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

Abstract

In contrast to the mature predictive frameworks applied to neutral cocrystals, ionic cocrystals, those including an ion pair, are difficult to design. Furthermore, they are generally excluded categorically from studies which correlate specific molecular properties to cocrystal formation, leaving the prospective ionic cocrystal engineer with few clear avenues to success. Herein ammonium nitrate, an energetic oxidizing salt, is targeted for cocrystallization in a potential coformer group selected based on likely interactions with the nitrate ion as revealed in the Cambridge Structural Database; six novel ionic cocrystals were discovered. Molecular descriptors previously identified as being related to neutral cocrystal formation were examined across the screening group but showed no relationship with ionic cocrystal formation. High packing coefficient is shown to be a constant among the successful coformers in the set and is utilized to directly target two more successful coformers, bypassing the need for a large screening group.Cocrystal design strategies are rapidly evolving to enhance predictive capabilities and improve the efficiency of coformer screening. However, many of the molecular descriptors associated with successful cocrystallization are ambiguous for salts, complicating coformer selection when designing ionic cocrystals. Herein, a study of ammonium nitrate ionic cocrystals reveals packing coefficient as a promising indicator for coformer success.