Formation pathways of ethynyl-substituted and cyclopenta-fused polycyclic aromatic hydrocarbons

Two novel classes of polycyclic aromatic hydrocarbons (PAH), those with ethynyl substituents (ethnyl-PAH) and those with externally fused five-membered rings (cyclopenta-fused PAH or CP-PAH), have recently been identified in the products of a variety of fuels and combustion/pcrolysis environments. However, the recently developed capacity for identifying these compounds has raised new questions about preferential reaction pathways. Specifically, across various fuels and operating conditions, experimentally observed products are (1) CP-PAH, which result from C2H2 addition to an aryl radical, followed by cyclization to a cyclopenta ring and (2) ethynyl-PAH, which result from C2H2 addition to locations on the aryl radical where cyclization is not possible. We have never observed ethynyl-PAH resulting from C2H2 addition to an aryl radical at a point where cyclization into a five-membered ring is possible. To explain this behavior, we have performed AM1 semiempirical quantum chemical computations with group correction in order to examine the potential energy surfaces of the reaction pathways that lead to ethynyl-PAH and CP-PAH. We have performed computations for the parent aryl radical, possible ethynyl-PAH products, possible CP-PAH products, as well as intermediates and transition states, for C2H2 addition to naphthalene, anthracene, phenanthrene, acenaphthylene, fluoranthene, and pyrene. Possible CP-PAH products are acenaphthylene, aceanthrylene, acephenanthrylene, pyracylene, cyclopenta[cd]fluoranthene, and cyclopenta[cd]pyrene In all cases, we have found that, although energy differences between ethynyl-PAH isomers are very small (similar to1 kcal/mol), the experimentally observed ethynyl-PAH is always the lowest energy isomer. Furthermore, the observed preference for cyclization to CP-PAH over formation of an ethynyl-PAH can be explained by the significantly lower energy barrier (23 vs. 36 kcal/mol) for the cyclization reactions. Finally we have determined that, while not prohibited. the isomerization of ethynyl-PAH to CP-PAH requires significantly higher energy than the aryl-vinyl cyclization reactions, and therefore is not expected to make a significant contribution to the product distribution. These results are sufficiently consistent that the computation of reaction pathway energy surfaces can be used to identify likely ethynyl-PAH and CP-P4H products from the addition of C2H2 to much larger parent PAH.