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

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.