Spin coupling rules for the ground state of the carbon or nitrogen biradicals of o-, m-, p-bimethoxymethanobenzene and its derivatives containing the carbon, nitrogen biradicals have been studied quantum chemical ab initio methods. It is proved that there are topological rules for the magnetic couplings between the radicals of nonplane conjugated systems: In conjugated system, the effective exchange integrals of radical sites through the even number of carbons with a π-electron network are negative in sign, which means the low spin state corresponds to a stable ground state, while those through odd carbon atoms are positive in sign, (i.e., the high spin state corresponds to a stable ground state). The properties of radical have obvious influence on the spin coupling. Stronger ferromagnetic coupling occurs between positive ion radicals, which provides theoretical guide for molecular design and experimental synthesis of organic magnetic materials.

Key words: METHOXY GROUP, BENZENE P, FREE RADICALS, SPIN, COUPLING, QUANTUM CHEMISTRY, AB INITIO CALCULATION, MAGNETIC MATERIALS