A variable-temperature X-ray diffraction and theoretical study of conformational polymorphism in a complex organic...

Area of Science:

• Solid-state chemistry
• Crystallography
• Computational chemistry

Background:

• 3-diethylamino-4-(4-methoxyphenyl)-1,1-dioxo-4H-1λ6,2-thiazete-4-carbonitrile (DTC) exhibits polymorphism.
• Understanding crystal polymorphism is crucial for material properties and drug development.

Purpose of the Study:

• To analyze two conformational crystal polymorphs of DTC.
• To investigate the role of electrostatic versus dispersion-repulsion forces in phase stability.
• To determine the influence of temperature on crystal structure and stability.

Main Methods:

• Single crystal X-ray diffraction at variable temperatures (100 K to room temperature).
• High-quality Density Functional Theory (DFT) calculations.
• Energy decomposition analysis.
• Quantum Theory of Atoms in Molecules (QTAIM) analysis of intermolecular contacts and dipole moments.

Main Results:

• Two polymorphs of DTC were identified, differing by a -OCH3 group flip, with similar energies and low interconversion barriers.
• The system is monotropic, with one phase consistently more stable.
• Energy decomposition revealed dominant electrostatic contributions, stabilizing with decreasing temperature.
• Dispersion and repulsion forces exhibited expected temperature dependencies.

Conclusions:

• Electrostatic interactions are the primary drivers of phase stability in DTC polymorphs.
• The subtle differences in conformation and intermolecular forces explain the observed phase behavior and stability trends.
• QTAIM analysis provides detailed insights into the intermolecular interactions governing phase stability.