According to the results of PBE0/cc-pVTZ quantum chemical calculations, the equilibrium mixture of alpha-diazocarbonyl bullvalene derivatives (C10H8N2O) contains an impurity of the corresponding bullvaleno-[1,2,3]oxadiazoles. The carbonyl group in the major tautomer is conjugated with the three-membered ring. The concentration of other tautomers in the equilibrium mixture of two bullvaleno[1,2,3]selenadiazoles is negligible.