Ferromagnetic coupling through m-phenylene. Molecular and crystal orbital study

As a study of organic ferromagnetism in a one-dimensional system, magnetic interaction between radical centers through a m-phenylene coupling unit is considered on the basis of the molecular orbital (MO) and the crystal orbital (CO) methods. The semiempirical MO calculations for dimer models show that the exchange integral between radical centers such as C^•-H, N^•, N^•+-H, and N-O^• through the m-phenylene coupling unit is positive both in the unrestricted Hartree-Fock (UHF) approach and in the configuration interaction (CI) approach. Although the UHF and the CI methods predicted quite similar triplet geometries, the UHF method was found to overestimate the preference for the triplet states. Moreover, the CO calculations with the UHF method gave a theoretical background for the realization of a ground state with macroscopic spin alignment on the one-dimensional model chains.