# CCL:Alternant hydrocarbons: "Pairing Symmetry"

*From*: "Jens Spanget-Larsen" <jsl (- at -)
virgil.ruc.dk>
*Organization*: Roskilde Universitetscenter
*Subject*: CCL:Alternant hydrocarbons: "Pairing
Symmetry"
*Date*: Tue, 18 Sep 2001 12:45:58 +0100

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
(1997).
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