Subject: CCL:Alternant hydrocarbons: "Pairing Symmetry"

From: "Jens Spanget-Larsen" <[email-masked]>
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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