Structural Origin of Anisotropic Thermal Expansion of Molecular Crystals and Implication for the Density Rule Probed with Four ROY Polymorphs

The objective of this work was to investigate the molecular origin differences in thermal expansivity four ROY polymorphs (Y, R, OP, and ON) using variable temperature single crystal X-ray diffractometry (VT-SCXRD). Thermal found be directly influenced by packing number type directional interactions, such as hydrogen bonds, involved packing. Polymorphs with layered packing, i.e., ON, show higher volume expansivity, where axial component expansion is largest directions perpendicular hydrogen-bonded layers smallest along layers. Polymorph Y shows least which corresponds presence a denser network structure crystal, absence apparent overall observed for polymorph ON that lacks intermolecular bonds exhibits pattern two axes. lead violations density rule stability prediction due crossover change temperature, means rank order temperature-dependent. Thus, at absolute zero, most thermodynamically stable predicted not have highest density, violates rule. Likewise, all enantiotropic undergoing phenomenon, valid only within range bracketed temperatures (Td) thermodynamic transition (Tt). For monotropic polymorphs, above Td.