Stone–Wales Rearrangements in Polycyclic Aromatic Hydrocarbons: A Computational Study

Mechanisms for Stone–Wales rearrangements (SWRs) in polycyclic unsaturated hydrocarbons containing a pentafulvalene core have been studied using density functional, coupled cluster, and multiconfigurational methods. At the BD­(T)/cc-pVDZ//(U)­M06-2X/cc-pVDZ level of theory, free energies of activation (at 1000 °C) range from ca. 70 kcal/mol for the model system pentafulvalene → naphthalene (1 → 2) to >110 kcal/mol for the degenerate SWR of pyracyclene (3). Systems studied that do not contain a pyracyclene subunit are predicted to have ΔG^⧧ less than about 90 kcal/mol and to proceed by a carbene-type mechanism. Substrates containing a pyracyclene subunit should proceed via a cyclobutyl mechanism, and appropriate benzannelation of 3 lowers the activation free energy considerably. Computed ΔG^⧧ values are consistent with experimental observations reported for known systems. SWRs of two untested substrates, cyclopent­[fg]­aceanthrylene (18) and dicyclopenta­[fg,op]­tetracene (21), are predicted to have ΔG^⧧ < 95 kcal/mol and thus to be accessible via flash vacuum pyrolysis.