The 1,2,3,5-dithiadiazolyl radical [HCN2S2] has a broad visible and near-infrared electronic transition spanning the 12 000-18 000 cm(-1) region. On the basis of ab initio molecular orbital calculations, the spectrum can be assigned as an allowed <(A)over tilde B-2(2)>-<(X)over tilde (2)A(2)> electronic transition. This transition corresponds to the promotion of an electron from the singly occupied molecular orbital (of a(2) symmetry) to the lowest unoccupied orbital (of b(2) symmetry); i.e., the transition has unusual pi*(S-S) to sigma*(S-S) character.