The relationship between the structures and electronic ground state properties of non-Kekule-type N-substituted m-phenylenes was studied utilizing density functional theory (DFT), with the aim of determining the factors that lead to ground state triplet diradicals. At B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) we identified octahydropyridoquinoline with an exceptionally large singlet-triplet energy separation of +27.9 kcal mol(-1), in favor of the triplet. ne high-spin structures can readily be obtained by interruption of the full cyclic pi-delocalization that avoids cross-conjugation of the nitrogen radical centers. Introduction of additional heteroatoms, on the other hand, preferentially stabilizes the singlet zwitterionic resonance contributors in these systems. The identified diradicals show strong ferromagnetic exchange interactions between two radical centers.