Acta Crystallographica Section B

Structural Science, Crystal Engineering and Materials

Volume 69, Part 1 (February 2013)


research papers



Acta Cryst. (2013). B69, 70-76    [ doi:10.1107/S2052519212049329 ]

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

C. Näther, C. Döring, I. Jess, P. G. Jones and C. Taouss

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), P21212 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 C2v 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)[R21(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).

Keywords: thermodynamics; polymorphism; hydrogen bonding; powder studies; solvent-induced phase conversion; differential scanning calorimetry.


pdfdisplay filedownload file

Portable Document Format (PDF) file
[ doi:10.1107/S2052519212049329/ps5022sup4.pdf ]
X-ray powder patterns amd DSC curve and table of refinement details at room temperatyre


Notes:

To open or display or play some files, you may need to set your browser up to use the appropriate software. See the full list of file types for an explanation of the different file types and their related mime types and, where available links to sites from where the appropriate software may be obtained.

The download button will force most browsers to prompt for a file name to store the data on your hard disk.

Where possible, images are represented by thumbnails.

 bibliographic record in  format

  Find reference:   Volume   Page   
  Search:     From   to      Advanced search

Copyright © International Union of Crystallography
IUCr Webmaster