Tetragonal polymorph of 5,5-dichlorobarbituric acid

The tetragonal polymorph of 5,5-dichlorobarbituric acid (m.p. 478 K), C4H2Cl2N2O3, forms an N—H⋯O hydrogen-bonded tape structure along [001]. Two tapes related by a twofold rotation axis are associated via Cl⋯O contacts [3.201 (1) Å], and four such chain pairs are arranged around a fourfold roto-inversion axis. The crystal structures of the monoclinic and orthorhombic polymorphs have been reported previously [Gelbrich et al. (2011 ▶). CrystEngComm, 13, 5502–5509].


Comment
The polymorphic nature of 5,5-dichlorobarbituric acid (I) is already mentioned in Groth's compendium on the chemical crystallography of organic compounds, published more than a hundred years ago (Groth, 1910). As part of our wider investigation of solid state forms of barbiturates, we have recently determined the crystal structures of a monoclinic (Ia) and an orthorhombic (Ib) form (Gelbrich et al., 2011), and herein we report on the tetragonal polymorph (Ic) of the title compound. The equilibrium melting points of (Ia), (Ib) and (Ic), determined by hot-stage microscopy, are 477, 490 and 478 K, respectively. All three modifications were obtained in sublimation experiments; (Ia) as plates, (Ib) as prisms and (Ic) as long needles.
In the crystal structure of (Ic), a single tape consists of two parallel strands. Neighbouring molecules forming a single strand are N-H···O═C bonded to one another via their C6 carbonyl groups. Two strands of a tape are linked together by a second set of N-H···O═C interactions in which the C2 carbonyl group is involved. These interactions result in two independent R 3 3 (12) rings (Etter et al., 1990;Bernstein et al., 1995). The molecules of a single strand are related to one another by a translation along [001]. Additionally, the tape possesses a glide mirror plane that is oriented perpendicular to its mean plane. The C4 carbonyl group is not involved in hydrogen bonding.
The three polymorphs of (I) can be readily distinguished from each other by their FT-IR spectra, which are depicted in respectively. Therefore, the order of decreasing densities is (Ia) > (Ic) >> (Ib). The density of the tetragonal form (Ic) is 1% lower than that of the monoclinic form (Ia) and 6% higher than that of the orthorhombic polymorph (Ib), which is also the form of (I) with the most complex H-bonded structure.
supplementary materials sup-2 Experimental Needle-shaped crystals of (Ic) were obtained in a sublimation experiment carried out at 473 K. On heating, (Ic) undergoes a transformation into (Ib). However, the melting of (Ic) can be observed in a thermomicroscopic experiment if the crystals are placed on a hot stage that is preheated to just below the melting temperature of (Ic).
The FT-IR spectrum of (Ic) (see Fig. 4) shows a strong and sharp N-H vibration at 3258 cm -1 and a weak one at 3152 cm -1 . In the C═O region the spectrum exhibits a weaker band at 1756 cm -1 with a shoulder at about 1769 cm -1 and a stronger band at 1729 cm -1 . These characteristics are consistent with the G5b-type spectrum in the IR classification schmeme for barabiturates (Zencirci et al., 2009). This type indicates the presence of the H-bonded tape connectivity C-3. Previous G5b examples include form I of alphenal and the metastable polymorph VIII of phenobarbital (Zencirci et al., 2009).