In Silico Prediction of Structural Properties of a Racemic Porous Organic Cage Crystal

Y Liu and GH Zhu and WQ You and HJ Tang and CW Jones and RP Lively and DS Sholl, JOURNAL OF PHYSICAL CHEMISTRY C, 123, 1720-1729 (2019).

DOI: 10.1021/acs.jpcc.8b08838

Porous organic cage (POC) solids are porous materials made up of individual porous molecules held together by noncovalent forces. Although many examples of POCs have been made in homochiral forms (i.e., crystals in which every molecule has the same chirality), crystallization of POCs that contain mixtures of chiral molecules can also yield useful properties. For instance, our previous work has demonstrated that a POC crystal cage crystal 3 (CC3)-racemic synthesized using a racemic mixture of diaminocyclohexane has improved sorption properties and stability than a similar POC crystal synthesized with homochiral diaminocyclohexane. The key aim of this paper is to predict the structure of CC3-racemic with atomic detail because it is challenging to fully resolve the structure of this racemic crystal experimentally since only subtle differences exist between the racemic structure and the known crystal structure of homochiral CC3. Here, we introduce an in silico prediction method that combines electronic structure calculations and atomistic calculations to predict the structure of CC3-racemic. We first enumerate types of cage molecules that can be present in this material and establish their concentrations. A key observation from these calculations is that CC3-racemic is not made up of only CC3-R and CC3-S molecules and that an additional class of heterochiral cages is also present. By studying the packing energy of cage pairs in CC3-racemic, a lattice model representation of the racemic crystal is developed and used in MC simulations to assess the structure of extended crystals. By expanding the lattice model into atomic detail, fully detailed CC3-racemic crystal models are obtained. These models provide the most thorough description available to date of the composition and cage packing of this interesting material.

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