Semiempirical MO Treatments of Planar Conjugated Molecules

Hie canonical MOs of a planar unsaturated organic molecule can be divided into r and 77 MOs according to whether the eigenvalue for reflection in the molecular plane is +1 or -1, respectively (Section 15.10). The earliest semiempirical methods for planar conjugated organic compounds (Sections 16.2-16.4) treated the tt electrons separately from the cr electrons. Coulson stated that the justification for the ct-tt separability approximation lies in the different symmetry of the cr and tt orbitals and in the greater polarizability of the tt electrons, which makes them more susceptible to perturbations such as those occurring in chemical reactions. 

In the TT-electron approximation, the n tt electrons are treated separately by incorporating the effects of the cr electrons and the nuclei into some sort of effective TT-electron Hamiltonian (recall the similar valence-electron approximation Sections 13.20 and 15.5)  

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