Polycyclic aromatic molecules electrophilic substitution

Both HMO calculations and more elaborate MO methods can be applied to the issue of the position of electrophilic substitution in polycyclic conjugated molecules. The most direct approach is to calculate the localization energy. This is the energy difference between the aromatic molecule and the o--complex intermediate. In simple Hiickel calculations, the localization energy is just the difference between the energy calculated for the initial tt system and that remaining after two electrons and one carbon atom have been removed from the conjugated system ... 

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