Modelling temperature-dependent properties of polymorphic organic molecular crystals

Area of Science:

• Solid-state chemistry
• Materials science
• Computational chemistry

Background:

• Polymorphism is common in molecular organic crystals, impacting material properties.
• Understanding temperature-dependent stability is crucial for crystal engineering and predicting phase transitions.

Purpose of the Study:

• To investigate the temperature-dependence of structural and thermodynamic properties of organic molecular crystal polymorphs.
• To quantify the influence of lattice vibrations and thermal expansion on polymorph stability.
• To identify potential enantiotropic phase transitions in a large dataset of crystal structures.

Main Methods:

• Calculated Gibbs free energy differences between polymorph pairs at 0 K and melting points.
• Employed an anisotropic multipole-based force field for high accuracy.
• Incorporated thermal expansion using negative thermal pressure.

Main Results:

• Re-ranking of thermodynamic stability observed in 21% of the 475 polymorph pairs, indicating potential enantiotropic phase transitions.
• Vibrational contributions significantly affect free energy differences and thermodynamic stability.
• Thermal expansion generally has a minor impact on polymorph free energy differences.

Conclusions:

• Lattice vibrations are a critical factor in determining the relative stability of organic crystal polymorphs across temperatures.
• Enantiotropic phase transitions are a common phenomenon in organic molecular crystals.
• Accurate computational methods are essential for predicting temperature-dependent polymorphic behavior.