Disorderly Conduct of Benzamide IV: Crystallographic and Computational Analysis of High Entropy Polymorphs of Small Molecules

Benzamide,
a simple derivative of benzoic acid and a common intermediate
of pharmaceutical compounds, was reported to form two polymorphs in
1832, but the single crystal structure of the more stable form was
not solved until 1959. Nearly 50 years later, the second form was
characterized by powder diffraction, followed shortly thereafter by
characterization of a third form, a polytype of the most thermodynamically
stable Form I. These two new forms, Forms II and III, are metastable. Herein, we describe a fourth
polymorph, Form IV, discovered by melt crystallization
concurrently with its crystallization under confinement at small length
scales (<10 nm), where it is stable indefinitely. Form III exists under confinement in larger pores, and melting point data
for different pore sizes corroborate the existence of Form IV below 10 nm. Form IV is highly disordered, precluding
indexing of powder diffraction data other than hk0 reflections. Nonetheless, a combination of powder X-ray diffraction
and computational crystal structure prediction reveals that Form IV contains a 2D motif resembling that of Form II, but with long-range order in the third dimension masked by ubiquitous
stacking faults. This approach relies on distilling a large number
of candidate structures to a few possible disorder models based on
benzamide tetrads that organize in 2D parquet-like tiles, with organization
along the third dimension, that can be modeled with various stacking
fault configurations having distinct intermolecular interactions and
translations in the dimension orthogonal to the tiling planes. These
observations reveal a bewildering crystallographic complexity for
such a simple molecule. Nonetheless, the approach described herein
demonstrates that challenging structures that may be abandoned prematurely
because of poor crystallinity, twinning, or disorder can be solved.