On constitutional isomers and tautomers of oxadiazolones and their mono- and disulfur analogues (C2H2N2XY; X, Y = S, O)

► Oxadiazolone, oxadiazolthione, thiadiazolone, thiadiazolthione isomers were studied. ► For each constitutional isomer five tautomers conserving five-membered ring were considered. ► The chalcogen atom in the ring determines differences between oxadiazolone O and S analogues. ► Presence of water do not change the general isomer relative stability picture. ► The π-electron system of the studied compounds is not sensitive to change of the external chalcogen.