Analysis of the Nucleus Independent Chemical Shifts of [10]cyclophenacene: Is It an Aromatic or Antiaromatic Molecule?

[10]cyclophenacene is an important synthetic target that shows a pair of NICS values for the center of mass and six-membered rings typical of an aromatic species. This is found in contrast with the global paratropic current density induced by a magnetic field parallel to the main symmetry axis. This apparent contradiction has been analyzed studying the tensor character of the magnetic response. It turns out that the molecule displays two characters: one paratropic (antiaromatic) and another one diatropic (aromatic), depending on the orientation of the inducing magnetic field. The paratropic response, which cannot be recognized from the NICS values, is associated to a well-defined destabilization of the belt closure, as witnessed by homodesmotic reactions. A scalar measure of magnetic aromaticity, the Field-Independent Current Strength (FICS) has been introduced, which allows reaching the conclusion that [10]cyclophenacene is indeed an aromatic molecule, although significantly affected by the paratropic response.