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Thermodynamic and structural relationships between the two polymorphs of 1,3-dimethylurea
Christian Näther1, Cindy Döring, Inke Jess
1Institut für Anorganische Chemie, Universität Kiel, Max-Eyth-Strasse 2, D-22418 Kiel, Germany.
Acta Crystallographica. Section B, Structural Science
|February 1, 2013
Summary
This study investigates the crystal polymorphism of a compound, revealing two distinct forms (I and II) with different symmetries and hydrogen-bonding patterns. Polymorph (I) is the stable form at room temperature, while polymorph (II) is metastable.
Area of Science:
- Crystallography
- Solid-state chemistry
- Materials science
Background:
- The title compound is known to exist in at least two polymorphic forms, designated (I) and (II), with different space groups and unit cell parameters.
- Previous studies have characterized these polymorphs, but further investigation at lower temperatures and detailed physical property analysis are warranted.
Purpose of the Study:
- To redetermine the crystal structures of polymorphs (I) and (II) at lower temperatures.
- To elucidate the relationship between the polymorphs and their thermodynamic stability.
- To characterize the phase transitions and identify the predominant form in commercial samples.
Main Methods:
- Single-crystal X-ray diffraction at 180 K for polymorph (I) and 100 K for polymorph (II).
- Differential scanning calorimetry (DSC) to study thermal behavior.
- Powder X-ray diffraction to analyze bulk samples and phase mixtures.
- Solvent-induced phase conversion experiments.
Main Results:
- The crystal structure of polymorph (I) was confirmed in the Fdd2 space group, with molecules exhibiting crystallographic twofold symmetry and associating into chains of rings via bifurcated hydrogen bonds.
- Polymorph (II) crystallizes in the P2(1)2(1)2 space group, also featuring similar molecular arrangements and hydrogen-bonding networks.
- Commercial samples predominantly consist of polymorph (II), which transforms endothermically into polymorph (I) upon heating.
- Polymorph (I) is the thermodynamically stable form at room temperature, with the transition temperature between 253 K and room temperature.
- An apparent third polymorph was identified as a mixture of (I) and (II).
Conclusions:
- The two polymorphs (I and II) are enantiotropically related, with (I) being the stable form at ambient conditions.
- Understanding the polymorphic behavior is crucial for controlling the solid-state properties of the compound.
- The study provides a comprehensive characterization of the polymorphism, including structural details and thermodynamic relationships.
Keywords:
differential scanning calorimetryhydrogen bondingpolymorphismpowder studiessolvent-induced phase conversionthermodynamicsMore Related Videos
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