Energetics of the naphthalene/azulene monocation isomerization: density functional and coupled cluster calculations†‡

Abstract

The energetics of isomerization between the azulene and naphthalene radical cations have been investigated using the hybrid density functional method B3LYP with the cc-pVDZ basis sets. CCSD/cc-pVDZ energy calculations were also carried out for selected points along the reaction coordinate. The transition state barrier energies for isomerization are lower than the dissociation limit of C₈H₆^˙+ (benzocyclobutadiene^˙+) + C₂H₂ deduced earlier. A key intermediate is a hydrogen shifted naphthalene isomer analogous to the intermediate suggested in the Dewar–Becker isomerization mechanism for neutral azulene. The norcaradiene isomer of the Dewar–Becker mechanism was found to be a transition structure in the ionic system. Results of the present density functional theory (DFT) and coupled cluster calculations are discussed in the light of recent experimental evidence.