CCL:Alternant hydrocarbons: "Pairing Symmetry"

CCL:Alternant hydrocarbons ...
 Just some additional comments:
 One very interesting aspect of the electronic structure of alternant
 hydrocarbons is the orbital and state pairing symmetry predicted by
 simplified model Hamiltonians (e.g., HMO and PPP). This symmetry has
 important consequences for the predicted electronic transitions for
 this class of compounds. For example, only dipole transitions between
 so-called plus and minus states are allowed by the pairing symmetry,
 and a mirror image relationsship is predicted for the spectra of
 paired cations and anions. There is strong experimental evidence for
 the presence of effective orbital and state pairing in alternant
 hydrocarbons, particularly in benzenoid PAHs, and there can be no
 doubt that the pairing theorem offers a unique insight into their
 electronic structure.
 However, perfect pairing symmetry is an abstract, purely mathematical
 concept, based an idealized model assumptions. In fact, the pairing
 properties predicted by a variety of popular quantum chemical
 procedures are highly sensitive to calculational details. For
 example, the degeneracies required by the pairing theorem are not
 even approximated by the CNDO/S method of Del Bene and Jaffe. It
 seems that a remarkable coincidence, an "accidental cancelation of
 terms", is responsible for the presence of near-perfect pairing
 symmetry in benzenoid hydrocarbons.*)
 Yours, Jens >--<
 J. Spanget-Larsen: "The alternant hydrocarbon pairing theorem and
 all-valence electrons theory. ..", Theor. Chem. Acc. 98, 137-153
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