Energetics of the [N]Phenylenes with Application to Helical Conformers

Ab initio HF/6-31G* and BLYP/6-31G* density functional calculations of [ N ]phenylenes are reported for all twelve [5]phenylenes and for helical [ N ]phenylenes with N = 6−8. The energetics of the [ N ]phenylenes are put on a systematic basis by means of a simple fragment-based additivity scheme, allowing both ab initio energies and heats of formation of larger phenylenes to be estimated without resort to extensive calculation. Helical [6]phenylene racemizes through a planar transition state with a barrier of 4−5 kcal/mol. A nonplanar [5]phenylene derivative is obtained by substitution of methyl for an interior hydrogen.