Quantum chemical calculations employing two-configuration self-cosistent field theory and density functional theory have been carried out for a new diradical cation composed of two ring systems, 1,4-thiazine and 1,2,3,5-dithiadiazolyl. These ring systems are coupled through a central C-N bond, and hence, the free rotation are allowed for this diradical cation. However, the singlet-triplet energy splitting has been predicted to be positive for every dihedral angle between two ring systems. Moreover, the cation diradical was found to be isoelectronic with trimethylenemethane in the planar conformation. This fact is deeply related with the robustness of high-spin correlation of the diradical cation. (C) 2004 Elsevier B.V. All rights reserved.