Organic open-shell systems with indirect magnetic interaction caused by the topology of the elementary units

The magnetic properties of a class of 1-D pi-systems having a frontier half -filled band (HFB) of degenerate non-bonding MOs (NBMOs) localized within t he elementary units (EU) have been studied theoretically. The localization of the MOs, determined by the topology of the pi-fragments which form the E Us is explained by means of the extended Coulson-Rushbrooke-Longuet-Higgins theorem. The perfect localization and non-overlapping of the NBMOs results in vanishing of the direct (Hund-) and the kinetic exchange interaction be tween the unpaired electrons. The indirect exchange of the unpaired electro ns (spin polarization exchange) occurs via the delocalized pi-electrons in the filled energy bands.