Through-Bond and Through-Space Interactions of Organic Radicals Coupled by m-Phenylene

Through-bond and through-space magnetic interactions in tricarbene and its related triradicals coupled through m-phenylene units are discussed with a semiempirical molecular orbital method including configuration interactions (PM3-CI method). The high-spin (HS) and low-spin (LS) states have quite similar geometries, but the methylene bridging angle is larger in the HS states. The HS states of these lie well below the corresponding LS states, except for one which contains nitroxide and aminiumyl radicals. Instability of the HS state of the heteroatomic system is a consequence of significant mixing between NBMOs and benzene e_lg-type orbitals, which can be explained in terms of simple perturbation theory. The energy difference between the HS and LS states is not explicitly influenced by the conformation of the molecules examined. This shows that such conformations have little effect on the magnetic properties in possible polymer ferromagnets which contain m-phenylene coupling units, as far as the second nearest neighbor interaction.