Five pseudopolymorphs of 6-amino-2-thio­uracil: absence of N—H...S hydrogen bonds

In order to study the preferred hydrogen-bonding pattern of 6-amino-2-thio­uracil, C₄H₅N₃OS, (I), crystallization experiments yielded five different pseudopolymorphs of (I), namely the dimethyl­formamide disolvate, C₄H₅N₃OS·2C₃H₇NO, (Ia), the dimethyl­acetamide monosolvate, C₄H₅N₃OS·C₄H₉NO, (Ib), the dimethyl­acetamide sesquisolvate, C₄H₅N₃OS·1.5C₄H₉NO, (Ic), and two different 1-methyl­pyrrolidin-2-one sesquisolvates, C₄H₅N₃OS·1.5C₅H₉NO, (Id) and (Ie). All structures contain R₂¹(6) N—H...O hydrogen-bond motifs. In the latter four structures, additional R₂²(8) N—H...O hydrogen-bond motifs are present stabilizing homodimers of (I). No type of hydrogen bond other than N—H...O is observed. According to a search of the Cambridge Structural Database, most 2-thio­uracil derivatives form homodimers stabilized by an R₂²(8) hydrogen-bonding pattern, with (i) only N—H...O, (ii) only N—H...S or (iii) alternating pairs of N—H...O and N—H...S hydrogen bonds.