously published for ionized naphthalene. A common pathway emerged involving the isomerization of the molecular ions to azulene-containing analogues, followed by the contraction of the seven-member ring into a five- and four-member fused ring system, leading to the cleavage of C 2 H 2 . The key transition state was found to be for this last process, and its relative energy was consistent going from naphthalene to tetracene. That for pyrene, though, was significantly higher due to the inability of the azulene moiety to achieve a stable conformation because of the presence of the three fused rings. Thus, C 2 H 2 loss is discriminated against in pericondensed PAHs. For catacondensed PAHs, C 2 H 2 loss also drops in relative abundance as the PAH gets larger due to the increase in the number of available hydrogen atoms, increasing the rate constant for H atom loss over that for C 2 H 2 loss as PAH size increases. The unimolecular reactions of four cyano-substituted polycyclic aromatic hydrocarbon (PAH) ions were also probed as a function of collision energy by collision-induced dissociation tandem mass spectrometry. As the size of the ring system increases, HCN loss decreases in importance relative to other processes (H and C 2 H 2 loss). 9-Cyanophenanthrene ions were chosen for further exploration by theory and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The calculated reaction pathway and energetics for C 2 H 2 loss were consistent with those found above. The calculations suggest that larger PAHs of interest in the interstellar environment will behave independently of a CN substituent.