Comparison of the hydrogen-bond patterns in 2-amino-1,3,4-thiadiazolium hydrogen oxalate, 2-amino-1,3,4thiadiazole succinic acid (1/2), 2-amino-1,3,4-thiadiazole glutaric acid (1/1) and 2-amino-1,3,4-thiadiazole adipic acid (1/1)

The X-ray single-crystal structure determinations of the chemically related compounds 2-amino-1,3,4-thiadiazolium hydrogen oxalate, C2H4N3S+center dot C2HO4-, (I), 2-amino-1,3,4-thiadiazole-succinic acid (1/2), C2H3N3S center dot 2C(4)H(6)O(4), (II), 2-amino-1,3,4-thiadiazole-glutaric acid (1/1), C2H3N3S center dot-C5H8O4, (III), and 2-amino-1,3,4-thiadiazole-adipic acid (1/1), C2H3N3S center dot C6H10O4, (IV), are reported and their hydrogenbonding patterns are compared. The hydrogen bonds are of the types N-H center dot center dot center dot O or O-H center dot center dot center dot N and are of moderate strength. In some cases, weak C-H center dot center dot center dot O interactions are also present. Compound (II) differs from the others not only in the molar ratio of base and acid (1:2), but also in its hydrogenbonding pattern, which is based on chain motifs. In (I), (III) and (IV), the most prominent feature is the presence of an R-2(2)(8) graph-set motif formed by N-H center dot center dot center dot O and O-H center dot center dot center dot N hydrogen bonds, which are present in all structures except for (I), where only a pair of N-H center dot center dot center dot O hydrogen bonds is present, in agreement with the greater acidity of oxalic acid. There are nonbonding S center dot center dot center dot O interactions present in all four structures. The difference electron-density maps show a lack of electron density about the S atom along the S center dot center dot center dot O vector. In all four structures, the carboxylic acid H atoms are present in a rare configuration with a C-C-O-H torsion angle of similar to 0 degrees. In the structures of (II)-(IV), the C-C-O-H torsion angle of the second carboxylic acid group has the more common value of similar to vertical bar 180 vertical bar degrees. The dicarboxylic acid molecules are situated on crystallographic inversion centres in (II). The Raman and IR spectra of the title compounds are presented and analysed.