Planar unsaturated organic molecule

We shall use the allyl radical to illustrate how one can treat planar unsaturated organic molecules using multiconfigurational methods. Some properties of the groimd state will be studied and, in addition, the electronic spectrum. The system has acmally been used as a example in a course given at the Department of Theoretical Chemistry in Lund called Quantum Chemistry at Work and a number of students have performed the calculations. We shall use their results. Some of them were recently published [69]. 

Hie canonical MOs of a planar unsaturated organic molecule can be divided into 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. 

Most organic molecules contain an even number of electrons and have zero spin in the ground state. On the other hand, radicals are systems with an odd number of electrons, and have at least one unpaired electron. The highest energy level (in the sense of the simplest IPM) is supposed to be occupied by the unpaired electron, and in most unsaturated planar radicals the corresponding orbital is a a orbital. 

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