Why are the Properties of Polycyclic Hydrocarbons Additive over Conjugation Circuits ? *

Properties as different as resonance energies and magnetic ring-currents are investigated and a non-empirical system of increments is proposed. It is shown that the :n:-electron properties of · conjugated, polycyclic hydrocarbons are additive over all conjugation circuits that can be identified in the molecule. The contribution from each individual conjugation circuit is calculated non-empirically on a simple, free-electron model for the associated annulene. Within a special form of the resonance theory, it is shown that all conjugation circuits should contribute with equal weight, and not only the »independent circuits« as proposed earlier by Randie or certain lower circuits as postulated by Herndon. Contributions from higher circuits turn out quite naturally, however, to be small. Results for a wide variety of polycyclic hydrocarbons, alternant and non-alternant, are presented. Both for resonance energies and for magnetically-induced ring-currents, very satisfactory agreement with conventional calculations has been obtained.