With involvement of substituents

All known examples of ring-chain tautomerism can be conveniently divided into two large groups (i) those without direct involvement of any substituent in the heterocyclic ring and (ii) isomerizations with participation of certain substituents. 

With a few exceptions simple 1,2,3-oxadiazole (245, X = 0) are not isolable because they are less stable than their open tautomers (246, X = O). The geometry and energy of 1,2,3-oxadiazole, calculated by an ab initio method (85AG(E)713) indicate that the heterocycle is unlikely to be isolated as a discrete species, even in a matrix at low temperature, because there is a low energy barrier to ring opening to di azoacetaldehyde. 

The sterically protected 1,2,3-oxadiazole (247) is the only known oxadiazole bearing alkyl substituents. It exists in the cyclic form in the crystalline state but as the diazoketone in chloroform 

3-thiadiazoles (245, X = S) exist as a remarkably stable neutral aromatic compound. It is isomeric with the ring opened a-diazoketones (246, X = S) and there is evidence that it can react through (246) as an intermediate. 

For mesoionic 1,2-dithioles, IR studies indicate that the cyclic structure (249) is favored over acyclic structure (250) except where R is an amino substituent (87PS(3l)109). 

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