Concomitant Polymorphism in an Organic Solid: Molecular and Crystal Structure and Intra- and Intermolecular Potential...

Area of Science:

• Solid-state chemistry
• Molecular dynamics
• Crystallography

Background:

• Polymorphism is common in organic molecules, impacting physical properties.
• Understanding molecular dynamics is crucial for predicting material behavior.
• Anthracene derivatives exhibit diverse structural and dynamic properties.

Purpose of the Study:

• To investigate the relationship between crystal structure and molecular dynamics in 2-(tert-butyl)-9-(4-(tert-butyl)phenyl)anthracene.
• To elucidate the factors contributing to polymorphism in this compound.
• To determine the rotational barriers of tert-butyl and methyl groups and their dependence on crystal structure.

Main Methods:

• Powder and single-crystal X-ray diffraction to identify polymorphs and analyze crystal structures.
• Ab initio electronic structure calculations to assess intramolecular energy barriers for molecular conformation.
• Solid-state 1H nuclear magnetic resonance (NMR) spin-lattice relaxation experiments to measure rotational dynamics and activation energies.

Main Results:

• At least four polymorphs were identified, with three characterized by single-crystal X-ray diffraction.
• The dihedral angle of the tert-butylphenyl group relative to the anthracene moiety varies among crystal structures.
• Low intramolecular energy barriers allow significant conformational changes, explaining polymorphism.
• NMR experiments provided average activation energies for tert-butyl and methyl group rotations, showing dependence on crystal structure.

Conclusions:

• Molecular flexibility is a key factor driving polymorphism in 2-(tert-butyl)-9-(4-(tert-butyl)phenyl)anthracene.
• Intermolecular interactions, influenced by crystal packing, significantly affect molecular dynamics.
• The study provides insights into structure-dynamics relationships in organic crystalline materials.