The functional groups at which nucleophilic or electrophilic substitution or addition occur in organic molecules may be considered as charge traps in the charged-radical transient (intermediate or transition) states which play a crucial part in such reactions. The role of the sigma framework in modifying the electronic properties of the functional-group trapping states may be referred to as sigma coupling. The general expressions for sigma coupling are derived in the context of a charge-excitation model for the transient species. They provide a quantitative definition of Hoffmann's through-bond concept.