Progress in Predicting Ionic Cocrystal Formation: The Case of Ammonium Nitrate.

Area of Science:

• Materials Science
• Crystallography
• Chemical Engineering

Background:

• Predictive frameworks for neutral cocrystal design are mature, but ionic cocrystals, containing ion pairs, remain difficult to engineer.
• Existing studies often exclude ionic cocrystals, limiting design strategies and understanding of structure-property relationships.

Purpose of the Study:

• To address the challenges in designing ionic cocrystals.
• To identify key molecular properties governing the formation of ionic cocrystals.
• To develop a more targeted approach for discovering novel ionic cocrystals.

Main Methods:

• Targeted ammonium nitrate for cocrystallization with coformers selected based on Cambridge Structural Database analysis of nitrate ion interactions.
• Screened potential coformers and analyzed molecular descriptors related to neutral cocrystal formation.
• Investigated packing coefficient as a potential predictor for ionic cocrystal success.

Main Results:

• Six novel ionic cocrystals containing ammonium nitrate were successfully synthesized.
• Previously identified molecular descriptors for neutral cocrystals did not correlate with ionic cocrystal formation.
• A high packing coefficient was consistently observed in successful ionic cocrystal coformers.

Conclusions:

• High packing coefficient is a critical and constant parameter for successful ionic cocrystal formation.
• This finding enables a more direct and efficient strategy for designing and discovering new ionic cocrystals.
• The study provides a new avenue for engineers working with energetic oxidizing salts and ionic cocrystal design.