Platt s nomenclature

AHOMO and ALUMO may be derived from a perturbational treatment of the union of the [I l]annulenyl cation and C or of the [I3]annulenide anion and C , respectively. This also shows that these molecules have further excited states in addition to those derived from a (4N + 2)-electron perimeter. This is true for the longest-wavelength transitions of both molecules, which therefore cannot be labeled within the framework of Platt s nomenclature. Hence, a prediction of their MCD sign on the basis of the perimeter model is also impossible. However, the next two bands correspond to the L, and Lj states of a (4Af + 2)-electron perimeter and show the expected behavior in the MCD spectra. In acenaphthylene the order of the B terms is -, + and in pleiadiene -f-, -. These signs are not changed by perturbing substituents since the difference between AHOMO and ALUMO is too large. Both molecules represent hard chromophores. 

The l Bzu excitation is polarized along the short axis of the naphthalene molecule. In Platt s nomenclature of excited states of polycyclic aromatic hydrocarbons (PAHs), l Bzu corresponds to the La state, which has more ionic character than the 1 B3 (or Lj,) state. Parac and Grimme have pointed... 

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