Thermodynamic and structural relationships between the two polymorphs of 1,3‐dimethylurea

The title compound exists as polymorph (I), Fdd 2 with Z = 8 [Pérez‐Folch et al. (1997). J. Chem. Cryst. 27 , 367–369; Marsh (2004). Acta Cryst. B 60 , 252–253], and as polymorph (II), P 2 1 2 1 2 with Z = 2 [Martins et al. (2009). J. Phys. Chem. A , 113 , 5998–6003]. We have redetermined both structures at somewhat lower temperatures [(I) at 180 K rather than room temperature; (II) at 100 K rather than 150 K]. For polymorph (I) the space group Fdd 2 is confirmed rather than the original choice of Cc . The molecular structures of both polymorphs are essentially identical, with exact crystallographic twofold symmetry, approximate C 2 v symmetry, and a trans orientation of the H—N—C=O moiety. In both polymorphs the molecules associate into chains of rings with graph set C (4)[ R 2 1 (6)] via bifurcated hydrogen‐bond systems C(N—H) 2 ...O=C. In the polar structure (I) the chains are necessarily all parallel, whereas in (II) equal numbers of parallel and antiparallel chains are present. Further physical investigations [differential scanning calorimetry (DSC), powder investigations, solvent‐induced phase conversions] were undertaken: these showed: (i) that the commercially available compound consists predominantly of polymorph (II), which on heating transforms into polymorph (I) by an endothermic reaction, so that both polymorphs are related by enantiotropism; (ii) that polymorph (I) represents the more stable modification at room temperature, where polymorph (II) is metastable, with the thermodynamic transition temperature lying somewhere between 253 K and room temperature. An apparent third polymorph, consisting of fibrous needles, was shown by powder diffraction to consist of a mixture of polymorphs (I) and (II).