Limiting properties of large [N]phenylenes

Geometries, energies, and magnetic properties are reported at the ab initio B3LYP/6-31G* level for three classes of [N]phenylene: helical ([N]heliphenes) (N = 14, 19, and 24); zigzag, and linear (N = 14 and 19). Bond lengths, proton chemical shifts, and nucleus-independent chemical shift (NICS) values are converged for all three forms. Limiting values for the proton shifts are delta 6.50, 5.92-5.97, and 5.87 for the zigzag, helical, and linear classes, respectively. NICS values have converged for zigzag and linear [19]phenylenes and for [24]heliphene. The inner four-membered rings of [19]heliphene and [24]heliphene are sandwiched between six-membered rings (and vice versa). NICS values for the heliphenes are small and positive along the helical axis. The length of [24]heliphene is ca. 14.6 Angstrom, and its internal diameter is ca. 5.3. For N = 14 and 19, the zigzag isomer is lowest in energy, the linear is the highest; the differences are, at most, 2 kcal/mol per ring. A similar ordering is found for isotropic magnetic susceptibilities.