Remarkable discrepancy in the predominant structures of acyl(or thioacyl)aminothiadiazoles, acyl(or thioacyl)aminooxadiazoles and related compounds having the potential for rotational, geometrical and tautomeric isomerism

X-Ray crystallographic structures of 5-ethyl-2-trifluorothioacetylamino-1,3,4-thiadiazole 2, 5-ethyl-3-p-nitrobenzyl-2-trifluorothioacetylimino-1,3,4-thiadiazoline 4 and 5-n-propyl-2-trifluoroacetylamino-1,3,4-thiadiazole 7 proved to be monomeric or dimeric bearing intermolecular hydrogen bondings and/or an intramolecular nonbonded 1,5-type S⋯S or S⋯O interactions. In contrast, the X-ray crystallographic structure of 5-ethyl-2-trifluoroacetylamino-1,3,4-oxadiazole 5 was shown to be dimeric involving ordinal intra- and intermolecular hydrogen bondings without any close contact. The ab initio computational studies (HF/6-311G*) of the four monomeric formylamino- and thioformylaminothiadiazoles(or -oxadiazoles) and formylimino- and thioformyliminothiadiazolines(or -oxadiazolines) structures 8–11 clarified the relative structure–stability order and a remarkable discrepancy in the predominant structures of the acylamino- and thioacylaminothiadiazoles and -oxadiazoles having the potential for rotational, geometrical and tautomeric isomerism.