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

Abstract

The title compound exists as polymorph (I), Fdd2 with Z = 8 [Pérez-Folch et al. (1997). J. Chem. Cryst. 27, 367-369; Marsh (2004). Acta Cryst. B60, 252-253], and as polymorph (II), P2(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 Fdd2 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(2v) 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).

Packing principles for urea and thiourea solvates: structures of urea : morpholine (1 : 1), urea : 1,4-dioxane (1 : 1), thiourea : morpholine (4 : 3) and thiourea : 1,4-dioxane (4 : 1)

“…The urea residues are connected to form chains of rings parallel to the a axis; the hydrogens trans to OLC across the N-C bond act as donors to the oxygen of the neighbouring molecule (via H bonds #1 and #3). This pattern, with the graph set 22 C(4)[R 1 2 (6)], is a robust supramolecular synthon that occurs in the structure of urea itself 23 and of many N,N9-substituted ureas (in which the NH donors trans to CLO are retained), 16,24 although it has not been realised in other urea solvates.…”

“…10 For thiourea, all ordered solvates are 3 : 1 clathrates with cyclic hydrocarbons or related materials; 11 to the best of our knowledge, no structure of a thiourea solvate involving a simple solvent has been published, 12 although the cell constants [a 9.7, b 7.2, c 14.2 Å, b 98u, space group P2/m, refcode FIBSIS] of a ''3.82 : 1 thiourea : 1,4-dioxane clathrate'' were reported in 1987. 13 We are interested in the structures and polymorphism of simple urea and thiourea derivatives; our recent publications include reports on the structures of trimethylurea 14 and tetramethylthiourea 15 and on polymorphism in 1,3-dimethylurea 16 and trimethylthiourea. 17 In all cases particular interest attached to the packing patterns, which are generally determined by classical hydrogen bonding.…”

Two polymorphs of 4-hydroxypiperidine with different NH configurations

“…For 1t, 1o and 3, which all crystallize in non-centrosymmetric but achiral space groups, the anomalous dispersion was negligible and Friedel opposite reflections were therefore merged; the Flack parameters are thus meaningless. Compound 4 was a nonmerohedral twin generated by 180°rotation about c* and was refined using the HKLF5 method; 24 the relative volume of the smaller component refined to 0.4150 (7). Because equivalent reflections are merged and reflections from both twin components are present, the number of reflections should be interpreted with caution.…”

“…ref. [6][7][8], whereby we have generally depended on serendipity and the recognition of alternative crystal habits as a precondition for more detailed investigations. Our chemical and crystallographic interests, including studies of amine complexes of gold 9 and solvates/ adducts of urea derivatives, 10 have provided numerous examples.…”

Inhibition of the Vapor-Mediated Phase Transition of the High Temperature Form of Pyrazinamide

“…DMU ( Figure 1 b) has two known polymorphic forms, which are enantiotropically related; form I is stable at high temperatures and form II at low temperatures. 23 The thermodynamic transition temperature lies around 298 K in the presence of water, even though it is observed in the DSC at 324 K, and at low relative humidity (RH) it increases to 331 K. 24 DMU is commercialized in form I. Form II of DMU transforms to form I at room temperature, but form II was not produced during these experiments.…”