Theory for a general system of increments for the properties of polycyclic hydrocarbons

A general non-empirical system of increments for the calculation of molecular properties of polycyclic, conjugated hydrocarbons is proposed. It is based on identifying the conjugation circuits in all Kekulé structures and assuming an increment from each circuit comparable to the magnitude of the property in the associated annulene. These increments are calculated by a simple free-electron theory with a Kuhn-type harmonic potential. No adjustable parameters are used to fit the property being calculated. The relation between this method and a very simplistic VB formalism is considered. The reason why the non-empirical parametrization of such crudely approximated formalism may lead to rather improved results is discussed in some detail. This novel system of increments is tested for two properties, resonance energy and magnetic ring-currents. The results obtained by this method correlate well with those of standard techniques. This system of increments for estimating local properties of molecules gave particularly gratifying results when used to predict ring-current intensities. It is hoped that this method, being equally applicable to other properties, will prove to be a valuable instrument for the rapid estimation of a wide range of properties of polycyclic, conjugated hydrocarbons. A preliminary account of this work was presented at the International Symposium on Aromaticity held at Dubrovnik, Croatia, Yugoslavia, September 3–5, 1979