From Pentalene to Dicyclopenta[b,g]naphthalene, or the Change towards Delocalized Structures

Using triples‐corrected coupled‐cluster methods as well as other high‐level theoretical approximations, the optimized parameters and isomerization barriers of the family of compounds cyclopentadiene–(benzene)x–cyclopentadiene (x=0, 1, 2) are computed. In contrast to previous studies, s‐indacene presents a localized C2h geometry. Also, the localized structure of pentalene is found to be the most stable, but when two benzene rings are intercalated between the five‐member rings of pentalene, the resulting molecule preferably adopts a delocalized D2h conformation. One ring is not enough: A detailed, high‐level ab initio study of a series of cyclopentadiene–(benzene)x–cyclopentadiene (x=0, 1, 2) molecules is presented. When x=0, 1, a localized structure is favored; but, for x=2, a delocalized structure is obtained (see figure).