A Simple Model for the Description of Biradicals

The antiaromatic geometry found along the concerted path of ground-state-forbidden pericyclic reactions, which is topologically equivalent to an antiaromatic Hiickel [4n]annulene or MObius [An + 2]annulene, is a particularly interesting type of biradicaloid geometry. (Cf. Section 4.4.) Other biradicaloid geometries and combinations of those mentioned are equally possible. 

Whereas at equilibrium geometries the electronic structures of even-electron molecules are in general quite well described by a single closed-shell 

POTENTIAL ENERGY SURFACES BARRIERS, MINIMA, AND FUNNELS 

for instance, the orthogonal nonbonding orbitals are AOs Xa and Xh located at the atoms A and B and prevented from interacting either by excessive separation, as in a dissociated single bond, or by symmetry, as in a twisted double bond—that is, if p = and q = Xh—fh configurations given in Equations (4.1) and (4.2) are the VB structures which may be represented by the formulae 2-7. 

For systems such as a dissociated H2 molecule (8) or twisted ethylene (9) the slnicliires B and B arc nonpolar and can be represented as A—B and 

---