Why Do Large Ionized Polycyclic Aromatic Hydrocarbons Not Lose C2H2

The reaction mechanisms for the loss of C2H2 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 C2H2. 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, C2H2 loss is discriminated against in pericondensed PAHs. For catacondensed PAHs, C2H2 loss also drops in relative abundance as the PAH gets larger due to the increase in the number of available hydrogen atoms, increas...