Structure, biradicaloid covalent

If all atoms involved in the reaction lie in the same plane, the unpaired electron of the acyl radical may be either in an orbital that is symmetric with respect to this plane or in an orbital that is antisymmetric—that is, either in a 0 or in a r orbital, whereas only a o orbital is available for the unpaired electron of radical R. Instead of just one singlet and one triplet covalent biradicaloid structure (Figure 4.5), there are now two of each, which may be denoted as B and B respectively. Similarly, there are also different zwitterionic structures to be expected. The increase in complexity and the number of states that results from the presence of more than two active orbitals on the atoms of a dissociating bond has been formalized and used for the development of a classification scheme for photochemical reactions ( topicity ), as is outlined in more detail in Section 6.3.3. 

In a general case, two or all three types of perturbation may be present simultaneously. A biradicaloid characterized by a simultaneous covalent and polar structural perturbation is called non-symmetric [6]. The eigenfunctions of the perturbing Hamiltonian Hi are obtained from those given in (4) or (5) by the substitution a p... 

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