Configuration interaction symmetry-forbidden reaction

A shortcoming of the orbital approach which underlies the formalism of Woodward-Hoffmann and similar schemes (see Chap. 4) is that they fail to take into account the electron repulsion. For this reason, the selection rules for pericyclic reactions do not depend on the multiplicity of the state and the configurational interaction cannot be taken into account in a sufficiently rigorous manner to be able to allow for electron correlation. This point is particularly inconvenient when analyzing symmetry-forbidden reactions. 

Xas->sa °f the same symmetry aa (dashed lines). Configuration interaction between xo of symmetry, v,v and the doubly excited configuration of the same symmetry splits the two crossing states of symmetry ss apart (full lines), but a symmetry-imposed barrier remains, which classifies the reaction as forbidden in the ground state. 

It turns out through our extensive studies that the electron wavepacket description of chemical reactions offers an interesting and novel way of comprehending chemical phenomena, in which the shift of nuclear configuration induces a qualitative change in the electronic states through nonadiabatic interactions. This situation is typically observed in chemical reactions that are non-concerted reactions or Woodward-Hoffmann symmetry forbidden... 

---