Topological dependence of ferromagnetic coupling strength in some π-conjugated radicals

The Lanczos method has been employed to solve the semi-empirical valence bond model exactly for a series of organic di- and poly-radicals of benzenoid type of up to 18 π electrons. The calculated energy gaps between the high-spin ground and first excited states have been applied to gain some insight into the topological dependence of the strength of ferromagnetic spin coupling in these molecules.