Structural and Energetic Origin of Elastic and Plastic Bending in Molecular Crystals

The ability of polymer and metallic materials to deform without breaking is one of their most useful properties, however, this behavior was only recently discovered in crystalline molecular materials. Attempts to categorize molecular crystals as either mechanically plastic or elastic have relied on basic qualitative observations. In this study, we employ X-ray quantum crystallography (QCr) to distinguish these distinct mechanical behaviors through a comparative quantitative analysis of three pairs of isostructural organic crystals that demonstrate both plastic and elastic bending. QCr analysis identified subtle yet consistent differences in lattice cohesive energies (LCE) of 1-4 kJ mol^{- 1}, with higher dispersion energy components and potential energy densities at the bond critical points of intermolecular interactions in elastic relative to plastic crystals. While the intermolecular force constants (k) of elastic crystals are systematically higher, k_elastic > k_plastic, the trend in the corresponding pairwise compressibilities ( $\tau$ ), $\tau$ _elastic > $\tau$ _plastic, is counterintuitive. Contrary to some intuitive considerations, our analysis of slip planes and rugosity does not effectively differentiate between elasticity and plasticity; instead, the QCr-derived atomic displacement parameters implicate more pronounced lattice vibrations and higher entropy and thus, shallower intermolecular potential wells, in plastically bending crystals.