Carbonyl diazide, (N-3)(2)CO (I), is a highly explosive compound. The isolation of the substance in a neat form was found to provide unique access to two other high-energy molecules, namely, N-3-NCO (III) and cycl-N2CO (IV), among the decomposition products of (I). To understand the underlying reaction mechanism, the decomposition reactions including the thermal conversion of two conformers of (I) were revisited, and the potential energy surface (PES) was computationally explored by using the methods of B3LYP/6-311+G(3df) and CBS-QB3. The most stable syn-syn structure (I) readily converts into the syn-anti conformer (Delta H-exptl = 1.1 +/- 0.5 kcal mol(-1)), which undergoes decomposition in two competing pathways: a concerted path to N-3-NCO (III) or a stepwise route to (III) via the nitrene intermediate N3C(O)N (1)(II). The calculated activation barriers (E-a) are almost the same (similar to 33 kcal mol(-1), B3LYP/6-311+G(3df)). Further decomposition of (III) occurs through a concerted fragmentation into 2 N-2 + CO with a moderate E-a of 22 kcal mol(-1), and this process is compared to the isoelectronic species N-3-N-3 -> 3 N-2 (E-a = 17 kcal mol(-1)) and OCN-NCO -> N-2 + 2 CO (61 kcal mol(-1)). No low-energy pathway leading to (IV) was found on the singlet PES. However, the intervention of triplet ground-state (3)(II) from the initially generated (1)(II) through an intersystem crossing (ISC) offers a likely approach to (IV); that is, (3)(II) can decompose in a concerted process (E-a = 30 kcal mol(-1)) by eliminating one N-2 to yield the disfavored OCNN (3)(VI). A careful intrinsic reaction coordinate analysis and a combined energy scan of the N-C-N angle reveals a bifurcation point on this triplet PES, which allows a spin crossover to the singlet PES along the reaction coordinate and eventually leads to the formation of the metastable diazirinone (IV).