Spin coupling rules for the biradicals of o-, m-,p-bimethoxymethanobenzeneand its derivatives

Spin coupling rules for the ground state of the carbon or nitrogen biradica ls of o -, m -, p - bimethoxymethanobenzene and its derivatives containing the carbon, nitrogen biradicals have been studied quantum chemical ab initi o methods. It is proved that there are topological rules for the magnetic c ouplings 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 pi - 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 obvio us influence on the spin coupling. Stronger ferromagnetic coupling occurs b etween positive ion radicals, which provides theoretical guide for molecula r design and experimental synthesis of organic magnetic materials.