Polymorphic phase transformations of 3-chloro-trans-cinnamic acid and its solid solution with 3-bromo-trans-cinnamic...

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• Polymorphism significantly impacts material properties.
• Understanding phase transformations is crucial for controlling material behavior.
• Halogenated cinnamic acids offer a model system for studying solid-state reactions.

Purpose of the Study:

• To investigate high-temperature polymorphic phase transformations in 3-chloro-trans-cinnamic acid (3-ClCA) and a 3-ClCA/3-bromo-trans-cinnamic acid (3-BrCA) solid solution.
• To compare the transformation pathways and resulting polymorph structures.
• To elucidate structure-property relationships in these halogenated organic compounds.

Main Methods:

• Variable temperature powder X-ray diffraction (PXRD) to monitor phase changes.
• Single-crystal X-ray diffraction to determine the precise structures of polymorphs.
• Differential scanning calorimetry (DSC) to identify transition temperatures.

Main Results:

• 3-ClCA undergoes a γ to β polymorphic transformation at 413 K.
• The resulting β polymorph of 3-ClCA is structurally distinct from the β polymorph of 3-BrCA, despite their isostructural γ phases.
• The solid solution also transforms from a γ-type to a β-type structure, with the latter being similar to the β polymorph of 3-BrCA.

Conclusions:

• The polymorphic transformations of 3-ClCA and its solid solution with 3-BrCA yield distinct crystalline structures.
• The presence of different halogen substituents influences the resulting β-phase structure.
• These findings highlight the sensitivity of solid-state transformations to molecular composition and packing.