Rationalizing Distinct Mechanical Properties of Three Polymorphs of a Drug Adduct by Nanoindentation and Energy Frameworks Analysis: Role of Slip Layer Topology and Weak Interactions

Three concomitant polymorphs of 3-((4-chlorophenyl)­imino)­indolin-2-one, a Schiff’s base, are identified and sorted based on morphology and mechanical response of their crystals. Form I grows as blocks and shows brittle fracture, Form II has long needles and shows plastic bending, and Form III also has long needles and shows elastic bending under similar qualitative mechanical deformation tests. Furthermore, the brittle Form I was found to exhibit thermosalient behavior (jumping) when heated on a hot plate. The distinct mechanical behavior of the three forms is rationalized by analyzing intermolecular interaction energies from energy frameworks analysis, slip layer topology, Hirshfeld surface analysis, and nanoindentation. The quantitative nanoindentation studies unveiled that Form III has higher elastic modulus and stiffness than Forms I and II, while the hardness was lowest for the plastic Form II. Despite high structural similarity in Forms II (plastic) and III (elastic), the E of elastic Form III was found to be 3 orders of magnitude higher than that of plastic Form II crystals, which is attributed to the subtle differences in interaction energies and slip layer topology in the two cases. Consideration of slip layer topology and interaction energies from the structures are very useful for rationalizing mechanical properties, but may not be always sufficient, and one may also need to know the topology of the potential energy surface of the slip layers for understanding the distinct mechanical behavior.