The Hidden Rules of Bendable Crystals: Quantifying the Energetics of Plastic Deformation at the Molecular Level

The exceptional mechanical flexibility of organic crystals has generated considerable interest for applications in flexible electronics, soft robotics, and photonic devices. While plastic bending has conventionally been attributed to anisotropic, layered structures, both the plastic α- and brittle δ-polymorphs of pyrazinamide satisfy these established criteria, yet exhibit fundamentally different mechanical behavior. Through integrated computational and experimental analysis, we demonstrate that the α-form’s (002) slip system uniquely combines low interlayer binding energy (∼20% of lattice energy) and an exceptionally low slip barrier (∼1–2 kJ/mol─5 times lower than the lowest barrier in the δ-form). These energetic descriptors complement conventional structural criteria, offering new insights into the mechanisms distinguishing plastic from brittle behavior in molecular crystals and suggesting a promising framework for future predictive studies.